Gurgling and air in central heating

[jim843]

Hello,

Around 2 years ago we had a very old, leaky Baxi combi replaced with a newer Baxi.

For background I live in a 3 storey terrace. First floor has 3 rads, second floor has 4 rads and third floor has 2 rads.

We noticed problems with noise and air in the system. One of the rads in the loft made gurgling noises. If the rad was bled, air came out. Also, at this time the pressure gauge was all over the place. Up to 3, back down to 1 and every number in between. The gauge didn’t appear to drop, though.

The installer wasn’t helpful over the phone so I ended up finding a different engineer.

Turns out the installer of the boiler didn’t do a particularly thorough job. He didn’t clean the old system, I found leaks on some of the nuts beneath the boiler, he didn’t fit a soak away outside so water drips out over our path (great in winter when it’s freezing).

He engineer found the expansion vessel was flat. He pumped it up, tightened the pipes, advised on a soak away. He told me to bleed all the rads individually for a few mins, top up the system and add more inhibitor. He also suggested power flushing the system. I did all this and also rented a machine and flushed the system myself.

I did all this but the problems remained. I ended up contacting Baxi who came out and replaced the expansion vessel.

This didn’t solve anything. More gurgling, more air.

We ended up getting the boiler serviced again (this is maybe a year after install) and I mentioned the problems to the engineer. He asked the usual questions (is there inhibitor, have you bled the system, etc) and mentioned we had an old radiator in the bathroom. He thought this might be the source of air.

We were planning on getting the bathroom refitted so I decided to wait and see if that solved any issues.

We got the bathroom refitted. Now, the gurgling and air does not go to the top of the house but is in the towel rail in the bathroom. I can bleed the rad once a day and there’s maybe a second or two of air and then water. The pressure does not drop on the boiler.

I contacted Baxi again who came out and replaced the expansion vessel with a bigger one. The engineer also suggested fitting an automatic bleed valve on the chrome towel rail because in his experience chrome towel rails suck in air. I bought one two valves (one for spare) and fitted it.

I noticed an odd thing while fitting the automatic bleed valve. I turned off the two rad valves, released the pressure. Once the air left the rad I could put my finger in the top of the rad and feel there was no water there at all. I’d expected that the water would be just below the top of the rad?

But you’ve guessed it — still gurgling and air in the system! The automatic bleed valve doesn’t appear to vent automatically, not sure why. Could be faulty. I can swap it over with the spare but haven’t gotten around to it yet.

The only other thing I noticed with the system is that if the system is off in the summer then there is minimal air in the rad. I assume because the pump isn't forcing water around the system?

If you got this far, thanks! I’d love to solve this issue and have a system that works! I’m not opposed to ripping out pipework and starting again (even though I’m nearing the end of a long renovation!). Ideally there are some simpler steps I can take to diagnose?

Any advice?

 

[SJB060685]

That's a bit of a length post 😂. When the system is cold what are you pressurising the system to?

 

[Last Plumber]

Whopper of a post!
I don't agree with the auto air vent idea. A heating system should not be producing or introducing gases. Once they are installed correctly they shouldn't really need 'topping up'.

How far vertically (feet or meters) is the top of the top rad/towel rail from the bottom of the boiler?
What pressure is in the boiler when it is cold and not running?
Just for information.

 

[jim843]

Thanks, both! I went for maximum detail!

From bottom of boiler to top of towel rail it’s about 3.6m. There’s another floor above as well.

Pressure from cold? Not too sure now as it hasn’t been refilled in ages.

I know that when the pump stops the gauge hovers around 1.5. When the pump comes on it’ll drop to about 1.2. There’s a slight variance.

 

[SJB060685]

When filling a pressurised system you have allow for the lift/height of the property. So if the boiler and pressure gauge are on the ground floor and the highest pipework above it is say 5m then you will have to fill to at least .5 bar plus a bit extra so the pipes at the highest point of the system don't draw/suck in air.

 

[jim843]

@SJB060685 thanks. I’ll let the system cool and pressurise to say, 2.2? If that number seems reasonable I’ll go ahead and do it.

Out of interest, why would air collect in the bathroom towel rail and not the two rads on the floor above?

 

[SJB060685]

@SJB060685 thanks. I’ll let the system cool and pressurise to say, 2.2? If that number seems reasonable I’ll go ahead and do it.

Out of interest, why would air collect in the bathroom towel rail and not the two rads on the floor above?

When the system is cold top up to about 1.5 bar and see if that has an affect. I wouldn't go as high as 2 bar as this is likely to cause the prv to discharge when the system is hot, the vessel condition and charge will have an affect on system pressure as well.
Why would air settle in the towel rails and not on the floor above? Air is less dense than water so you are correct to assume that it would settle at highest point of the system and usually does, however by design towel rails are taller and it's possible the air is getting stuck at the top and the water is circulating below it, happy to be proved wrong there though?

 

[Deleted member 123817]

Check that the flow/return piping are connected correctly to the boiler ie not reversed , would normally expect pressure to rise when the circ pump starts,

 

[jim843]

Check that the flow/return piping are connected correctly to the boiler ie not reversed , would normally expect pressure to rise when the circ pump starts

Gotcha.

I was under the impression that the flow and return didn’t matter? Like, doesn’t matter which way the flow and return is attached to bidirectional TRVs, etc?

I think I’m missing something! 🙂

 

[jim843]

Check that the flow/return piping are connected correctly to the boiler ie not reversed , would normally expect pressure to rise when the circ pump starts,

Or if I ask another way: how can I tell if the flow and return and round the wrong way?

All I can see is some pipe work going from the boiler. I don’t know which should be the flow, which should be the return…

 

[Deleted member 123817]

The boiler won' be very happy if the flow & return are swapped over however unlikely that it was installed this way. Just feel the hotter of the two pipes and check in your users manual if correct.

 

[jim843]

When the system is cold top up to about 1.5 bar and see if that has an affect. I wouldn't go as high as 2 bar as this is likely to cause the prv to discharge when the system is hot, the vessel condition and charge will have an affect on system pressure as well.
Why would air settle in the towel rails and not on the floor above? Air is less dense than water so you are correct to assume that it would settle at highest point of the system and usually does, however by design towel rails are taller and it's possible the air is getting stuck at the top and the water is circulating below it, happy to be proved wrong there though?

I jumped the gun a bit and pressurised to 2.2. So far no air in the bathroom rad.

I’ll check the flow and return when I have a sec.

So far the upstairs rads seem much hotter (a good thing).

 

[jim843]

When the system is cold top up to about 1.5 bar and see if that has an affect. I wouldn't go as high as 2 bar as this is likely to cause the prv to discharge when the system is hot, the vessel condition and charge will have an affect on system pressure as well.
Why would air settle in the towel rails and not on the floor above? Air is less dense than water so you are correct to assume that it would settle at highest point of the system and usually does, however by design towel rails are taller and it's possible the air is getting stuck at the top and the water is circulating below it, happy to be proved wrong there though?

Boiler was at 1.5 when cooling. I pressurised to 2.2 (based on your advice of .5 and a bit).

The bathroom towel rail seems much hotter, which is good.

There is still a tiny bit of air in the bathroom towel rail every day. I want to say quite a bit less but there’s nothing scientific about it. I turn the bleed valve and I get a bit of air for half a second/second.

Other rads in the house, when bled, are just instant water.

Still need to check if flow and return on boiler have been flipped accidentally.

Any ideas?

 

[jim843]

When the system is cold top up to about 1.5 bar and see if that has an affect. I wouldn't go as high as 2 bar as this is likely to cause the prv to discharge when the system is hot, the vessel condition and charge will have an affect on system pressure as well.
Why would air settle in the towel rails and not on the floor above? Air is less dense than water so you are correct to assume that it would settle at highest point of the system and usually does, however by design towel rails are taller and it's possible the air is getting stuck at the top and the water is circulating below it, happy to be proved wrong there though?

@SJB060685 checked system at about 9am this morning and there was a 1sec burst of air. Checked at 10:15am and there was another 1sec burst of air.

So air somehow being pulled/generated in the system?

 

[SJB060685]

@SJB060685 checked system at about 9am this morning and there was a 1sec burst of air. Checked at 10:15am and there was another 1sec burst of air.

So air somehow being pulled/generated in the system?

Would appear so yes mate. Are you sure it's just air and not hydrogen?

 

[jim843]

Would appear so yes mate. Are you sure it's just air and not hydrogen?

I’m not sure.

Is this the old lighter trick? Not quite sure how best to test it out.

 

[jim843]

Would appear so yes mate. Are you sure it's just air and not hydrogen?

@SJB060685 just reading How to | Circulation Issue Causes Central Heating | Sentinel - https://www.sentinelprotects.com/uk/support/how-diagnose-circulation-issues

• If there is a reddish tinge to the water, it indicates that iron oxide is present, suggesting active corrosion. The system may be drawing in air.
• If there are black particles present in the water, these are most likely to be magnetite. As this is the final stage of corrosion, large deposits can be expected within the system.

I'd say I've seen both of these in the system.

I bled the rads last week and the water was reddish in places.

I also bled the towel rail on the automatic air vent side (you have to unscrew the whole thing). The pressure was high and a jet of black water shot out of the rad.

Could it be as simple as the system needs a thorough clean?

 

[SJB060685]

That is an indication you have sludge (magnetite) in the system. This is caused where you have large amounts of air entering the system and it and the water reacts with the iron content on the steel radiators causing corrosion. Hydrogen may be produced as a byproduct of this corrosion as well. The lighter or match trick would reveal if hydrogen yes. Next time you vent a radiator vent a small sample of the gas into an upturned cup and offer a flame to it, if it pops it's hydrogen. Don't vent too much into the cup though.

 

[jim843]

That is an indication you have sludge (magnetite) in the system. This is caused where you have large amounts of air entering the system and it and the water reacts with the iron content on the steel radiators causing corrosion. Hydrogen may be produced as a byproduct of this corrosion as well. The lighter or match trick would reveal if hydrogen yes. Next time you vent a radiator vent a small sample of the gas into an upturned cup and offer a flame to it, if it pops it's hydrogen. Don't vent too much into the cup though.

Thanks @SJB060685! I'll give it a few days and vent into a cup so there's a bit more air/hydrogen.

All radiators in the property are less than 5 years old. Most are column rads.

Does the air/hydrogen in the system mean that air is entering the system now?

Or can the air/hyrdrogen be a consequence of a prior leak and subsequent corrosion?

 

[SJB060685]

Hydrogen build-up is a by product of the rads corroding, I believe it can form from galvanic corrosion as well, however I can't find anything to stare that for sure. Hydrogen Sulphide however is caused by a bacterial problem but is also accompanied by a rotten egg smell, unlike hydrogen which is odourless. If it's air in the system then it must be coming from somewhere.
I take it it was another combi installed?
The sludge/magnetite needs to be removed. Ideally one would have a thorough powerflush, then suitable inhibitor added to the system afterwards, plus a filter on the return going back into the boiler. When the new boiler was installed the engineers should have taken into consideration the quality of the existing system water and pipework for foulness, you probably would have been better off separating the boiler from system side with a plate heat exchanger, this would have prevented any fouled water and debri from entering the boiler.

 

[jim843]

Hydrogen build-up is a by product of the rads corroding, I believe it can form from galvanic corrosion as well, however I can't find anything to stare that for sure. Hydrogen Sulphide however is caused by a bacterial problem but is also accompanied by a rotten egg smell, unlike hydrogen which is odourless. If it's air in the system then it must be coming from somewhere.
I take it it was another combi installed?
The sludge/magnetite needs to be removed. Ideally one would have a thorough powerflush, then suitable inhibitor added to the system afterwards, plus a filter on the return going back into the boiler. When the new boiler was installed the engineers should have taken into consideration the quality of the existing system water and pipework for foulness, you probably would have been better off separating the boiler from system side with a plate heat exchanger, this would have prevented any fouled water and debri from entering the boiler.

Yes, previous boiler was a combi. New boiler is also a combi.

The installer added an inhibitor but didn't powerflush or add any cleaner. I remember asking him about cleaning the system and he said he'd do "everything required".

When I cleaned the system (a few months after install) there was quite a bit of black water coming out.

There is a filter on the return side (assuming the return side is correctly plumbed!).

The pipework on the bottom two floors of the property are fairly old as far as I know. From the second floor to third floor it's JG Speedfit.

If I understand I need to:

• see if it's hydrogen in the system
• if it is hydrogen, something is corroding in the system
• if it is corrosion, then I need to powerflush the system and add inhibitor

If it's not hydrogen... argh!

 

[SJB060685]

That was another consideration, did you have any plastic pipework? Which you say you do. If the plastic pipework is non barrier then that could very well be the point of air ingress.

 

[jim843]

That was another consideration, did you have any plastic pipework? Which you say you do. If the plastic pipework is non barrier then that could very well be the point of air ingress.

Just checked and it's Barrier PB 15 x 1.5mm.

This pipework starts below the towel rail and goes upstairs in the wall and services two rads above.

I can isolate the plastic pipework fairly easily (I'll have to drain and cap off the system) to see if this is the cause of air? Not sure why the air would end up back up on the second floor and not on the third?

 

[SJB060685]

If it's barrier pipe then it should not be a means of air ingress. The problem with non barrier pipe is air can pass through it and enter the system and would continue to do so until a pressure equilibrium is met. You could isolate it but I doubt it will reveal anything, might be wrong though.
The reason I asked if it was another combi installed is because they are internally laid out so the pump and vessel locations provide a positive pressure throughout the system and therefore shouldn't be any points where air would be drawn in, rather a leak would be visible if there was a failed connection somewhere.

 

[jim843]

If it's barrier pipe then it should not be a means of air ingress. The problem with non barrier pipe is air can pass through it and enter the system and would continue to do so until a pressure equilibrium is met. You could isolate it but I doubt it will reveal anything, might be wrong though.
The reason I asked if it was another combi installed is because they are internally laid out so the pump and vessel locations provide a positive pressure throughout the system and therefore shouldn't be any points where air would be drawn in, rather a leak would be visible if there was a failed connection somewhere.

I was confused about that, too: but you say the entire system is positive pressure, not negative.

I'd been wondering if some part of the system was negative and somehow pulling in air.

If there was a leak, the pressure would drop, right?

 

[SJB060685]

If there was a weak point in the system at low pressure you would draw in air yes.
The expansion vessel will be positioned before the pump, with the pump pumping away. The point the vessel connects to the system is the neutral point or point of no pressure change. Between the vessel and pump will be a very short section of pipework under negative pressure, the pump will then add it's differential pressure to the discharge side, this will supply a positive pressure throughout the system back to the point of no pressure change at the vessel connection point.

 

[Deleted member 123817]

Pressure won't drop with a leak if the filling loop has a PRV with the isolation valve open, so called auto filling.

 

[SJB060685]

Pressure won't drop with a leak if the filling loop has a PRV with the isolation valve open, so called auto filling.

A crude means of auto top up yes but if there was a leak I'm sure he would of noticed somewhere, that would be a lot of fresh water being added to the system.

 

[jim843]

If there was a weak point in the system at low pressure you would draw in air yes.
The expansion vessel will be positioned before the pump, with the pump pumping away. The point the vessel connects to the system is the neutral point or point of no pressure change. Between the vessel and pump will be a very short section of pipework under negative pressure, the pump will then add it's differential pressure to the discharge side, this will supply a positive pressure throughout the system back to the point of no pressure change at the vessel connection point.

Yeah, so if the system is at less than 1 bar and there's a weak point, air would be pulled in.

A crude means of auto top up yes but if there was a leak I'm sure he would of noticed somewhere, that would be a lot of fresh water being added to the system.

Isolation valve is a manual turn on/off thingy.

As far as I can tell there is no pressure drop based on the gauge.

Conversely, if air is being generated in the system, shouldn't I see a pressure increase on the gauge?

 

[SJB060685]

If you read .5 bar on the pressure gauge at the boiler then the pressure at the highest point of the system will be less, perhaps no water up there at all and yes air can enter.
Would air being admitted into the system cause the pressure to rise? Yes it would.

 

[jim843]

If you read .5 bar on the pressure gauge at the boiler then the pressure at the highest point of the system will be less, perhaps no water up there at all and yes air can enter.
Would air being admitted into the system cause the pressure to rise? Yes it would.

Pressure is now between 2.2 and 2.5 (I've not seen it drop below 2.2).

There's defo water in the rads at the top of the house.

When I pressurised to a higher bar I got the tado app to open all valves so everything was open. I only forgot about the two rads in the house in the hall that have normal TRVs.

 

[jim843]

Would air being admitted into the system cause the pressure to rise? Yes it would.

So the pressure never seems to drop or rise over time.

That said, I haven't looked too much at the gauge going up as I have been looking for drops and bleeding the bathroom rad.

 

[SJB060685]

2.2 bar cold?

 

[jim843]

2.2 bar cold?

Not stone cold, cool-ish. Because the heating has been on because of cold weather it's never quite cold.

As an example, when the pump comes on the pressure drops to about 2 bar. When the pump switches off, it goes to 2.2 or slightly higher.

Based on what @johntheo said I checked the flow and return pipes...

Facing the boiler, the left most pipe seems to be the flow. The filling loop goes from the cold water supply to the return. The return has the magnetic filter on.

I checked the schematics in the installation and service manual and this seems to match.

 

[Deleted member 123817]

How often have had to top it iup?

 

[jim843]

How often have had to top it iup?

Never.

I bleed the air. That’s it.

 

[Deleted member 123817]

So after bleeding the air off each time the pressure returns to normal ?. I know it doesn't add up but could/can you remove the filling loop to absolutely rule it out.

 

[jim843]

So after bleeding the air off each time the pressure returns to normal ?. I know it doesn't add up but could/can you remove the filling loop to absolutely rule it out.

I’m not sure that the pressure rises. I will try and monitor.

It’s hard because it fluctuates a little bit. Baxi engineers said this is normal.

Not sure how to remove the filling loop exactly. Would I have to drain the system?

 

[Deleted member 123817]

There should be a flexible hose somewhere where it connects to the boiler return with isolating valves or can you post a photo of the isolation valve you described previously?

Do mean by PG fluctuations that it changes fairly rapidly while watching it and have you watched it for any unusual behaviour while using hot water?

 

[jim843]

There should be a flexible hose somewhere where it connects to the boiler return with isolating valves or can you post a photo of the isolation valve you described previously?

Do mean by PG fluctuations that it changes fairly rapidly while watching it and have you watched it for any unusual behaviour while using hot water?

Will see if there are isolating valves behind.

The pressure only drops/rises when the pump goes on/off. Not during heating.

Haven’t checked when using hot water but will check tomorrow.

 

[jim843]

@johntheo while I was looking I noticed this.

Also some of the copper pipes into the boiler have this.

Anything to worry about?

 

[John.g]

Will see if there are isolating valves behind.

The pressure only drops/rises when the pump goes on/off. Not during heating.

Haven’t checked when using hot water but will check tomorrow.

That is the filling loop with a non return valve where shown and a isolating valve where shown on the other end, ensure the isolation valve is closed "across (at right angles to) the pipe" then just slacken the flexible hose where attached to the NR valve but do not remove it, when slackened a few turns water should only dribble out for a few seconds and then stop.
There may be another isolation valve on top of the NR valve but I don't think so, if there is, shut it as well

 

[John.g]

@johntheo while I was looking I noticed this.

Also some of the copper pipes into the boiler have this.

Anything to worry about?

I dont't think so, these look like isolating valves for the boiler flow/return.

 

[jim843]

That is the filling loop with a non return valve where shown and a isolating valve where shown on the other end, ensure the isolation valve is closed "across (at right angles to) the pipe" then just slacken the flexible hose where attached to the NR valve but do not remove it, when slackened a few turns water should only dribble out for a few seconds and then stop.
There may be another isolation valve on top of the NR valve but I don't think so, if there is, shut it as well

Ok, I’ll try this tomorrow along with the hydrogen check!

Do you think air could be somehow being pulled into the system here?

If so, would it be the loop or the NRV?

 

[John.g]

No, I think it highly unlikely because even if the flexible hose is perished/holed then with a sealed system the pressure should still keep the NR valve shut. I also find it difficult to imagine that you are pulling in air anywhere in the volumes that you are getting especially if the boiler pressure is normally 1bar or higher. Lets hope in one way that its hydrogen!.

 

[jim843]

No, I think it highly unlikely because even if the flexible hose is perished/holed then with a sealed system the pressure should still keep the NR valve shut. I also find it difficult to imagine that you are pulling in air anywhere in the volumes that you are getting especially if the boiler pressure is normally 1bar or higher. Lets hope in one way that its hydrogen!.

I’ll take a video of the amount of air/hydrogen coming out so it’s easier to see.

Is it worth doing the bleed when the system is cold? And by cold, how long should the system be off for?

 

[John.g]

I've only had to vent some air from my open vented system after a complete drain down and refill, I would stop the circ pump/boiler and then vent when hot. Hot or/& cold better IMO to vent with pump off.

 

[jim843]

I vented the bathroom towel rail today. Heating was turned off. System was still a bit warm.

I got about 7 seconds worth of air with a quarter turn of the bleed valve.

Here is the pressure before bleeding:

And pressure after bleeding:

Thing is, the pressure gauge usually varies a bit.

From earlier in the week:

@jim843 I tried the hydrogen test. There was no pop.

@John.g do you still think it's worth removing the filling loop?

@johntheo I turned on the hot water and the pressure does drop. It went from about 2.4 to about 2. Is this a problem?

Thank you all for your help. It's been very useful for me to vent (no pun intended?!) and discuss this.

 

[John.g]

Yes, definitely remove (after first slackening) that filling loop, it's only hand tightened at the connection to the NRV, obviously ensure that the isolaton valve is shut, as it appears to be, then post back.

 

[jim843]

Yes, definitely remove (after first slackening) that filling loop, it's only hand tightened at the connection to the NRV, obviously ensure that the isolaton valve is shut, as it appears to be, then post back.

Ok. Just so I'm clear, remove loop and leave it a couple days and see if there's any air in the system?

 

[John.g]

Correct, but let us know when you do disconnect it if there is any dribble of water from it just now.

 

[jim843]

Correct, but let us know when you do disconnect it if there is any dribble of water from it just now.

Ok, just slackened it and yes there was a dribble of water.

I've left a container underneath to see if it keeps leaking...

 

[John.g]

Now remove it completely and if its still dribbling from the hose then that's your problem.

 

[jim843]

Now remove it completely and if its still dribbling from the hose then that's your problem.

I don't think it's leaking from the hose but the NRV appears to be leaking:

I watched it and I can see the water filling up and then leaking.

Could this be the issue?

 

[John.g]

I would't have thought so as difficult to see how this would pull in air so as one would expect the pressure to slowly decay with time, just leave the hose disconnected and see what happens to the pressure.

 

[jim843]

I would't have thought as difficult to see how this would pull in air so as one would expect the pressure to slowly decay with time, just leave the hose disconnected and see what happens to the pressure.

Ok, will do.

 

[jim843]

I would't have thought so as difficult to see how this would pull in air so as one would expect the pressure to slowly decay with time, just leave the hose disconnected and see what happens to the pressure.

So filling loop has been off since about 1400 (5 hours or so).

Pressure was at 2.5 a while ago. I heard some bubbling sound in bathroom rad. So bled rad again - few seconds of air - and now pressure is 2.4.

Still confused why the pressure drops when the pump comes on…

 

[Deleted member 123817]

The expansion vessel is normally teed into the pump suction so will rise by the generated pump head when it starts so if connected to the pump discharge will fall but when when off should return to the original pressure if water temp doesn't change,
Can you bring on the hot water very briefly say for 10 secs and then switch off and see if pressure returns to its original.

 

[jim843]

The expansion vessel is normally teed into the pump suction so will rise by the generated pump head when it starts so if connected to the pump discharge will fall but when when off should return to the original pressure if water temp doesn't change,
Can you bring on the hot water very briefly say for 10 secs and then switch off and see if pressure returns to its original.

Ok. Pressure drops slightly when hot water comes on.

I waited about 30seconds and then the pressure goes (more or less) back to what it was before.

 

[Deleted member 123817]

Was the new E.vessel installed internally in the boiler or is it external where you can see it!.

 

[jim843]

Was the new E.vessel installed internally in the boiler or is it external where you can see it!.

Pretty sure it’s internal. The engineer took the cover off. There’s nothing outside the boiler as far as I know.

 

[John.g]

In the attachment is that a PRV on the filling system??.

It's also possible I suppose but very unlikely as I said above that the pump is pulling in a tiny amount of air through that leaking NRV each time it starts especially if the E.vessel is on the discharge side of the pump, the closer to the boiler the greater the chance, if the boiler is under warranty then that should be covered? its the only part that has direct communication to the atmosphere via the leak even though pressurized.

 

[jim843]

In the attachment is that a PRV on the filling system??.

It's also possible I suppose but very unlikely as I said above that the pump is pulling in a tiny amount of air through that leaking NRV each time it starts especially if the E.vessel is on the discharge side of the pump, the closer to the boiler the greater the chance, if the boiler is under warranty then that should be covered? its the only part that has direct communication to the atmosphere via the leak even though pressurized.

Maybe I’m mixing this up: the leak is on the valve that has an arrow on it.

The arrow you have drawn on the photo: do you mean a pressure release valve? I don’t know what that black thing is.

 

[jim843]

In the attachment is that a PRV on the filling system??.

It's also possible I suppose but very unlikely as I said above that the pump is pulling in a tiny amount of air through that leaking NRV each time it starts especially if the E.vessel is on the discharge side of the pump, the closer to the boiler the greater the chance, if the boiler is under warranty then that should be covered? its the only part that has direct communication to the atmosphere via the leak even though pressurized.

This is where the leak is (see attached).

Do you think it’s worth replacing that NRV?

 

[John.g]

Yes, I would replace it as every other possibility seems to have been reasonably exhausted. Maybe its under warranty?.

The valve thats leaking is a non return valve (NRV). the PRV I'm referring to is a pressure reducing valve called a PRV (as is the safety valve also called a PRV, a bit confusing) the pressure reducing valve above is/should be connected dirrectly to the filling hose?.

Forget my reference to that PRV, thats the isolating valve for the filling hose.

 

[John.g]

As a temporary measure you could attach another filling hose which has isolating valves at both ends, connect one end to the NRV and shut it.

 

[jim843]

Yes, I would replace it as every other possibility seems to have been reasonably exhausted. Maybe its under warranty?.

The valve thats leaking is a non return valve (NRV). the PRV I'm referring to is a pressure reducing valve called a PRV (as is the safety valve also called a PRV, a bit confusing) the pressure reducing valve above is/should be connected dirrectly to the filling hose?.

Forget my reference to that PRV, thats the isolating valve for the filling hose.

I seem to remember from a couple years back a Baxi engineer saying that this filling loop is old and should have been replaced when the boiler was replaced. Maybe I’m misremembering, though.

You’d think the boiler would be under warranty but the installer has moved and I’ve no idea where they live now. Considering the mess they made - leaking joints under the boiler, not fitting the soak away - I should probably get someone else in.

 

[jim843]

As a temporary measure you could attach another filling hose which has isolating valves at both ends, connect one end to the NRV and shut it.

Ok, I’ll have a look and see what screwfix have tomorrow.

 

[jim843]

As a temporary measure you could attach another filling hose which has isolating valves at both ends, connect one end to the NRV and shut it.

Something like R24 Filling Loop - https://www.NoLinkingToThis/p/r24-filling-loop/83905 with some pipe and compression fittings to connect to that new loop?

 

[jim843]

As a temporary measure you could attach another filling hose which has isolating valves at both ends, connect one end to the NRV and shut it.

Another thought: the isolation valve with the handle before the filling loop would stop the leak, if any, right?

Or are you thinking that the leaking NRV is somehow pulling air into the system even though the filling loop isolation valve is closed?

 

[John.g]

Yes, maybe through the leaking NRV but very unlikely.

The most likely place for air ingress one might think would be at at attic rads even though these should still have ample positive pressure even if boiler pressure was =<1.5bar.
If you vent them with pump running does water definitely flow out after a burst of air? and does the same apply to all the other rads?

 

[jim843]

Yes, maybe through the leaking NRV but very unlikely.

The most likely place for air ingress one might think would be at at attic rads even though these should still have ample positive pressure even if boiler pressure was =<1.5bar.
If you vent them with pump running does water definitely flow out after a burst of air? and does the same apply to all the other rads?

The attic rads never have any air in them. They only had air in them before the bathroom was fitted. After the bathroom was fitted, air appeared in the bathroom towel rail instead.

I can always cap off the upstairs rads on the second floor and run the system for a bit to see if the air stops?

If the attic rads are pulling in air, why would it end up one floor down?

 

[John.g]

Its a mystery to me why any air is getting in!, does the towel rad vent water??.

 

[jim843]

Its a mystery to me why any air is getting in!, does the towel rad vent water??.

Yep, it vents water.

There’s some air. Then water.

I can repeat this every day.

 

[jim843]

Its a mystery to me why any air is getting in!, does the towel rad vent water??.

I re-read your previous comment: did you say vent with the pump running? I always vent when the pump is off.

Also, air only appears in the towel rail when the heating system has been on. In summer there’s no air in the towel rail. This might be expected, I don’t know.

 

[John.g]

No, vent with pump off but when water is circulating in any pressurized/sealed system then there should always be water pressure at the vent when opened, if no water then ensure that air is actually venting and not being drawn in.

I have two rads in a upstairs attic conversion that always run with a slight vacuum (open vented system) but it has never caused any problems because there are no leaks anywhere associated with them.

 

[jim843]

No, vent with pump off but when water is circulating in any pressurized/sealed system then there should always be water pressure at the vent when opened, if no water then ensure that air is actually venting and not being drawn in.

I have two rads in a upstairs attic conversion that always run with a slight vacuum (open vented system) but it has never caused any problems because there are no leaks anywhere associated with them.

There’s always pressure. The air is always forced out and I get water.

 

[John.g]

OK, Thanks, stumped for sure, can you please post the exact model of the Baxi Combi you have.

Also, what is boiler flow temperature on CH.

 

[jim843]

OK, Thanks, stumped for sure, can you please post the exact model of the Baxi Combi you have.

Also, what is boiler flow temperature on CH.

Sure, it’s a Baxi Duo-tec Combi 33.

I think it’s anything from about 60 to 80. Will look more closely.

 

[John.g]

At least one small query is answered in the schematic, the E.Vessel is on the flow side, the reason why the pump pressure drops on start up.

 

[SJB060685]

If that schematic is correct then perhaps we have the answer. The pump is pumping towards the point of no pressure change and therefore the system is under negative pressure. Any weak joints will be drawing air in, alternatively the the system being under negative pressure means you will pull any dissolved gases out of solution.

 

[John.g]

The point of no pressure change is where the EV connects into the system and is the EV pressure itself, with the circ pump off then this pressure will be exactly the same throughout the circuit, after allowing for elevation. With the circ pump running, while the pressure where the EV connects to the system will still be exactly the same then its possible to get negative pressure(s) somewhere but only due to pipe friction losses due to blockages?.
Maybe all combis have their EVs connected in likewise, someone might care to watch their own boiler pressure gauges on start up and note whether it rises or falls with pump start and also its magnitude, the one above certainly moves more than I would have expected, even though theoretically possibly correct, possibly pointing to restriction somewhere?

If the system isn't ingesting air then all this air must be trapped since initial filling but as its being slowly vented then the boiler pressure should gradually fall and require top up but it isn't. Perhaps drain the whole system again and fill very slowly stopping to vent from the bottom up.

The only other thing maybe worth checking out is the PHE, if it had a pinhole maybe possible to keep the system nicely topped up from the mains and if its at or near the system pressure then no problem with boiler PRV lifting. but plenty of oxygenated air gaining access.
Suggest shutting off the mains feed at the boiler and open a hot tap and leave open for a few hours with CH in service and see if any change in behaviour, presume the diverter valve isolates the primary water from the PHE but something may still show up.

A interesting riddle alright.

 

[SJB060685]

If the pump is pumping towards the vessel then the entire system will be under a negative pressure, from the pump suction side right the way back to the vessel.

 

[John.g]

Don't see it that way SJB but could be wrong, pages 36/37 of the attached explains even though they use very low pressures to demonstrate how a negative pressure may occur.

https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_16_na_0.pdf

 

[SJB060685]

Yes, so my above comment wasn't worded correctly. The pump will maintain a differential pressure though. Therefore if it's only able to increase pressure slightly on the discharge the rest will be seen as a negative pressure on the suction side.

 

[jim843]

The point of no pressure change is where the EV connects into the system and is the EV pressure itself, with the circ pump off then this pressure will be exactly the same throughout the circuit, after allowing for elevation. With the circ pump running, while the pressure where the EV connects to the system will still be exactly the same then its possible to get negative pressure(s) somewhere but only due to pipe friction losses due to blockages?.
Maybe all combis have their EVs connected in likewise, someone might care to watch their own boiler pressure gauges on start up and note whether it rises or falls with pump start and also its magnitude, the one above certainly moves more than I would have expected, even though theoretically possibly correct, possibly pointing to restriction somewhere?

If the system isn't ingesting air then all this air must be trapped since initial filling but as its being slowly vented then the boiler pressure should gradually fall and require top up but it isn't. Perhaps drain the whole system again and fill very slowly stopping to vent from the bottom up.

The only other thing maybe worth checking out is the PHE, if it had a pinhole maybe possible to keep the system nicely topped up from the mains and if its at or near the system pressure then no problem with boiler PRV lifting. but plenty of oxygenated air gaining access.
Suggest shutting off the mains feed at the boiler and open a hot tap and leave open for a few hours with CH in service and see if any change in behaviour, presume the diverter valve isolates the primary water from the PHE but something may still show up.

A interesting riddle alright.

If I understand: the EV can cause negative pressure in the system. So a weak point/pin hole leak could be anywhere?

Your suggestion is:

1. switch off the main feed (the cold water?)
2. switch CH to service (not sure how to do this? Just switch to hot water only?)
3. Run hot tap in the kitchen for a couple hours
4. Watch for pressure drop/change?

 

[jim843]

@SJB060685 and @John.g...

I'm off next week so will fit an isolation valve to the leaky NRV. I'll also try the steps suggested above.

Sounds like I'm getting closer to the dreaded "replace all the pipework in the house"?

Supposed to be getting carpets fitted in 3 weeks. The last in the house after a 7 year renovation in my spare time. Oh well, better know now before getting the carpet fitters in... 🙃

 

[John.g]

If I understand: the EV can cause negative pressure in the system. So a weak point/pin hole leak could be anywhere?

Your suggestion is:

1. switch off the main feed (the cold water?)
2. switch CH to service (not sure how to do this? Just switch to hot water only?)
3. Run hot tap in the kitchen for a couple hours
4. Watch for pressure drop/change?

I don't think the EV is causing negative pressure Jim.

Re 1 to 4 above .

Shut either valve 13 in schematic post 80, or just shut the main stopcock, either will cut off the cold water supply to the combi side of the boiler, open any hot tap, (hot) water flow should stop.
Ensure boiler on CH say like now, just watch the pressure gauge for few hours for any change.
When complete shut hot tap and restore the cold water.

Just trying to establish if there is a leak between the mains and the boiler "water" via the HW heat exchanger. (PHE).

 

[jim843]

I don't think the EV is causing negative pressure Jim.

Re 1 to 4 above .

Shut either valve 13 in schematic post 80, or just shut the main stopcock, either will cut off the cold water supply to the combi side of the boiler, open any hot tap, (hot) water flow should stop.
Ensure boiler on CH say like now, just watch the pressure gauge for few hours for any change.
When complete shut hot tap and restore the cold water.

Just trying to establish if there is a leak between the mains and the boiler "water" via the HW heat exchanger. (PHE).

Ok, gotcha. Thanks.

I can do this tomorrow (probably -- weather dependent!).

Do you want me to run the CH as normal (Tado switches various rooms on/off during the day)?

 

[John.g]

Just as normal.

 

[jim843]

Just as normal.

Ok, I’ve turned the cold feed off. Hot water tap is open.

I’ve left the boiler settings as normal (heating and hot water on).

 

[jim843]

Just as normal.

Water has been off for a bit two hours. The gauge looks the same as before. About 2.2.

I haven't bled the towel rail today.

 

[jim843]

@John.g

Hot water was off for about 3 hours.

Central heating on as normal.

The gauge looked fine, no drop.

This is the reading:

I then bled the towel rail (quite a lot of air -- maybe 10 seconds worth):

I'd say there's a tiny drop there?

 

[John.g]

Don't think there is any marked difference there than with HW enabled, but leave it off for as long as you can and see do you see any difference in behaviour. The air build up is stiil the same or worse so cant't blame the mains water/PHE for that.

 

[jim843]

Ok, will leave it off overnight and see what happens. Thanks!

 

[hometech]

Hello,

Around 2 years ago we had a very old, leaky Baxi combi replaced with a newer Baxi.

For background I live in a 3 storey terrace. First floor has 3 rads, second floor has 4 rads and third floor has 2 rads.

We noticed problems with noise and air in the system. One of the rads in the loft made gurgling noises. If the rad was bled, air came out. Also, at this time the pressure gauge was all over the place. Up to 3, back down to 1 and every number in between. The gauge didn’t appear to drop, though.

The installer wasn’t helpful over the phone so I ended up finding a different engineer.

Turns out the installer of the boiler didn’t do a particularly thorough job. He didn’t clean the old system, I found leaks on some of the nuts beneath the boiler, he didn’t fit a soak away outside so water drips out over our path (great in winter when it’s freezing).

He engineer found the expansion vessel was flat. He pumped it up, tightened the pipes, advised on a soak away. He told me to bleed all the rads individually for a few mins, top up the system and add more inhibitor. He also suggested power flushing the system. I did all this and also rented a machine and flushed the system myself.

I did all this but the problems remained. I ended up contacting Baxi who came out and replaced the expansion vessel.

This didn’t solve anything. More gurgling, more air.

We ended up getting the boiler serviced again (this is maybe a year after install) and I mentioned the problems to the engineer. He asked the usual questions (is there inhibitor, have you bled the system, etc) and mentioned we had an old radiator in the bathroom. He thought this might be the source of air.

We were planning on getting the bathroom refitted so I decided to wait and see if that solved any issues.

We got the bathroom refitted. Now, the gurgling and air does not go to the top of the house but is in the towel rail in the bathroom. I can bleed the rad once a day and there’s maybe a second or two of air and then water. The pressure does not drop on the boiler.

I contacted Baxi again who came out and replaced the expansion vessel with a bigger one. The engineer also suggested fitting an automatic bleed valve on the chrome towel rail because in his experience chrome towel rails suck in air. I bought one two valves (one for spare) and fitted it.

I noticed an odd thing while fitting the automatic bleed valve. I turned off the two rad valves, released the pressure. Once the air left the rad I could put my finger in the top of the rad and feel there was no water there at all. I’d expected that the water would be just below the top of the rad?

But you’ve guessed it — still gurgling and air in the system! The automatic bleed valve doesn’t appear to vent automatically, not sure why. Could be faulty. I can swap it over with the spare but haven’t gotten around to it yet.

The only other thing I noticed with the system is that if the system is off in the summer then there is minimal air in the rad. I assume because the pump isn't forcing water around the system?

If you got this far, thanks! I’d love to solve this issue and have a system that works! I’m not opposed to ripping out pipework and starting again (even though I’m nearing the end of a long renovation!). Ideally there are some simpler steps I can take to diagnose?

Any advice?

ask the experts on here to explain hydronic flow under variable speed pumps (of which your combi will most certainly have one if its only a few years old)

 

[jim843]

ask the experts on here to explain hydronic flow under variable speed pumps (of which your combi will most certainly have one if its only a few years old)

i had a quick read. Won’t pretend to understand but do you mean air is a product of this system?

 

[jim843]

Started watching

@John.g HW off for the night. Will check in the morning!

 

[jim843]

@John.g HW off from about 10pm last night to 09:15am this morning.

Heating was just cooling when I checked gauge.

It was at 2.5 at 69 degrees. Over the next few mins the pressure dropped back to normal level at about 47 degrees.

I haven’t bled the towel rail yet.

 

[John.g]

So, does this indicate to you so far, that the behaviour is the same as with the HW enabled, ie that the system will still "never" need top up no matter how much air is vented?. If this is the case, then that would indicate that you are pulling air into the system and not venting residual air. If the PHE was causing the problem then you would expect the overall boiler pressure to gradually decay? now that its (the PHE) out of the picture.