Air in CH system and hot water returning to FE tank via the cold feed

[Tiggy1955]

I have a problem with air being sucked into my CH system creating airlocks in the radiators. The water is flowing back into the FE tank via the cold feed pipe. I can feel warm water coming through the feed pipe back into the tank as the tank water level slowly get higher. I can hear the overflow gurgling in the FE tank.

The system is 20 years old but the Worcester Bosch boiler is only 9 years old. All has worked perfectly well. I did have the classic blockage where the cold feed meets the hot return but the clogged pipes have been cut out and replaced.

I have fitted a new pump and it is set to the lowest speed. All of the radiators have been bled many times and balanced.

So the heating is working fine except that I have this problem of the air displacing the water back to the FE tank. The water it getting back into the tank a quite a rate - I would say about 2 litres per hour. This means that the rads need to be bled constantly.

Any ideas? Could it be an problem with the boiler itself?

 

[sammathias]

If the coil has failed in the cylinder the small feed and EXPANSION tank would constantly be overflowing due to the hot water cistern being higher/bigger. It's normal for the water to rise slighlts in the f+e. After all where else is the expanded water going to go????

 

[Tiggy1955]

If you were to isolate the ballcock and let the system cool, bleed the air out and add back the water you have scooped out, then you should have the same level you started with.

Looking at your photo, I am wondering where your vent is? I am assuming there isn't one? If so, then the system isn't pumping over.

Assuming your pump is somewhere on the boiler flow then anything between the cold feed and the pump inlet is running at a vacuum when the pump is on and there could be a minor weep which is admitting air to the system. Could be a radiator valve, the pump inlet gate valve stem, the pump inlet washer, or one of the joints in or near the boiler.

Alternatively, if there was a blockage, then there must be some degree of scale or rust in the system. Could be the boiler is kettling and creating steam, but I wouldn't imagine that steam could continue to survive as such for very long in the sort of quantity you are experiencing.

You seem to enjoy problem solving, so hopefully this will give you some ideas, but I am beginning to think you may need a professional to attend and have a good look at the system and how it is all arranged.

Hi Ric2013 - Thanks for all the info so far. To answer some of your points:

The vent (and by that I assume you mean the pipe that dangles over the FE tank?) is T’d into the boiler flow and is plastered into the wall about 20” to the right of the cold feed shown in the photo.


The pump is in the airing cupboard and I assume is attached to the boiler flow (pretty sure it is - the pipes come up through the floor boards). Above the pump is the 3 way motorised valve - which appears to be functioning correctly when you switch between CH and HW.


As for enjoying solving problems - yes I do - but this one is driving me nuts!!! 🙁

Thanks again.

 

[Ric2013]

Ah, so your arrangement is something like this then?

If so, the pump is pulling on the vent and could be pulling in air if it runs too fast. Unfortunately, in PP mode, the pump will start fast even when it then slows down. What you have there is probably a residual of an old legacy system much more than 20 years old that worked very well with the old boilers that had negligable resistance to flow, not so well with a modern boiler. Possibly has never been fully modernised. The pump may well have been on the return once upon a time.

You can try setting down the pump as other have suggested, but my feeling is that this kind of setup is prone to this sort of problem and you might want to look at getting the system reconfigured.

One important question: Did it used to work okay and when did this change?

View attachment 36394

 

[John.g]

Ah, so your arrangement is something like this then?View attachment 36395

If so, the pump is pulling on the vent and could be pulling in air if it runs too fast. Unfortunately, in PP mode, the pump will start fast even when it then slows down. What you have there is probably a residual of an old legacy system much more than 20 years old that worked very well with the old boilers that had negligable resistance to flow, not so well with a modern boiler. Possibly has never been fully modernised. The pump may well have been on the return once upon a time.

You can try setting down the pump as other have suggested, but my feeling is that this kind of setup is prone to this sort of problem and you might want to look at getting the system reconfigured.

One important question: Did it used to work okay and when did this change?

View attachment 36394

My own system has a combined vent and cold feed, ie the cold feed is teed directly into the vent where it passes by the Feed & Expansion tank, I know of a number of systems like Tiggy1995,s above that were modified to this combined system, this obviously isn't allowed in a solid fuel system. However he did say his system worked well as is and the boiler was renewed 9 years ago, I don't think he has had this problem for the past 9 years.

Re the pump speeding up on PP setting, it does, but only for a few seconds to find its steady state condition, its only the press of a button to try it out and it may alleviate if not cure the problem until a permanent solution is found.

 

[Tiggy1955]

Ah, so your arrangement is something like this then?View attachment 36395

If so, the pump is pulling on the vent and could be pulling in air if it runs too fast. Unfortunately, in PP mode, the pump will start fast even when it then slows down. What you have there is probably a residual of an old legacy system much more than 20 years old that worked very well with the old boilers that had negligable resistance to flow, not so well with a modern boiler. Possibly has never been fully modernised. The pump may well have been on the return once upon a time.

You can try setting down the pump as other have suggested, but my feeling is that this kind of setup is prone to this sort of problem and you might want to look at getting the system reconfigured.

One important question: Did it used to work okay and when did this change?

View attachment 36394

Yes that diagram is correct. The system has never been reconfigured. We have lived here since the house was built. The only thing that has changed is the boiler. The original was an Ideal now a Worcester Bosch (I think the builder might have used BG to install the heating system at the time - over 20 years actually).

You are also correct in that this problem has just started in the last few weeks. I have also just realised something else this evening. When the system is set to CH and HW the heating hardly gets hot. On CH only it heats up nicely. I'm at the end of what I think I can do so I'm going to get a heating engineer to have a look at it now. I'm even considering replacing the whole lot with a combo boiler. We live in a bungalow and there are only two of us so a combo should be OK. I'll get advice.

 

[Ric2013]

I think getting someone in may be a good idea. No point fitting a new boiler as a soution until you can be sure what the problem is. Good luck, and please do let us know how you get on - makes it worth our while, you know.

 

[Tiggy1955]

OK guys here is an update on my situation...............

Action taken yesterday:

• I fully drained the system.
• I fitted a brand new diverter mid-position valve. I did this because a) it was old, b) the CH and HW were not working together c) the system was empty so do it now as a preventative measure.
• I have filled the system with Sentinel X400 and that is currently still in the system.

Outcome:

• The system is working very very well - getting a lot hotter that it used to.
• The CH and HW work together now.
• The whole system seems quieter and more efficient.

But:

• The rads still need bleeding within 30 minutes of the CH starting up. One in particular is gurgling and this is the one loosing most of it’s water back to the FE tank.
• The FE tank is still getting water pushed back into it via the flow outlet at the bottom of the tank. I know this because, apart from the rads getting cold at the top, if you watch it for 5 minutes you can actually see the levels rising and you can see murky water entering via the flow outlet. The tank water was perfectly clear yesterday, now it’s rusty red (well at least I guess the X400 is doing it’s job 🙂.
• When I bleed the rads the water level in the FE drops back to the correct level.

Planned next steps:

• Leave the X400 in for a couple more days and see if this makes any difference.
• Drain, flush and add some X100 inhibitor.
• Test for another couple of days.
• If I still have the back fill/bleeding problem call a good Heating Engineer for advice.
• Go off and resolve the Brexit Crisis because that looks a lot easier 😉

Thank you all for your help - I’ll keep you posted.

 

[John.g]

OK guys here is an update on my situation....

Action taken yesterday:

• I fully drained the system.
• I fitted a brand new diverter mid-position valve. I did this because a) it was old, b) the CH and HW were not working together c) the system was empty so do it now as a preventative measure.
• I have filled the system with Sentinel X400 and that is currently still in the system.

Outcome:

• The system is working very very well - getting a lot hotter that it used to.
• The CH and HW work together now.
• The whole system seems quieter and more efficient.

But:

• The rads still need bleeding within 30 minutes of the CH starting up. One in particular is gurgling and this is the one loosing most of it’s water back to the FE tank.
• The FE tank is still getting water pushed back into it via the flow outlet at the bottom of the tank. I know this because, apart from the rads getting cold at the top, if you watch it for 5 minutes you can actually see the levels rising and you can see murky water entering via the flow outlet. The tank water was perfectly clear yesterday, now it’s rusty red (well at least I guess the X400 is doing it’s job 🙂.
• When I bleed the rads the water level in the FE drops back to the correct level.

Planned next steps:

• Leave the X400 in for a couple more days and see if this makes any difference.
• Drain, flush and add some X100 inhibitor.
• Test for another couple of days.
• If I still have the back fill/bleeding problem call a good Heating Engineer for advice.
• Go off and resolve the Brexit Crisis because that looks a lot easier 😉

Thank you all for your help - I’ll keep you posted.

Thanks for the feed back, did you try the pump on PP control?.

 

[Tiggy1955]

Thanks for the feed back, did you try the pump on PP control?.

Going to do that today. Will let you know - thanks.

 

[Tiggy1955]

Thanks for the feed back, did you try the pump on PP control?.

John G you are a star!!! Set the pump to PP2 and all is sweet! I thank you sir.

Would you mind spending 2 mins explaining to me in laymans terms exactly what the Proportional Pressure setting does? It must be more that just a lower pressure setting - thanks.

 

[doitmyself]

Yes the tank is full. As the heating runs I can see the sediment being moved around the FE tank. I put my hand in the tank and you can feel warn water coming back in via the feed outlet.

I see that you have 'solved' the problem by setting the pump to PP2. However this may be a compromise.

When the water is heated it will expand so the water level in the FE tank will rise and feel warm (hot water rises).

The level should be adjusted, by bending the ballcock valve stem, so the tank is only half full when the water is hot.

If the level is too high water will escape via the overflow pipe. So when it cools down the level in the tank drops and has to be replenished via the ballcock valve. This introduces more air to the system meaning more bleeding.

 

[John.g]

John G you are a star!!! Set the pump to PP2 and all is sweet! I thank you sir.

Would you mind spending 2 mins explaining to me in laymans terms exactly what the Proportional Pressure setting does? It must be more that just a lower pressure setting - thanks.

Good to hear that the pump over has stopped, I assume that you checked it with CH only & HW only. One concern I have is that you may have a partial blockage of the boiler heat exchanger as you ran for years with no pump over on a fixed speed setting, if there is a blockage and it gets progressively worse the the pump on PP control will just keep modulating down and the boiler deltaT will keep increasing for any given firing rate, I think some boilers cut out if the deltaT exceeds 25C or 30C, on the other hand you may have no blockage and/or you may have cleared it out with the chemical addition. Pump PP control is not recommended for boilers with a ABV (auto bypass valve) which opens on circ pump over run (if installed) for a short time on boiler shutdown to remove the residual heat from the exchanger, this is no problem with a fixed speed pump as the pump head will rise to its max (4M in your case) when all the zone valves shut so th ABV can be set at say ~ 3M so that it will open only on boiler shutdown, On PP control, the pump will reduce its speed to maintain its minimum head of ~ 1.2M but if you set the ABV down to say 0.8M then when the boiler comes back on the pump will ramp up to maybe 2M or so and there will be excessive bypass at this pressure. Again your boiler may not be fitted with this or it may use the post air purge to cool down the exchanger.
Now for the good news....pump PP control.
PP control was introduced on A rated circ pumps to save energy and in some cases to reduce noisy TRV,s when throttling down.
I have one myself (Wilo) which consumes 21/23 watts with everything opened up and falls to 12/14 watts on CH only when some of the TRVs are throttled down.
They work like this, depending on the PP setting you select the pump then sets a minimum head at zero flow and a maximum head at its full flow. For example assume that PP3 setting is selected on the UPS2, at that setting (see page 11 of the manual), the pump will modulate between 0 flow at 1.2M & 2.15M3/h (36 LPM) at a 3M head.
Now assume that the pump is running on fixed speed1 and the system with everything opened up requires a 2.9M head at a f/rate of 0.9 M3/h (15 LPM).....you now change over to PP3 control, the pump will go minimum speed and then starts ramping up, for any given point on that "curve" the pump calculates the exact power needed and compares this with the actual absorbed pump power and because there is only one point on this (or any) curve where that calculated power and the absorbed power are exactly equal then the pump will stop ramping and remain at this setting until the system conditions change. I am able to calculate this in a spreadsheet (unfortunately that I am unable to post on here) so if we go back to our original requirement of a 2.9M head at 15 LPM and c/o to PP control then the pump head and f/rate will fall to 1.8M head & 11.6 LPM, as the system requirements fall then the pump will modulate still further. Again if you are back to fixed speed and you shut off "1/2" the rads you might need a 2M head for a f/rate of 7.5 LPM, change back to PP control and the head will fall to 1.52M & the f/rate to 6.7 LPM. If you "dead headed" the pump by say shutting the pump discharge isolating valve, the pump head will fall to 1.2 M at zero F/rate, if you fitted a controllable by pass around the pump and started opening it, the pump would ramp up to a 3 M head & a f/rate of 2.15 M3/h or 36 LPM.

Edit: Should have asked, what PP setting are you using??, thanks.
Have just seen that you are on PP2, I will adjust the numbers above shortly to reflect this, I would suggest PP3 if still no pump over.
I have left the above PP3 and shown the PP2 settings below, very little difference, one of the draw backs of UPS2 PP control.

PP2 SETTING
assume that the pump is running on fixed speed1 and the system with everything opened up requires a 2.9M head at a f/rate of 0.9 M3/h (15 LPM).....you now change over to PP2 control, the pump will go minimum speed and then starts ramping up, for any given point on that "curve" the pump calculates the exact power needed and compares this with the actual absorbed pump power and because there is only one point on this (or any) curve where that calculated power and the absorbed power are exactly equal then the pump will stop ramping and remain at this setting until the system conditions change. I am able to calculate this in a spreadsheet (unfortunately that I am unable to post on here) so if we go back to our original requirement of a 2.9M head at 15 LPM and c/o to PP2 control then the pump head and f/rate will fall to 1.58M head & 11.1 LPM, as the system requirements fall then the pump will modulate still further. Again if you are back to fixed speed and you shut off "1/2" the rads you might need a 2M head for a f/rate of 7.5 LPM, change back to PP2 control and the head will fall to 1.35 M & the f/rate to 6.2 LPM. If you "dead headed" the pump by say shutting the pump discharge isolating valve, the pump head will fall to 1.1 M at zero F/rate, if you fitted a controllable by pass around the pump and started opening it, the pump would ramp up to a 2.7 M head & a f/rate of 2.34 M3/h or 39 LPM.

 

[doitmyself]

I am able to calculate this in a spreadsheet (unfortunately that I am unable to post on here)

You can post it if you zip it first. I would be interested in seeing the spreadsheet.

 

[Ric2013]

Just in case you didn't follow that, and I'm not sure I did, here is another very rough explanation of PP as explained to me by a Grundfos engineer.

Very basically, a pump could run faster and slower to maintain the same available head as rad valves are open and shut. When a valve is opened, more water can flow so the pump runs faster to maintain the same pressure. This would be a constant pressure setting.

Proportional pressure slightly over-compensates, so as the flow increases when you (or a TRV) open the flow to a rad, the pressure at the pump is not only maintained by increasing the pump rotation speed, but the pressure is actually raised a little. The reason for this is that the primary circuit (the pipe from the boiler to the motorised valve and the common return to the boiler) creates more drag when the water velocity is higher. So if the pump merely maintained a constant pressure at the pump, the pressure at each radiator would drop when all valves were open and each radiator would still receive a little less water than if only half were turned on. By over-compensating, the pump makes up for the increased drag on the primary circuit as velocity increases.

On the UPS2 (and I would imagine most pumps are similar), the PP1,2, and 3, differ in that the PP3 does the most overcompensation, and the PP1 does the least.

 

[John.g]

You can post it if you zip it first. I would be interested in seeing the spreadsheet.

There you go, works exactly as the spreadsheet shows.

 

[John.g]

Just in case you didn't follow that, and I'm not sure I did, here is another very rough explanation of PP as explained to me by a Grundfos engineer.

Very basically, a pump could run faster and slower to maintain the same available head as rad valves are open and shut. When a valve is opened, more water can flow so the pump runs faster to maintain the same pressure. This would be a constant pressure setting.

Proportional pressure slightly over-compensates, so as the flow increases when you (or a TRV) open the flow to a rad, the pressure at the pump is not only maintained by increasing the pump rotation speed, but the pressure is actually raised a little. The reason for this is that the primary circuit (the pipe from the boiler to the motorised valve and the common return to the boiler) creates more drag when the water velocity is higher. So if the pump merely maintained a constant pressure at the pump, the pressure at each radiator would drop when all valves were open and each radiator would still receive a little less water than if only half were turned on. By over-compensating, the pump makes up for the increased drag on the primary circuit as velocity increases.

On the UPS2 (and I would imagine most pumps are similar), the PP1,2, and 3, differ in that the PP3 does the most overcompensation, and the PP1 does the least.

Just in case you didn't follow that, and I'm not sure I did, here is another very rough explanation of PP as explained to me by a Grundfos engineer.

Very basically, a pump could run faster and slower to maintain the same available head as rad valves are open and shut. When a valve is opened, more water can flow so the pump runs faster to maintain the same pressure. This would be a constant pressure setting.

Proportional pressure slightly over-compensates, so as the flow increases when you (or a TRV) open the flow to a rad, the pressure at the pump is not only maintained by increasing the pump rotation speed, but the pressure is actually raised a little. The reason for this is that the primary circuit (the pipe from the boiler to the motorised valve and the common return to the boiler) creates more drag when the water velocity is higher. So if the pump merely maintained a constant pressure at the pump, the pressure at each radiator would drop when all valves were open and each radiator would still receive a little less water than if only half were turned on. By over-compensating, the pump makes up for the increased drag on the primary circuit as velocity increases.

On the UPS2 (and I would imagine most pumps are similar), the PP1,2, and 3, differ in that the PP3 does the most overcompensation, and the PP1 does the least.

I would like to ask that grundfos engineer why in a 6M pump that they only allow a max 3M PP head, even the "cheap" pumps allow a 5 M PP head. I think that even the UPS3 only has a max PP setting of 3.5M.
If you take the above example that I used and assume a perfectly normal CH system. Ideally to set up the PP control, one should open up all the zone valves including HW and open all or any TRVs fully to get the maximum flow, you can then calculate the head and flow rate, I did mine by measuring the boiler deltaT and because I knew its output I was able to calculate the flow rate, I then looked at the (fixed) speed pump curve and was able to read off the head.
So if the head & flow required are 2.9M @ 15 LPM, you should then be able to set the PP control high enough to give you this but because the UPS2 can only be set to a max of 3M the head and flow will fall to 1.8M & 11.6 LPM so you are getting 77% of the max flow. IF you could increase that PP setting to 4M (3.8M to be exact) then you would have the required head & flow of 2.9M & 15 LPM on change over to PP mode. I know this is exactly how it works because I can set my Wilo pump any where between 0.5 M & 5.5 M PP head in increments of 0.1 M so if I set it to the 3.8 M above I will get that 2.9M head & 15 LPM, it will never go any higher but will ramp down as I described on reducing heat demand.

 

[John.g]

You can post it if you zip it first. I would be interested in seeing the spreadsheet.

Here is very interesting article on Grundfos AuotAdapt.

 

[Tiggy1955]

I would like to ask that grundfos engineer why in a 6M pump that they only allow a max 3M PP head, even the "cheap" pumps allow a 5 M PP head. I think that even the UPS3 only has a max PP setting of 3.5M.
If you take the above example that I used and assume a perfectly normal CH system. Ideally to set up the PP control, one should open up all the zone valves including HW and open all or any TRVs fully to get the maximum flow, you can then calculate the head and flow rate, I did mine by measuring the boiler deltaT and because I knew its output I was able to calculate the flow rate, I then looked at the (fixed) speed pump curve and was able to read off the head.
So if the head & flow required are 2.9M @ 15 LPM, you should then be able to set the PP control high enough to give you this but because the UPS2 can only be set to a max of 3M the head and flow will fall to 1.8M & 11.6 LPM so you are getting 77% of the max flow. IF you could increase that PP setting to 4M (3.8M to be exact) then you would have the required head & flow of 2.9M & 15 LPM on change over to PP mode. I know this is exactly how it works because I can set my Wilo pump any where between 0.5 M & 5.5 M PP head in increments of 0.1 M so if I set it to the 3.8 M above I will get that 2.9M head & 15 LPM, it will never go any higher but will ramp down as I described on reducing heat demand.

So in really simple non-technical terms:

A pump set to a Fixed Speed will apply a constant speed/pressure regardless of what’s ahead of it in the system. The result being that it could be too much pressure (particularly when a change takes place such as some rads being closed off) and it is also inefficient.

A pump set with Proportional Pressure will apply a variable speed/pressure depending on what is ahead of it (when a rad is closed/opened the pump compensates accordingly). The result being a more appropriate pressure is applied and it is more efficient.

Is that about right?

 

[Tiggy1955]

I have one final question on this topic:

I replaced the old Grundfoss UPS pump (because it was old) with the Grundnfos UPS2. I got the UPS2 because that seemed to be the direct replacement for the UPS.

Until John G mentioned PP I had never heard of it. What’s more, I did not even know that the UPS2 had PP.


My question is: If I had not replaced the pump I would have been left with a pump that only had Fixed Speeds. In which case, how could I have resolved the pump over problem if the slowest speed of the pump was still too much?

 

[John.g]

I have one final question on this topic:

I replaced the old Grundfoss UPS pump (because it was old) with the Grundnfos UPS2. I got the UPS2 because that seemed to be the direct replacement for the UPS.

Until John G mentioned PP I had never heard of it. What’s more, I did not even know that the UPS2 had PP.


My question is: If I had not replaced the pump I would have been left with a pump that only had Fixed Speeds. In which case, how could I have resolved the pump over problem if the slowest speed of the pump was still too much?

With your system like Ric2013 has shown above, cold feed before the boiler and vent after it I am a bit surprised that you wern't get some pump over "all the tine" depending on the number of rads in service. The cure: ideally I would ensure that the boiler heat exchanger has no (or partial) blockage but of course you need a RGI to do this. If you are happy not to inspect it then if you want to retain the gravity system you could shift the cold feed from before the boiler to after it adjacent to the vent but not more than 150mm away from it OR fit a fully pressurized system with a expansion vessel.

If the PP control is "curing" the problem then why worry too much?.

Are all your rads etc heating up to your satisfaction?.

I don't know if you have a boiler flow temperature indication but (if so) sometime you might watch it when the boiler shuts down and see what the temperature rise is over the next few minutes.
All this has been and is very very interesting, thanks for the feed back.