Where oh where is this air coming from?

[Ric2013]

Help please.

My heating system at home is hardly an urgent issue. That said, I've had to admit defeat on how on earth air is getting in! Any ideas welcome.

The original pipework on this system was a bit messy and there were some redundant runs, but basically sound as far as I can see. There were points where the flow from the boiler to the vent dropped below horizontal, but I think I have got rid of these. Air has been a problem for some time, and a year ago I removed some of the bad and obsolete pipework and re-plumbed it in a more rational way. This has helped to some degree.

There is no evidence of leakage on the system that I have seen.

The boiler is 25' horizontal distance from the close coupled vent arrangement. System is a Y plan with heat only boiler on ground floor, horizontal(ish) primary runs in first floor void to close coupled vent arrangement and 3 port valve in airing cupboard on first floor. The heating and DHW returns merge just before the Magnaclean, and I have fitted the Magnaclean above this confluence so that the Magnaclean is accessible and so it acts as the highest point on the return (doubling as an air vent point).

Boiler (Potterton Netaheat Profile 40e) kettles badly, and probably needs descaling. Every time I run the boiler, there is air in the Magnaclean which I bleed out. There is air in the Magnaclean even if I leave the system to cool before bleeding the air, so that rules out it being steam. The air does not appear to be hydrogen. The air is only present if the system has been run. System was refilled with fresh inhibitor 3 months ago (after trying out the efficacy of MC3 cleaner) and has been bled several times a week since, so this surely must be new air, not residual from filling! Quantity is probably about a teaspoon of air for every time I heat the cylinder up.

I'm clutching at straws now. Could the possible bubbles of steam passing the vent be somehow dragging in air? Could they be dragging in air through a micro-weep somewhere hidden? Could the boiler heat exchanger be letting in air somehow if a seal is defective? Could the compression unions on the boiler tails be letting in air?

I spend £300 a year on gas, so the idea of buying a new efficient boiler and replumbing the entire system has limited appeal. I'm not so much worried about curing the problem as understanding what is wrong... but curing the problem would be nice.

Thank you if you are still reading 🙂

Ric

 

[Ric2013]

Experiment to see if air is being drawn down the vent:

System bled at all points. Air only found in one radiator (very small quantity).

Turn boiler stat to OFF. Run system at DHW only for one hour. No noise of gases running around system noted, however it sounded like there may have been a bubble or two in the pump due to the pathetic pump-down arrangement. After an hour, the system was switched off for an hour and bled again. NO AIR FROM ANY POINT.

This would seem to indicate that the system is not drawing in air from the vent when cold.

At this point, a yoghourt pot approximately full of water was tied to the open vent so the vent dipped into it. and the boiler set to fire at the usual setting (4 1/2). The vent was checked soon after switch-on. It is possible that the vent may have caused pressure at switch-on, (it would have been better to have left the pot not totally full of water to see if the level rose and to have checked the level accurately) but all I can say is that the water seemed still when I got into the loft to check.

The pot was checked every ten minutes:

0.10 boiler started to kettle and noise of gases flowing around system. Slight see-sawing of water (1/16") in pot suggests some steam was rising up vent and condensing.

0.20 System less noisy (as boiler had begun to cycle and was cooler than at 0.10?) and water in pot seemed static.

0.30 As 0.20

0.40 As 0.20

0.50 As 0.20 (boiler now firing less due to reduced heat transfer as cylinder warms)

0.60 as 0.50. System switched off. Pump over-run did not occur.

Level in yoghourt pot within 1/4" of top. Had slight pressure previously caused it to overflow and now the level has dropped back, or was it like that at start-up?

After an hour cooling and settling, the yoghourt pot level was still the same, there was no air in radiators and the Magnaclean contained the usual amount of air that generally I find when I heat my water cylinder.


Would people agree this is a failed experiment as I have failed to prove that the vent has sucked in any air whatsoever due to the fact that I was not entirely certain of how full the youghourt pot was at fire-up, or were people suggesting the vent was drawing in air expecting an appreciable quantity to be being dragged in, and so the idea that the vent is sucking has been disproven?

Chuck - a thought: if the boiler is kettling then some of the water is boiling, thus its ability to hold dissolved air drops to zero. This will increase the quantity of air being released when the boiler heats up.

 

[Chuck]

Chuck - a thought: if the boiler is kettling then some of the water is boiling, thus its ability to hold dissolved air drops to zero. This will increase the quantity of air being released when the boiler heats up.

I doubt it is significant. It's the heating of the bulk water that will allow the air to be released and turbulence, e.g. in a pump or air separator will speed up the process of reaching equilibrium. The amount of water boiling during kettling is tiny compared with the total volume of water in the system. The noise is tiny bubbles of steam being created in hot crevices at the HX surface, moving out into the relatively cool bulk water in the HX and then collapsing as the steam condenses.

Did you observe how much the height of the level in the expansion tank rose and fell between the system being cold and the system being hot? My theory (well, more of a hypothesis really) relies on that volume difference being larger that the volume of the pipe connecting the tank to your 'H'.

 

[Chuck]

Thanks, Chuck. You've really made me see open-vented systems in a new light. I was of the opinion that a good open-vented system didn't have air in it, once it had been bled and any dissolved air had reacted with the steel in the system. But now you have me wondering whether a good open-vented system simply has less air in it, and appears to have no air in it because the air gets out easily.

I think that's probably the case. It's why unvented systems invariably fill up with of magnetite sludge after a couple of years operation. Fernox slows the corrosion but it doesn't stop it.

Not totally convinced by your comment about the textbook, as the ONLY close-coupled system it shows has an air separator, but I suppose the point is that the design of my system is such that microbubbles will always find their way to Magnaclean after the system is at rest, and from there they can only escape if the vent is opened.

The textbook wasn't supposed to be crucial to my point, just to show I wasn't completely making it up out of thin air.

In any case, Chuck, the basic problem is, in your opinion, that the water in the F&E is moving into the system (by eddy currents, if nothing else) and will inevitably bring air with it as the water in the F&E is able to absorb air. So either I need to put a layer of paraffin on top of the water in the F&E (but oil in the system is bad news, so that rules that out), or a de-aerator on the boiler flow, or an air separator as you suggest.

I think I'd call it a 'hypothesis' at the moment. It was just attempt to explain how the behaviour you observed could occur in a leak-free system.

I would definitely not try putting oil or anything like that into the tank.

It is my opinion that installing an air separator, as shown in the text book, where your 'H' is should be high on your 'worth trying' list. It's a conventional thing to have, will cost about a tenner and should be easy to install. If it doesn't work you can leave it in place and it won't be doing any harm.

 

[Ric2013]

I would definitely not try putting oil or anything like that into the tank.

It is my opinion that installing an air separator, as shown in the text book, where your 'H' is should be high on your 'worth trying' list. It's a conventional thing to have, will cost about a tenner and should be easy to install. If it doesn't work you can leave it in place and it won't be doing any harm.

I'll add the air separator onto my to-do list. I expect it can't prevent the air coming in as that happens when the system cools after use, but it will probably help get it out again.

Fernox only slows corrosion?! And there was me thinking that quality inhibitor is 100% effective in preventing rust and cures cancer if you rub it into your skin on a Tuesday 🙂. Or so Sentinel's online 'training' would probably like me to think lol.

Don't worry about the oil, I was just thinking it would probably work albeit with so many undesirable other effects I wasn't planning on actually doing it.

 

[Chuck]

Fernox only slows corrosion?! And there was me thinking that quality inhibitor is 100% effective in preventing rust and cures cancer if you rub it into your skin on a Tuesday 🙂. Or so Sentinel's online 'training' would probably like me to think lol.

I recently tried the 'nail through a piece of pipe in a small jar of water' test with water half-filling the jar so there was some air available to dissolve. One with tap water another with sytem water treated with the correct amount of Fernox F1. The untreated nail showed signs of rusting within 24 hours and was very rusty within a week. The Fernox protected nail showed signs of rusting after a month and was very rusty after six months.

So, yeah, in a vented system I'd describe F1 as 'slowing' not 'stopping' corrosion. In practice, if you can slow the corrosion rate by a factor of 20 it's not going to be a problem.

Don't know about Sentinel, I've never tried the same test with it, but I'd expect it to behave in a rather similar way. The laws of chemistry and thermodynamics are pretty consistent between brands.

 

[Best]

I found the original Fernox back in the day to be exellent on open vented systems, often with little corrosion present after many years. But the modern Fernox seems different.
What puzzles me is the 4 litre Fernox is not the same as the concentrate.

 

[ShaunCorbs]

I found the original Fernox back in the day to be exellent on open vented systems, often with little corrosion present after many years. But the modern Fernox seems different.
What puzzles me is the 4 litre Fernox is not the same as the concentrate.

Watering it down mate

 

[Ric2013]

I recently tried the 'nail through a piece of pipe in a small jar of water' test[...].

I assume that's to test the effect of dissimilar metals too?

I must say I took a water sample from a customer's system recently, having drained to add a new radiator, and did the plain nail in a jar test, just using the system water undiluted and, in spite of 2l Sentinel to 13 radiators, the nails, in 10 days, are not quite as bright as when new.

But better than whatever inhibitor the RGI had used two months prior, His let the nail go a dark orange and pitted in three weeks and seemed to encourage slime to grow in the F&E (which I had thoroughly cleaned prior to his installation). I was trying to show the customer what inhibitor did, but it didn't really work as a demonstration. When refilling, I asked the RGI which inhibitor he had used so I could refill with the same and he said, 'Oh, "Range" boilers don't mind. Just use any good-quality inhibitor.' I took this as an evasion, and let it slide.

 

[Chuck]

I assume that's to test the effect of dissimilar metals too?

It's to model the Galvanic corrosion that occurs in heating systems. I suppose one should include some aluminium in the mix these days but this was the quick and dirty test the scientists that developed Fernox used to decide which formulations were worth investigating further.

To demonstrate the effect of the protector, you need to use a second pot with tap water as a control. With a nail through 10mm of 15mm Cu tube there'll be a noticeable amount of rust after 24 hours.

The RGI should have specified the inhibitor used on the Benchmark statement for the boiler. If you've drained and refilled with something else don't forget to update the docs and any labels on the system.

 

[Best]

Watering it down mate

It seemed to be a different product entirely compared with the concentrate. Different colour and had a smell to it that the 500ml doesn't.

 

[Ric2013]

The RGI should have specified the inhibitor used on the Benchmark statement for the boiler. If you've drained and refilled with something else don't forget to update the docs and any labels on the system.

Yes, he wrote '1l'. No mention of the brand. And he left the lid and the lagging off the F&E. I wasn't impressed, especially since I was the one that got him the contract to fit that boiler in the first place. I left the Benchmark as he had left it, stuck a Sentinel label on the boiler casing, and wrote '2 litres' on the label.

Did you observe how much the height of the level in the expansion tank rose and fell between the system being cold and the system being hot?

I've had a go today at checking the water level in the F&E. It rises from 5.5" to 5.6" over a 17 x 11.5" tank. I make this 0.56pt, or 318ml, compared with 358ml in the cold feed run, if we assume the pipe contains 130ml/m.

The pipe run from the close-coupled tee to the tank is about 1.25m vertical and then 1.5m across the loft joists, giving a total of 2.75m.

So, while the expansion is not quite as great as the quantity in the cold feed, it isn't far off, and I can see that air could potentially get into the system this way, particularly when central heating is warming up a far greater quantity of water than the small amount in the primaries, boiler, and cylinder coil.

 

[Ric2013]

The noise is tiny bubbles of steam being created in hot crevices at the HX surface, moving out into the relatively cool bulk water in the HX and then collapsing as the steam condenses.

This is how I've always had kettling explained to me. However, in the case of my system, you hear air/steam bubbles running the full length of the primaries. Sometimes they even are enough to be dragged back into the pump. Can steam possibly remain as steam for this distance? It's 25' (8m) horizontal in 22mm...

 

[Chuck]

This is how I've always had kettling explained to me. However, in the case of my system, you hear air/steam bubbles running the full length of the primaries. Sometimes they even are enough to be dragged back into the pump. Can steam possibly remain as steam for this distance? It's 25' (8m) horizontal in 22mm...

They can't be steam, unless your flow temperature is above 100°C, which I assume you would have mentioned if it were the case. The boiler is one place where dissolved air is likely to come out of solution because that's where it's heated. The pump is another because that's where there'll be turbulance and cavitation.

From your description of the noise, however, I suspect the long horizontal run may have a slight downhill gradient and be partially air locked as a result. Imagine the pipe half full of air. Is this possible? If so it could take a very long time to bleed all the air out of a few ml at a time.

 

[Ric2013]

The horizontal run used to dive down under the airing cupboard just before the pump arrangement, thus airlocking much of the run. I changed that, but, yes, there may be other problem areas.

What I don't understand is, if it isn't steam, why does this air running noise not happen at all until the boiler starts to kettle?

 

[Chuck]

Yes, he wrote '1l'. No mention of the brand. And he left the lid and the lagging off the F&E. I wasn't impressed, especially since I was the one that got him the contract to fit that boiler in the first place. I left the Benchmark as he had left it, stuck a Sentinel label on the boiler casing, and wrote '2 litres' on the label.

At least that's sorted now.

So, while the expansion is not quite as great as the quantity in the cold feed, it isn't far off, and I can see that air could potentially get into the system this way, particularly when central heating is warming up a far greater quantity of water than the small amount in the primaries, boiler, and cylinder coil.

It's not impossible, but I'm not convinced that with this mechanism is getting enough transfer of aerated water down into the system to explain the amount of air that you are getting out.

The 1.25 m head is lower than I'd guessed. (You might like to check the pump is okay with an inlet pressure this low.)

Has the system always needed air bleeding from it, or did it only start after you drained it for some reason? (I'm thinking that there may be a partial airlock acting as a reservoir of air.)

 

[Ric2013]

The system saw a lot of fills and drain last year, so I can't be certain, but in general, the radiators have not been quick to fill with air. They probably get bled each autumn: I lose track.

What used to happen (long-term issue) is that the air would run around the system, get caught under the airing cupboard where there was a pass-under on the flow under another pipe and then every now and then there would be a GLUG sound and then a load of air would get to the H arrangement. Most would go up the vent, but some would get into the pump which would then make an airy sound.

My response was to re-design all the pipework in the 12x10 landing-room, remove several radiators (who needs a radiator in a draughty porch or in a loft?), remove a run that ran all the way around the landing for no apparent reason before going to a bedroom radiator, add a radiator in a drying cupboard, drop the pump a few inches to see if that helped, replace the lounge radiators with underfloor heating (on the same zone as everything else but done for technical and practical reasons), design the return to ensure reverse circulation was impossible, and put a Magnaclean on the highest point of the return to act as an air-vent point, and as a filter.

The GLUG sound no longer happens and the air seems to be finding its way into the vent. It is no longer usual for the pump to get air in it. Everything seems to be working as it should. The only exception is the running air sound and this small quantity in the Magnaclean (only with a hot system).

Even if the pump were able to cavitate as the water temperature increased, as you say, the resultant steam could not survive for long. Though, at 75°C, it should be okay at a 0.5m head inlet pressure, according to Grundfos data, the only risk being cavitation.

 

[Ric2013]

Thanks to all those who commented and for the advice given. Considering that this system has been problematic for at least the last 10 years, and my various modifications have improved the various failings without yet solving this air problem completely, I knew it was never going to be easy.

I think Shaun's suggestion, to seal the system, might solve a lot of problems (or find the leaks!) but otherwise it remains a head-scratcher as I've tried to check if the vent was drawing air as suggested and it seems not to, and I've tried to see if this is only air coming out of solution and we think it mostly isn't. I will try the air separator though.

If everyone is agreed that kettled steam CANNOT survive long in water at 75°C, then the gasses running up and down must be air and are is probably getting in at a joint near the boiler, or from the boiler H Ex itself (I hope not).

If no one has any further suggestions, I'll try to update this thread when I get around to the next stage in investigations.

Thanks again.

 

[SavageW]

Honestly one of the most interesting threads I have read! Next time I come home from work and tell the Mrs I feel knackered & she replies "work is easier than having the kids"I will show her this thread good luck mate would be interested to how you get on

 

[Ric2013]

Just an update. The system seems still to have some air, though not quite as much. Interestingly, since the boiler is running (on and off) for central heating for most of the day, but only fires for DHW in the morning, the problem seems less severe than when it was firing for DHW only.

From this, it seems reasonable to speculate that this is another nail in the coffin of the idea that the problem could be due to air being absorbed into the F&E water and then coming in via the feed pipe, as the volume of water in 3 radiators and the UFH is much higher than just in the cylinder coil. If the problem were air absorbed via the F&E, I would expect it to be worse during the CH season?

 

[Stigster]

It could be pulling in air on the vacuum side of the circulating pump, past the washer. I had one like that a few years ago where I found the most minute of weeps on the pump valve. It had fibre washers and I changed them to rubber and the air ingress problem went away.

Just an idea!