Where oh where is this air coming from?

[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!