Still getting air in central heating...

[sjashford]

Hello:

Sorry to bother everyone again

I asked a question a while ago about how to deal with air getting into my heating system. The solution recommended here and by others was to convert it to a pressurised system. This I have now done - BUT still the problem persists.

System details:

Potterton Suprima wall-mounted boiler (just over a year old). Fully-pumped hot water and heating. 5 radiators: one just over a year old, a couple maybe 6 years old and a couple more perhaps 20 years old. Pump and main pipes to/from boiler renewed at the same time as the boiler.
Expansion vessel and filling loop feeding into the system approx 1.5 metres from boiler (I think on the flow side). Two auto air vents, one just after the pump on the output side, the other just after the boiler on the flow side.

Symptoms:

System can be pressurised up to any (sensible) level and when left idle will hold the pressure permanently. When running it appears to behave normally, the pressure increases a little as it heats up, then returns to the original level on cooling down. HOWEVER, the problem is that after a couple of weeks a significant amount of air can be found in the radiators (taking say 20 seconds to vent). Where it accumulates depends on which radiators are turned on the most; a small amount also accumulates at the vent point above the boiler. After venting the system pressure has dropped, and requires extra water adding to bring the pressure back up.

There are no signs of any leaks anywhere in the system. It is a first floor flat with mainly wooden floors and the person below is sure they are not being flooded.

Have taken advice from two plumbers so far, one of whom insisted there definitely must be a leak somewhere in the system, the other took some floorboards up to examine the only hidden pipes and concluded there was definitely no leak anywhere.

The only pipes which cannot be examined are a couple which are buried in a concrete floor in the bathroom area. But there is no sign of any leakage where they emerge from the concrete.

This problem is driving me to despair and beyond.

Anyone here with a magic wand to wave at it...?


Thanks in advance.


Simon Ashford.

 

[weareleeds]

Are you sure it is air as it may be hydrogen. if so you have corrosion taking place within system(confirmed by black water in rads etc)

 

[sjashford]

Have tried testing with a match but whatever it is, it doesn't seem to burn readily.

When system was drained for conversion to pressurised system, the water coming out looked clean and fresh.

I should mention this problem has been ongoing now for at least 2 years. Presumably if anything was corroding it would have finished by now?!?

 

[Bahco]

The only pipes which cannot be examined are a couple which are buried in a concrete floor in the bathroom area

Mmmmm.

Can these pipes be isolated from the rest of the system i.e put isolaters
before they enter concrete and turn off. You can then run your system and see if the prssure remains constant.

If it does you have found your problem.

 

[sjashford]

Thanks, might have to try that.

But am still mystified as to exactly what might be going on...

 

[ricarbo]

Every time you introduce new water, you introduce a little air, because it is dissolve in the water. When you heat the water, the air comes out . That's why you see bubbles rising in a saucepan or kettle, well before it boils.
Has the system got enough inhibitor to prevent corrosion? You can test this with the steel nail in a jar routine.
Corrosion has finished when there is no metal left!! (actually not quite true as it will stop when there is no free oxygen left, but you get the point - which is don't change the water if you can avoid it).
I don't think pipes leaking into the concrete would cause air in the rads. Surely that would just cause a loss of pressure?
Are the two vents manual or automatic? It may be that uou are still letting out the air you introduced when (Iassume) you drained down to convert to pressurised, If there were automatic, you could expect them to release the air so the system pressure would drop back untill all the air comes out, then stabilise.

 

[budlo]

Had a very similar problem with air entering the system but could find no leaks anywhere. I converted the system from open vent to sealed thinking that this would cure the problem, or at least indicate where the leak was, but with no success.....air still leaking into system. pressurising to 2bar and going around all the pipes/valves/radiators looking for water leaks also drew a blank. Leaving the system pressurised, but off, for a couple of days did not show any pressure drop on the gauge. The next step was to replace the pump, but to no avail....still the same problem.Cutting a long story short I eventually replaced all the lockshield and thermostatic valves on the radiators and this cured the problem instantly.

I still cant understand how you can have a 'one way' [air getting in,but no sign of water leaking out]leak in a sealed pressurised system, and I probably would not have believed it had I not experienced it myself.I suspect there are very strange 'venturi' effects going on when water is circulating past valves in a system........very frustrating

 

[arundel gas services]

i would sugest if it is now a sealed system that it is hydrogen try treating system with a good quality inhibitor