Air getting into sealed central heating system

My open vented central heating system suffered from air ingress so I had it converted to a sealed system. However, this has not cured the problem, air still collects in one of the upstairs radiators. I've done the hydrogen test and it really is air. Any idea how it can be getting in? The at cold system pressure is constant, so I presume there are no leaks. As the air collects the pressure doesn't rise but when I bleed it out I then have to replenish the system via the filling loop to restore the normal "at cold" pressure. It makes no sense to me!

Reply to
Honeymonster
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It means you have a leak somewhere. Even with a pressurised system, air will leak in through leaks. Such leaks can be hard to find as the moisture can dry on hot pipework before it gets noticed.

If the leak is slow, I suggest waiting until you switch the heating off for summer, and then use the filling loop to increase the cold pressure to match what the highest hot pressure is. Then go round looking for leaks which will hopefully become more obvious. Also bear in mind the leak uses up your inhibitor more quickly than normal and you might want to schedule replenishing that at the same time, when you've fixed the leak.

One other remote possibility -- what is your cold pressure, and how high above the pressure guage are the highest heating pipes or radiators? If you have the guage in the basement of a 3 or more story house such that there's more than 30' head of water above it, you might need to increase the cold pressure of the system to ensure it's all pressurised all the time.

Reply to
andrew

I reluctantly have to agree that there *must* be a leak in the system somewhere. I guess what I really need is for it to get worse. For the record the pressure gauge is at the same level as the highest radiator. Since air gets in only in CH mode (not DHW mode) I guess the problem might be close to the suck side of the pump?

Reply to
Honeymonster

Are you sure that it is air? If you do not have enough inhibitor you could be getting a reaction that is releasing hydrogen and this would have the same effect in the radiators.

I don't know how safe it would be to test with a match as you bleed a radiator.

Andrew

Reply to
Andrew

I know the system has plenty of inhibitor in it. Maybe I should squirt in some Fernox leak sealer and see what happens.

Reply to
Honeymonster

Check static pressure is adequate for the system. What is the height, where is the fill point and pressure gauge?

Any automatic air vents, where are they?

Check system layout. You should be pumping away from the expansion tank. The pump generates a pressure difference, PD.

The expansion tank connection is the point of no pressure change, the pressure change is negligible with the pump on or off. There is a constant static pressure, SP, applied at the expansion tank.

If pumping away from the expansion tank, the pressure at the pump inlet will be SP, pressure at pump outlet will be SP+PD.

If pumping towards the expansion tank pressure at pump outlet will be SP. Pressure at pump inlet will be SP-PD. NB this could be negative at the pump inlet and for some way upstream, especially at high level. Air will leak into the system.

Reply to
Aidan

You can't make any such assumption.

When the system is at its hottest, the water holds the minimum of dissolved air and the excess is forced out into top of the radiators (most commonly just one or two, depending on pipe layout). When the system cools down, the water is ready to dissolve more air. An air molecule just next to the leak will see a 50% vacuum in the water in the heating system, even if the water is still pressurised, and will be sucked into solution in the water. Next time the water is heated, half the air in solution will again be forced out into bubbles. Air, being much less viscous than water, can leak in faster than the water leaks out in some cases, although its likely to be limited to the speed it can diffuse through the water in the leak in most cases.

Reply to
Andrew Gabriel

plenty of background info in the FAQ below.

Reply to
Ed Sirett

Where is the expansion vessel in relation to the pump? It might just be that on its highest setting with the expansion vessel near the outlet that the pressure on the inlet side can get seriously reduced.

Running the system with a higher than normal starting pressure (provided the highest pressure is not too high) might help a lot.

Reply to
Ed Sirett

the fill point and pressure gauge?

I think the above answer is a very good answer. Many people cannot understand that a "pressurized" system can draw in air due to the suction effect upstream of the pump.

Reply to
Rob Wright

Wow that's a blast from the past to see Ed Sirett's name pop up, mind you it was a thread from 2006. Anyone know what happened to him, I do miss his sage wisdom.

Richard

Reply to
Tricky Dicky

Always vent radiators with the pump turned *off* in case some of the hiss that you hear is air being drawn *in* rather than escaping.

Reply to
NY

Its not particularly since it goes to much effort describing the effect around the header tank - something a sealed system does not actually have!

Reply to
John Rumm

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