air in radiators.

air is getting into my central heating , as fast as i can bleed it out. i noticed the presssure had risen a few weeks ago, and found air in the upstairs rads. after expelling it, the pressure rose again over the next few days. i have checked everywhere for leaks, and found none. i have replaced the expansion vessels ( i know the domsetic water (( white one)) isnt connected to the heating system, but i found no pressure in it , and upon disassembly found a split diaphragm ). The red one had a small amount of water come out the shrader valve when i emptied the air from it. I took it to mean a possible split, so replaced. Air pressue in said vessel is correct, but the problem has got progressively worse each day. Now if i bleed the system,and close the valve on the last ( and furthest rad ) as soon as air stops coming out, if i give it just ten seconds, and reopen the valve , there will be no water come out again for several seconds. I havent topped up the water in the system since installing the expansion vessels several weeks ago. If i had a leak , i would surely have needed to. Its as if the system is creating the air. If the pump was faulty, would that put air into the system without water loss? I usually bleed whilst the pump is running ( as suggested by my local plumber ). If i bleed cold, then i can expel all the air, without finding more, just seconds later, but as soon as the system is turned on the air reappears immediately.. The system is unvented, and powered by air source heat pump. Any ideas welcome . cheers
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On 04/01/2018 17:44, paul Roberts wrote:

Are you sure that it's air - and not hydrogen caused by corrosion in the system. Do you have any corrosion inhibitor, and when was its concentration last checked?
What happens if you hold a lighted taper in the stream of 'air' being bled from a radiator? If it's hydrogen, it will burn with a blue flame.
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Roger
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From the description though that amount of hydrogen should mean a rather large amount of corrosion, which in the end would I would have thought eventually lead to a leak.
Brian
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On 04/01/2018 17:44, paul Roberts wrote:

Could be a bad joint just before the pump allowing air to be sucked in.
Mike
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On 04/01/2018 17:44, paul Roberts wrote:

One possible cause of unexpected pressure rise could be the taps on the filling loop letting by.
However that would not explain the gas...
Corrosion might - not not in the space of minutes.

That sounds like an unvented cylinder - is it just heated by immersion then and not from the heat pump?

Possibly. Do you still see the pressure rise without the pump running?

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John.
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On 04/01/2018 17:44, paul Roberts wrote:

I remember my mate couldn't bleed his radiators, turned out the flow of water was pulling air into the system.
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