Air getting into unvented cylinder

[Puzzled]

Hi - hope someone can shed some light on this problem:

I have an Ariston unvented hot water system in a flat which is sometimes left vacant for a few weeks. During that time the mains feed to the flat is turned off.

When the mains feed is turned on again and the hot taps run - even without switching on the boiler - some very violent spitting and almost explosive emissions of air from the taps happens for a few minutes, then it all settles down and behaves perfectly. I you neglect to do this before switching on the boiler, then the pressure relief valve opens due to the air in the cylinder heating up.

The expansion vessel still has air in it, and for the life of me I cannot see how air can be getting into the tank over the period when I'm away. The local plumber s scratching his head. We tried replacing the sacrificial anode in case it was electrolysing the water, since it seemed to me that the emitted gas smelt odd, but the problem is still happens most times.

The flat is in a block where water is electrically pumped up from ground level - I wondered if perhaps in a power cut the pump might stop and water be sucked back down past the "off" mains tap (?) in through the pressure valve, but I am clutching at straws...

Has anyone come across anything like this?

 

[rpm]

How many floors in the block of flats and on what level are you?

 

[Puzzled]

How many floors in the block of flats and on what level are you?

On the 5th floor of 6.

My other thought is: if the tank is hot when the mains is turned off, presumably when it cools the water contracts and creates a vacuum which gradually pulls in air from "somewhere". Could air get in backwards through pressure relief valves under those circumstances?

 

[PhoenixServices]

When the mains is turned off are any taps opened? Or literally mains turned off?
Any dripping taps? Even a slight drip.

 

[chris watkins]

Mains water carries dissolved gasses, boil a pan of the stuff & watch it being driven off, thats all them little bubbles around the edge.

Under pressure it will remain dissolved but drop the pressure & it will come out & accumulate at the high points in the system.
You could cut it an AAV.

You need to get the system checked by someone who has their Part G ticket & knows what they are doing. The safety valve should not open when the system is heated, even if there is air in there & it gets heated.

 

[Puzzled]

The safety valve should not open when the system is heated, even if there is air in there & it gets heated.

Chris - many thanks for that. Further investigation suggests that the zone valve controlling boiler input to the heating coil in the cylinder is not properly closing when the thermostat triggers, hence the water carries on heating until the release valve triggers not from pressure but from temperature. So I'm going to get the zone valve fixed and hopefully that will help.

As far as the air in the system, I'm just going to have to be aware that it's necessary to run it off for a bit before firing things up, to be on the safe side.

 

[chris watkins]

First off please don't be tempted to play around with this system yourself it is both illegal & dangerous. Find an engineer who has their Part G ticket (ask to see it) & has experience in these, as I said in first post (& sorry for spelling, it was the beer).

Second it will almost certainly not be the T&P going off under temp because a zone valve is letting by, I take it you have a gas fired boiler ? If so, max temp on its stat is 82deg C, T&P is set 90-95degC, heating water is therefore not hot enough to overheat domestic hot water.

Trust me, get someone in who understands what they are doing more than likely they could sort out both issues at once.

 

[Puzzled]

Will do - thanks for your help.