Boiler Pressure Drop Only When Heating On

[markjg123]

My combi boiler is a Vokera Easi-Heat Plus. It's losing pressure when the heating is on but not when running hot water. The pressure drop doesn't affect the heating, ie the rads warm up and stay warm. I simply top up the pressure each time.

What's the mostly likely cause of the pressure drop and how much typically to repair? I want to be armed with some knowledge before a heating engineer tries convincing me that the boiler needs replacing.

 

[Harry Bloomfield]

My combi boiler is a Vokera Easi-Heat Plus. It's losing pressure when the heating is on but not when running hot water. The pressure drop doesn't affect the heating, ie the rads warm up and stay warm. I simply top up the pressure each time.

What's the mostly likely cause of the pressure drop and how much typically to repair? I want to be armed with some knowledge before a heating engineer tries convincing me that the boiler needs replacing.

Check your expansion vessel (EV), it should be filled with air, which compresses, to allow the water in your system, somewhere to expand into when it gets hot. It uses a Schrader car tyre type valve, and you can, assuming no water is released from the valve, top up the air pressure, with an ordinary tyre pump.

What will be happening, is the water instead is being forced out of your system via the relief valve. The water will appear outside, dripping from a short pipe, with two elbows, so the heated water points back at the brickwork of your wall somewhere.

 

[CountryFan]

As above, likely an expansion issue, often combined with a prv passing.

 

[markjg123]

Check your expansion vessel (EV), it should be filled with air, which compresses, to allow the water in your system, somewhere to expand into when it gets hot. It uses a Schrader car tyre type valve, and you can, assuming no water is released from the valve, top up the air pressure, with an ordinary tyre pump.

What will be happening, is the water instead is being forced out of your system via the relief valve. The water will appear outside, dripping from a short pipe, with two elbows, so the heated water points back at the brickwork of your wall somewhere.

Why is it doing that?

 

[polesapart]

https://www.diynot.com/wiki/Plumbing%3Afaq7

 

[gas112]

when the heating has been on for about 20 mins check to see if pressure gauge is rising into the red if so expansion vessel needs recharging .

.

 

[markjg123]

https://www.diynot.com/wiki/Plumbing%3Afaq7

Thanks. I'll give it a go.

 

[Harry Bloomfield]

Why is it doing that?

Heat up water, and it expands, needing more space. Your pipes, and radiators etc., cannot expand, to accommodate the extra water volume, so your system includes an expansion vessel to take care of that. An EV, has a rubber diaphragm across the middle, separating water at the bottom, from air at the top. The pressure of air, must be approximately the same as the water, so it can compress, as the water warms and expands.

If water comes out of the Schrader valve, by pressing the centre pin in, that means the diaphragm has split, so the EV has to be replaced.

If the EV fails to work, then pressure could build up in your system, enough to cause radiators, or pipes to burst. To avoid that, all such systems include a PRV / pressure release valve. The valve opens to vent the pressure at 3bar. Your system normally operates at between 1 and 2 bar.