Boiler Pressure Increasing Mystery

andrewsfoster67

Jeepers_Creepers said:

To check if it's releasing excess pressure, find out where the 15mm copper discharge pipe exits your house and rubber-band a clear bag over the end.

If it was losing water through this, the pressure would drop to below what it started from when it cooled down again as there would now be less water in the sealed system.

Money is still on EV :-)

This makes perfect sense, thanks for the explain.

So when you say it must be the EV, do you mean that it needs recharging? when the engineer did the check there was no water release from the schrader valve and the gauge connected to the EV showed approx 2 bar (vs. cold system pressure of 1.3) and he said it was fine

However if I understand correctly to check the true EV pressure, i'd need to bleed enough water from a radiator until system pressure was zero and then check the EV pressure, which should be per boiler guidelines i.e. approx 1bar?)

neilmcl

grumbler said:

neilmcl said:

grumbler said:

IMO, the expansion vessel is the only possible explanation.

ETA: is there one radiator without TVR in your house? There must be one.

Not the "only" possible explanation. My dad's Worcester Bosch combi's pressure was constantly on the rise when the heating was on and he was constantly having to drain off the system to keep it down. We too thought it was the expansion valve and had that checked as well as forking out for a new PRV, but after the third visit from a different heating guy it turned out to be a faulty filling loop which wasn't closing properly.

expansion vessel

The essential difference I see is that in your case the pressure "was constantly on the rise"

I meant vessel, it was just a mistype.

Jeepers_Creepers

andrewsfoster67 said:

Jeepers_Creepers said:

To check if it's releasing excess pressure, find out where the 15mm copper discharge pipe exits your house and rubber-band a clear bag over the end.

If it was losing water through this, the pressure would drop to below what it started from when it cooled down again as there would now be less water in the sealed system.

Money is still on EV :-)

This makes perfect sense, thanks for the explain.

So when you say it must be the EV, do you mean that it needs recharging? when the engineer did the check there was no water release from the schrader valve and the gauge connected to the EV showed approx 2 bar (vs. cold system pressure of 1.3) and he said it was fine

However if I understand correctly to check the true EV pressure, i'd need to bleed enough water from a radiator until system pressure was zero and then check the EV pressure, which should be per boiler guidelines i.e. approx 1bar?)

Spot on.

Inside the vessel is a rubber diaphragm which separates the water side from the air side (which has the valve). The fact that no water came out of the valve is good as that suggests the dia is intact. However, what usually happens with EVs is that they slowly lose pressure over a few years - just general leakage from the valve, seams, perhaps even percolating through the dia. That means the air volume reduces, with the space taken up by the water pushing against the dia. This in turn means there's no room for the expanding water to escape to, so the system pressure soars instead.

The guy measured 2 bar on the air side? Cool. But what does that actually mean?! There could be half an EV full of air there at 2 bar (great), or a teaspoon's worth at 2 bar (pants). (In any case, 2 bar is too high...)

So, as you've sussed, the only way to recharge an EV is to release the pressure from the water side, and keep releasing it as you recharge the EV. The simplest way is via a bleed screw, and if you keep this open you'll find water squirting out with each pump of air into the EV as the dia finds its correct position.

The air side is usually charged to just 0.75b to 1 bar depending on the model. Once done, close bleed screw, repressurise water system - to 1 bar or fractionally above.

Jeepers_Creepers

Page 34, section 12.15 - it should be 0.75bar.

https://www.vaillant.co.uk/downloads/product-manuals/ecofit-pure-1/combi-5/ecofit-pure-combi-installation-and-maintenance-instructions-1943272.pdf

andrewsfoster67

Jeepers_Creepers said:

Page 34, section 12.15 - it should be 0.75bar.

Thought i'd close this thread out -

I drained the system and the EV air pressure was 1.65, so in the ended up letting air out until it was 0.75 bar (not sure how it could become overpressurised to begin with)
In any case the system pressure seems fine now and sits at 1.2 when cold and moves to 1.7 when CH is on, and 1.9 when CH and HW is on.
Thanks vm for your help !

Jeepers_Creepers

Nicely sorted, Andrew. :-)

Can't think of how the air side was that high either, unless the service guy did it. You can see how this caused the large pressure increase on the water side - the expanding water couldn't force itself in to the vessel as it effectively had a 1.65bar blockade!

You are officially more competent that your service guy - he measured 2bar on the EV valve, and thought that ok...