Boiler pressure increasing

[Bart C]

I have a pressurised CH combi boiler, the system pressure has gone up by around 2 bar over the last few weeks.

Bleeding the radiators reveals pale water that seems to have some tiny air bubbles.

Any ideas what's causing this? And can I just keep bleeding to get the pressure down? The boiler hasn't minded working at 3.5bar or so, so far.

Thanks.

Bart

[Christian McArdle]

Most likely either a not disconnected filling loop that is passing water (you hope) or a leak in the combi water to water heat exchanger (you hope not). An outside chance of a dodgy pressure gauge.

Christian.

[John Rumm]

The problem will be that the pressure release valve will open shortly, and these don't always reseal correctly. You then have a drip outside. Also it suggests you are continously introducing fresh (i.e. oxygenated) water into the system that will dilute the inhibitor and cause corrosian.

Is the filling loop still attached to the pipework? If so remove this and check the filling tap is not letting by a constant trickle of water.

If this is not the case, then the only place where the domestic hot water and the heating system water come close to each other is in the plate heat exchanger in the boiler. If this had a pinhole leak between primary and secondary sides it could also cause your problem.

(this all assumes you don't have a pressurised stored hot water cylinder run from the heating side of the combi)

[Bart C]

Disconnected this and there is a leak from the inlet service valve of about

1 drop every 10-15 seconds.

If that's the cause then it's amazing such an innocuous looking drip should pressurise my CH system.

Thanks for both your replies.

Bart

[Christian McArdle]

Result! Believe me, fixing the isolation valve (or just turning it off properly) will be much cheaper than changing the heat exchanger.

Christian.

[Roger Mills]

Yes, but think about it. Four or five drops per minute - how many litres is that is several weeks? Introducing that amount of extra water will

*certainly* pressurise it!

[adder1969]

It's not really that many drops entering the system over a period of time more that the pressure in the two systems will try to equate over a period of time. If the mains is the same pressure as the "sealed" system then no water will enter. The reason the filling loop *must* be disconnected in the opposite problem - if the mains pressure drops water from your boiler system could be introduced into the mains water.

[Roger Mills]

But the mains is likely to be at 4 bar or more, and the sealed system at 2 bar or less. Granted the flow will be less than when the tap is open to atmosphere - but if it's only at *half* the drip rate, it will still be enough to pressurise the system over a period of time.

[John Rumm]

Unlikely I would ahve thought. Most combis will have the blow off valve set at between 3 and 3.5 bar. Mains water will frequently be more than that.

That is also the reason there must be a double check valve fitted to the heating circuit so that water can not escape from it even if the tap were open and the mains pressure dropped below the system pressure.

[Roger Mills]

"Try to equate" is correct. It will take a long time at a very small flow rate, and the safety valve will operate before it actually gets there. From what the OP said, it was getting that way before he decided to do something about it!