Combi boiler, pressure dropping slowly

Roonaldo · 7 Mar 2010

Hi,

I moved into a new property 4 months ago, which is heated by way of normal radiators in the old part of the house and underfloor heating in the new extension.

These are fed from a standard gas combi boiler.

T the end of last year, i had to top up the combi boiler (up to 1.5 bar) as the pressure was around 0.7 bar.

Over a period of about 3 months since I topped it up. It seems to have dropped to around 1.2 bar.

There are no obvious leaks, no wetness or stains by the radiators etc.

Is this something I need to be worried about?

Red_Man · 7 Mar 2010

Not overly, keep an eye out for wet patches but it may be under first floor floorboards so you'll never find it.

Something that small isn't anything to worry about.

Try inserting some Fernox Leak Sealer into your system. May work on something that small.

Roonaldo · 7 Mar 2010

Red_Man said:
Not overly, keep an eye out for wet patches but it may be under first floor floorboards so you'll never find it.

Something that small isn't anything to worry about.

Try inserting some Fernox Leak Sealer into your system. May work on something that small.

Thanks, where would you insert this? Is this something I could do myself, or need a plumber?

Cheers

Red_Man · 7 Mar 2010

Drain your higest radiator... remove the blanking cap at the top of the radiator... not the bleed valve.

Buy a dosing kit and insert... seal the system, repressurise... bobs your uncle, fanny... is a naughty word.

MANDATE · 7 Mar 2010

Lots of combis lose pressure over a period of time.
This is usually down to the 'air' side of the expansion vessel.
The expansion vessel is pre charged with air to about 0.8 bar, so it does not contain any water (or very little) before the heating is on.
The excess water produced by expansion when heating is on pushes against the diaphragm and compresses the air, which produces a increase in pressure.
If the air capacity of the vessel is reduced too far, then pressure rises higher and opens the Pressure Relief Valve which discharges water out through tthe discharge pipe.
Your boiler may not yet have reached that stage, but if the vessel has taken in some water, it would explain why you can't find a leak.
Expansion vessels need to be large enough to deal with the volume of water in the system. If you have added additional volume by way of more radiators or pipe work, it raises the question whether the existing expansion vessel is still large enough.
Expansion vessels can be re charged using a pump, but it's important that any water in the vessel is removed during the procedure.

Roonaldo · 7 Mar 2010

MANDATE said:
Lots of combis lose pressure over a period of time.
This is usually down to the 'air' side of the expansion vessel.
The expansion vessel is pre charged with air to about 0.8 bar, so it does not contain any water (or very little) before the heating is on.
The excess water produced by expansion when heating is on pushes against the diaphragm and compresses the air, which produces a increase in pressure.
If the air capacity of the vessel is reduced too far, then pressure rises higher and opens the Pressure Relief Valve which discharges water out through tthe discharge pipe.
Your boiler may not yet have reached that stage, but if the vessel has taken in some water, it would explain why you can't find a leak.
Expansion vessels need to be large enough to deal with the volume of water in the system. If you have added additional volume by way of more radiators or pipe work, it raises the question whether the existing expansion vessel is still large enough.
Expansion vessels can be re charged using a pump, but it's important that any water in the vessel is removed during the procedure.

I have noticed some copper coloured discharge on the floor directly under the pressure release valve. Not very much, but looks like its released at some point over the last few weeks. Could this be related/ a symptom?

MANDATE · 7 Mar 2010

Well it could be related. If the PRV has opened due to pressure reaching 3 bar, on the other hand, engineer may have opened it during service.
Problem also is that when opened and closed, the valves get dirt on the valve seat and don't close properly so allowing a small leak.
Two things worth doing. Take note of the pressure rise when using the heating, ideally it should be about 1 bar. Also does it get anywhere near the 3 bar needed to open the PRV?.
If you secure a plastic bag on the discharge pipe you may find evidence of a leak.
The amount of pressure loss in your case is very little compared with others, I would not bother topping it up from 1.2 bar. The higher the pressure the more the leak.
There may be a low pressure which prevents the boiler from lighting, but if it operates ok say from 1 bar rising up to 2 bar, then I don't see any harm.

Roonaldo · 7 Mar 2010

MANDATE said:
Well it could be related. If the PRV has opened due to pressure reaching 3 bar, on the other hand, engineer may have opened it during service.
Problem also is that when opened and closed, the valves get dirt on the valve seat and don't close properly so allowing a small leak.
Two things worth doing. Take note of the pressure rise when using the heating, ideally it should be about 1 bar. Also does it get anywhere near the 3 bar needed to open the PRV?.
If you secure a plastic bag on the discharge pipe you may find evidence of a leak.
The amount of pressure loss in your case is very little compared with others, I would not bother topping it up from 1.2 bar. The higher the pressure the more the leak.
There may be a low pressure which prevents the boiler from lighting, but if it operates ok say from 1 bar rising up to 2 bar, then I don't see any harm.

It only rises a bit when the heating is on I think, maybe 0.2 bar, but most of our heating is the underfloor heating. It seems to rise most when heating water, ie when the shower is on, then it rises about 0.6 -0.8.

I've never seen it above 2 bar.

airtrackinc · 1 Apr 2016

It is simple to say that you have a leak on the combination boiler or the central heating system. Personally what I would do is look for a leak as can be seen visible, it may be useful to check the pressure release pipe that goes from the boiler to the outside but you MUST not place a cap on it as this will become a safety hazard. If your unable to find the leak visible the I would advice you call an emergency plumber that is gas safe registered.

Madrab · 1 Apr 2016

Welcome to the forum from across the pond, a word of advice tho ... worthwhile checking the date of the post and the OP that posted it. This thread is 6 years old and the OP hasn't been on since then so has probably sorted the problem by now

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Combi boiler, pressure dropping slowly

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