Problem with new combi boiler

arciere

Our combi boiler was recently replaced as the old one, 16-years old, was (according to the plumber) uneconomical to repair).

The problem we now have is that the pressure keeps dropping, often from 1.5 bar to 0.0 in a couple of hours or less, but only when the heating is on or after that. However, if I top it up to 1.5 and leave it for several hours without using the heating (only hot water), it remains at 1.5. I've bled all radiators twice and topped it up at least 6-7 times.

Called the plumber and he suspects a leak but, there's no leak visible anywhere (not even the neighbours downstairs can see anything), we didn't experience any leaks with the old boiler (it just stopped working) and there doesn't seem to be a loss of pressure if we don't turn on the heating.

Any ideas?

Le_Kirk

Have you looked at the pressure relief valve outlet pipe? It usually comes out of the wall near the boiler with a short copper pipe that points at the ground or sometimes turns back on itself. Does water come out of there when the boiler is calling for heat?

MikeJXE

I'm no plumber but if you can increase the pressure and it holds when the boiler is switched off 

Then the pressure goes down when the boiler is switched on and pumping water round the system

That tells me you have a leak either a water leak or an air leak 

grumbler

You do keep your eye on the pressure discharge pipe, don't you?

Also,does it take a reasonably big amount of water to top up? If not, it can be a faulty expansion vehicle (full with water with very little amount of air if any).

bris

If you cant see an obvious leak then either the heat exchanger is cracked and the water is going down the condensate pipe or the expansion vessel needs charged and as the pressure rises then dumps it out the PRV.
Although if the heat exchanger was cracked it usually empties it even if the heating isn't on so unlikely to be that.

When you top it up and put the heating on keep an eye on the pressure and see if it rises high towards 3bar.

The prv could also be to blame. Its easy to get a bit of dirt in them which means they never fully seal and a drip can empty the system. Have a look outside and see if there is any signs of a drip, its usually easy to see.

Any way it goes get the installer back to remedy it, if he cant then the warranty should kick in.

Le_Kirk

It is a new boiler according to the OP, hard to imagine a faulty expansion vessel or cracked heat exchanger.

grumbler

MikeJXE said:

Then the pressure goes down when the boiler is switched on and pumping water round the system

That tells me you have a leak either a water leak or an air leak 

Pumping water through a pressurised system has no effect on leaking. Heating water in the system can have effect because water expands and pressure rises.

Le_Kirk said:

It is a new boiler according to the OP, hard to imagine a faulty expansion vessel

It's easy to imagine a vessel not charged properly or not sealed properly after charging.

bris

Le_Kirk said:

It is a new boiler according to the OP, hard to imagine a faulty expansion vessel or cracked heat exchanger.

Fitted two with faulty heat exchangers over the years.

ThisIsWeird

arciere said:

Our combi boiler was recently replaced as the old one, 16-years old, was (according to the plumber) uneconomical to repair).

The problem we now have is that the pressure keeps dropping, often from 1.5 bar to 0.0 in a couple of hours or less, but only when the heating is on or after that. However, if I top it up to 1.5 and leave it for several hours without using the heating (only hot water), it remains at 1.5. I've bled all radiators twice and topped it up at least 6-7 times.

Called the plumber and he suspects a leak but, there's no leak visible anywhere (not even the neighbours downstairs can see anything), we didn't experience any leaks with the old boiler (it just stopped working) and there doesn't seem to be a loss of pressure if we don't turn on the heating.

Any ideas?

Does the pressure drop after drawing just DHW for a reasonable time, say having a shower? (With the CH off)

A standard test for them to make could be to isolate the CH system - all the external pipework and radiators - using the two valves under the boiler, and if the pressure loss still occurs, the leak must be from within the boiler itself.

Le_Kirk

grumbler said:

Le_Kirk said:

It is a new boiler according to the OP, hard to imagine a faulty expansion vessel

It's easy to imagine a vessel not charged properly or not sealed properly after charging.

bris said:

Le_Kirk said:

It is a new boiler according to the OP, hard to imagine a faulty expansion vessel or cracked heat exchanger.

Fitted two with faulty heat exchangers over the years.

In this case either call back the installer (assuming they supplied the boiler) or the boiler supplier.

arciere

So after all it was a pipe leak, although even the plumber said it was strange.
The pressure never went above 1.6 bars. Left overnight at 1.5 and in the morning it stayed at 1.5. As soon as you turn on the heating, it gradually drops to 0.0.

Downstairs neighbours saw a tiny patch of wet floor inside a cupboard. Called the plumber back, he spotted a pipe with a plastic joint that was very slowly dripping (1 drop every couple of minutes or so).

Long story short, cut some wall panels, downstairs ceiling, but he couldn't reach the pipe. He left after 4 hours or so saying that he needs more time and more digging/cutting. Quoted for around £2,000 plus VAT for a day of work (plus materials), which is something that we definitely can't afford right now, after the boiler replacement.

For now, I guess the only option is to keep the boiler on just for hot water and wait until before next winter to fix that pipe...