Baxi Combi 105e: CH lost 1 bar of pressure in a few hours

[sonfre]

Hi everyone

Summary is in the title. Found the boiler was at 0.5 bar earlier today so topped up to 1.5 bar. Checked before bed and its back to 0.5 bar.

There are no signs of a leak around any radiators nor any suspicious water damage on the downstairs ceiling.

Before I rip up all the flooring downstairs to find a leak going into the cavity under the house, what other things could be causing this?

 

[John.g]

Your boiler will have a PRV (pressure relief valve) which lifts at 3.0 bar, you should see a small bore waste pipe from this PRV maybe exiting the wall at the rear of the boiler. Tiewrap or tape a plastic bag on the end of this and check for water regularly, also watch boiler pressure while the system is heating up and note the pressure rise, a 0.5/0.7 bar would be typical but if approaching 2.8bar then problem somewhere, probably E.vessel.

 

[Deleted member 120897]

Your boiler will have a PRV (pressure relief valve) which lifts at 3.0 bar, you should see a small bore waste pipe from this PRV maybe exiting the wall at the rear of the boiler.

waste pipe? Usually done in copper.

 

[sonfre]

Your boiler will have a PRV (pressure relief valve) which lifts at 3.0 bar, you should see a small bore waste pipe from this PRV maybe exiting the wall at the rear of the boiler. Tiewrap or tape a plastic bag on the end of this and check for water regularly, also watch boiler pressure while the system is heating up and note the pressure rise, a 0.5/0.7 bar would be typical but if approaching 2.8bar then problem somewhere, probably E.vessel.

Thanks for the reply.

The only copper pipe comes out of the outside wall below the flue, and then goes straight back inside the wall.

 

[John.g]

If this is less than say 1/2 ins then it might only seem to go back in, it might only be turned back in with its open end almost up against the wall.

 

[sonfre]

If this is less than say 1/2 ins then it might only seem to go back in, it might only be turned back in with its open end almost up against the wall.

You're absolutely correct, have just tied a bag to the end of that pipe and topped system up again to 1.5 bar (was just below 0.5 bar). With heating on, pressure rose to close to 2.5 bar within 10 minutes, then boiler shut off even though thermostat was not at its target temperature. Does this support the expansion vessel being the issue?

 

[John.g]

No, if no water in the bag. Also, 2.5bar shouldn't cause the boiler to shut down.

 

[sonfre]

No, if no water in the bag. Also, 2.5bar shouldn't cause the boiler to shut down.

My mistake about the thermostat, it was at target temp when it stopped. Pressure has stayed at about 2.2 bar and no water in the bag after a few hours. Will check again in the morning.

 

[John.g]

If no water bagged by morning and if no system leak then possibly boiler heat exchanger, if you can locate the boiler condensate pipe (on the instreuction/user manual) then with boiler shut down for a few hours you might be able to break a connection on this pipe and see if water dripping from it.

 

[sonfre]

If no water bagged by morning and if no system leak then possibly boiler heat exchanger, if you can locate the boiler condensate pipe (on the instreuction/user manual) then with boiler shut down for a few hours you might be able to break a connection on this pipe and see if water dripping from it.

This isn't a condensing boiler as far as I know, so don't think it would have a condensate pipe?

 

[sonfre]

If no water bagged by morning and if no system leak then possibly boiler heat exchanger, if you can locate the boiler condensate pipe (on the instreuction/user manual) then with boiler shut down for a few hours you might be able to break a connection on this pipe and see if water dripping from it.

Pressure has dropped back to about 0.8 bar and there is some water in the bag. Does this look about right for the amount of water for a c. 1.5 bar drop in pressure?

 

[Deleted member 120897]

Hard to tell because of the bag and the photo, but that doesn’t look right for a pressure relief valve safety discharge pipe.

 

[John.g]

Hard to say but good in one sense that the PRV is leaking.
So... assume a 8ltr EV with precharge and filling pressures of 0.8/1.5bar, the EV/system would have to loose 2L to fall to 0.8bar but if the EV effective volume is only 1/2 due to loosing air out of the EV (airend) then it it only has to loose 1L under the same conditions and so on.
Would suggest fill to 1.5bar, then open a rad vent and keep bleeding water into a container until the pressure falls to 0.8/1bar and measure it, that will tell (almost) all.