central heating loses pressure at night but not in the day

[Colin Green]

My central heating is losing pressure over night. If I fill it in the morning, it is still at pressure in the evening, but by the following morning the pressure is zero.

There's no sign of dampness in any of the walls, floors or ceilings. I've had the PRV overflow pipe off and a bowl underneath it. Not a drop has come out. The expansion vessel is not leaking.

I've tried closing both valves on all the radiators and the pressure still drops. I've tried closing both the central heating valves on the combi boiler. The pressure in the boiler was still good the next day, but as soon as I opened the valves, the pressure was lost. I seems I have a leaky pipe somewhere.

Why would a pipe hold good pressure all day but lose it overnight?

For reference, the whole system is 4 years old and the boiler is a Worcester Bosch Greenstar 29CDi

 

[CBW]

It could be a small leak, if your heating is used n the day, and not over night might be the reason. I once attended a leak, whereby, the expansion was enough to seal the pipe, but as soon as it cooled down, the leak appeared.

Try taking the pressure up to approx 2.5 bar and don’t use it, see if this shows anything.

 

[JohnD]

"If I fill it in the morning, it is still at pressure in the evening, but by the following morning the pressure is zero."

If the pressure is high when hot, and low when cold, it might be your expansion vessel.

Try turning it off to cool and refilling it in the evening. Leave it off until morning. What's the pressure? Then turn it on. What does the pressure do?

 

[terryplumb]

As you isolated flow and return valves on the boiler ,and pressure held until the next day,only dropping immediately the valves were opened, thats confirmation that the pressure is being lost on the system ( pipework and rads). Is any pipework buried in or under flooring?

 

[Colin Green]

As you isolated flow and return valves on the boiler ,and pressure held until the next day,only dropping immediately the valves were opened, thats confirmation that the pressure is being lost on the system ( pipework and rads). Is any pipework buried in or under flooring?

Yup. 2 downstairs rads. Suspended timber floor with Celotex glued between the joists. The dining room is Kearndean tile over plywood over chipboard flooring over Celotex. The Mrs is not going to be happy with that being taken up.

The boiler is in a side extension. The CH pipes come into the original house under the bathroom floor, which is tiled over plywood over chipboard flooring. I suspect I know which bit of stud wall the pipes come down from there to the ground floor rads. Is there any mileage in putting a hole in the plasterboard, cutting those pipes and fitting isolators, to lock off the ground floor circuit? That would prove that the leak is not upstairs, before ripping up the floor.

I've filled up the boiler to 2 bar and switched it off. The house is warm and the rads cold. I'll monitor the pressure as the house cools.

 

[terryplumb]

How long have you been losing pressure ,days,weeks, longer ?

 

[Colin Green]

How long have you been losing pressure ,days,weeks, longer ?

A couple of weeks. It was fine before then.

 

[Colin Green]

update: The system has lost 1.5 bar in 2 3/4 hours with the boiler switched off.

 

[terryplumb]

Have you made a careful check of all rads ,their valves and all visible pipework ? If you have ,and nothings visible on ceilings etc ,then your leak is likely below floor level.

 

[MeldrewsMate]

If you're convinced that what Terryplumb has written is correct

Have you made a careful check of all rads ,their valves and all visible pipework ? If you have ,and nothings visible on ceilings etc ,then your leak is likely below floor level.

then your idea of isolating just the two downstairs rads through the ceiling void is a valid and sensible way to isolate the problem. If it shows an underfloor leak then the Mrs will have to decide whether she likes the floor more than or less than heating.

Just by way of a reminder to be sure you've done what you said and isolated both ends of every rad (this only isolates the rads, not the pipework) I once visited a job where the builder was pulling his hair out over a persistant drop in pressure, even to the extent of replacing the boiler. His fault finding skills were crap, but still he insisted he'd tried 'everything'. When I looked there were no signs of leaks; when I applied 10 bar to the pipework (having isolated the boiler) the tiny pinhole in the middle of one radiator opened up just enough to show a wet mark on the wall behind it.

So before you tear open the ceiling...

1. How long does it take to re-pressurise the system? 1 second? 10 seconds? 1 minute? This gives us an estimate of how much water has been lost.
2. Have you tried having the pipework pressure tested? Will it hold overnight at 5 to 10 bar?
3. Have you any automatic air bleed valves? Have you had the one inside the boiler tested?

And a reminder of your first post. Is that all still true?

My central heating is losing pressure over night. If I fill it in the morning, it is still at pressure in the evening, but by the following morning the pressure is zero.

There's no sign of dampness in any of the walls, floors or ceilings. I've had the PRV overflow pipe off and a bowl underneath it. Not a drop has come out. The expansion vessel is not leaking.

I've tried closing both valves on all the radiators and the pressure still drops. I've tried closing both the central heating valves on the combi boiler. The pressure in the boiler was still good the next day, but as soon as I opened the valves, the pressure was lost. I seems I have a leaky pipe somewhere.

Why would a pipe hold good pressure all day but lose it overnight?

For reference, the whole system is 4 years old and the boiler is a Worcester Bosch Greenstar 29CDi

 

[Colin Green]

If you're convinced that what Terryplumb has written is correct

then your idea of isolating just the two downstairs rads through the ceiling void is a valid and sensible way to isolate the problem. If it shows an underfloor leak then the Mrs will have to decide whether she likes the floor more than or less than heating.

Just by way of a reminder to be sure you've done what you said and isolated both ends of every rad (this only isolates the rads, not the pipework) I once visited a job where the builder was pulling his hair out over a persistant drop in pressure, even to the extent of replacing the boiler. His fault finding skills were crap, but still he insisted he'd tried 'everything'. When I looked there were no signs of leaks; when I applied 10 bar to the pipework (having isolated the boiler) the tiny pinhole in the middle of one radiator opened up just enough to show a wet mark on the wall behind it.

So before you tear open the ceiling...

1. How long does it take to re-pressurise the system? 1 second? 10 seconds? 1 minute? This gives us an estimate of how much water has been lost.
2. Have you tried having the pipework pressure tested? Will it hold overnight at 5 to 10 bar?
3. Have you any automatic air bleed valves? Have you had the one inside the boiler tested?

And a reminder of your first post. Is that all still true?

To answer your questions:

1. To fill up from zero to 1.5 Bar takes about 30 seconds.
2. Not tried a pressure test
3. No external air bleed valves. Didn't know the boiler had one!

I've just run my fingers over all the rad and TRV joints. All dry and no visual signs of damp on walls etc.

I'll put some pressure in the system then lock off the boiler and all the rads at both ends. Just to be sure.

 

[JimCrow]

Have you checked any drain cocks outside.

 

[Colin Green]

Have you checked any drain cocks outside.

The drain is inside on one of the rads. There's no sign of a leak.

 

[Colin Green]

OK, so now I'm confused. Yesterday at 7pm I filled the system to 2.0 bar, I turned the the boiler off, the flow and return off, all rads off at both ends. This morning, I opened the flow and return and the pressure only dropped to 1.9 bar. I opened each rad in turn with no further pressure drop. How can it lose all its pressure in 2 hours over night if it is all on, but only 0.1 bar if everything is off?

Edit to say that after running the heating for 10 minutes, the pressure is back up to 2.0 bar. The pressure drop in the system could just be that it had gone cold over night.

 

[sbgasman]

If your main heat exchanger is leaking it could be running down the condense pipe , if you can see the pipe outside see if it’s dripping when the boiler is off.