Boiler losing pressure only when heating is off

[mikehzzzz]

I have a Baxi Duo-tec 40 Combination Boiler installed in June 16. It loses pressure every night. We top up the pressure to 1.5 bar in the morning and the heating runs fine without a loss of pressure until the heating goes off late in the evening. 1 hour later the pressure has dropped below 1 bar and in 2 hours it is 0 bar.
The Baxi engineer has been out twice. He replaced the factory fitted expansion vessel which he said was the wrong size for the boiler and fitted a larger one. He checked the pressure release valve.
There is no sign of a leak. I have had a plumber out who has checked all visible and under floorboard pipes and used a thermal imaging camera to check a few feet of underfloor pipe but cannot detect a problem. There is no sign of damp and yet this much water loss would surely be showing somewhere?
I don't understand how, if there is a leak in the system, the pressure would be unchanged throughout the day when the heating is on and only drop once the heating goes off.
Any thoughts on what to try next?

 

[picasso]

Isolate the boiler by turning off the valves under the boiler when you have switched the boiler off, that will tell you if its the boiler or the system losing pressure, do you have any pipes in concrete screed ?

 

[mikehzzzz]

Hi, Thanks for helping. I have just isolated the boiler and will see what happens over the next couple of hours. I do have a short length, about 5 feet, under concrete. The pipes are Hunter plastic pipes which the plumber told me are obsolete and a plumbers nightmare. If the leak is in the system why would it maintain pressure all day and not leak when the heating is on?

 

[jackthom]

It's most likely it isn't really maintaining pressure all day. As the pressure naturally rises when it's hot it won't be so obvious how much water has been lost until it cools down and contracts in the evening.

 

[mikehzzzz]

I had considered that but thought the drop was too much to be accounted for by contraction, especially as doesn't appear to drop at all throughout the day. I suppose I could see what happens if I leave it on for a couple of days.

 

[jackthom]

How high does the pressure rise when the heating is on?
I know the engineer has checked the PRV but it may still be a good idea to tie a plastic bag on the PRV discharge pipe for the day to check for any water release.

 

[mikehzzzz]

It only rises slightly when on. Now that I have isolated it the pressure has increased slightly. Nearly an hour gone and no loss of pressure at the boiler. I'm guessing this is looking more like it is in the system. If so then wonder if I could use a length of pipe to see if pressure is maintained with that concreted section bridged?

 

[Hason]

Likely to be either leak on pipe work/system or main heat exchanger is passing and leaking down condense run, I've had quite a few main heat exchangers pass, hope this helps good luck

 

[slippyr4]

If you're losing pints and pints every day into a 5' patch of concrete, that floor will probably be wet.

 

[mikehzzzz]

Thats what I thought but it is not. There is no visible damp anywhere. I was told it could be soaking straight down.

I isolated the heating for about 6 hours yesterday and then turned the heating off. The pressure in the boiler stayed at 1.5 bar until I opened the valves. It then dropped all the way down.