CH system losing pressure - but getting it back by itself

[munchkinsue01]

We have just had a loft conversion / extension done where they took out the water tank in loft and installed an unvented indirect Megaflow cylinder. We have kept our old boiler, which is a Baxi Bermuda back boiler.

For a while [about a month] all was ok. Since they have completed work in the extension, however, we have a problem with pressure dropping on the central heating system.

There is a little red expansion tank next to the Megaflow, with a dial over it. When the central heating / hot water are switched on [they are on a timer and always set to come on together] this pressure reads somewhere between 1 and 1.5 bars. But when the heating / water go off the pressure falls to zero within the space of about 20 minutes. The strange thing is that when we switch the heating / water back on again the pressure goes back up to somewhere between 1 and 1.5 bar, without us having to top up the system at all.

Two plumbers have told us that we must have a leak under the floor downstairs. Work in the extension did include work to the central heating system, fitting a new radiator and 2 fan heaters that take heat off central heating pipes, so this is certainly possible. But does this make sense if the pressure is going up again all by itself?

I know absolutely nothing about central heating systems, so please be patient with me. What I do know is that I don't want to have to dig up my newly laid wooden floor in extension.

Many thanks

 

[twgas]

the pressure should be about 1bar when cold, this will increase a bit as the system heats up, typically up to 2bar. When it cools it should go back to 1bar. If you try bleeding the rads (and I'm not advising you to), the pressure will drop a bit.

I'm assuming your boiler is suitable for a sealed system - some BBUs are not

 

[bengasman]

one month would be short enough to validate warrantee call back.
don' do anything yourself or hire anyone to do something.

was the conversion to sealed heating done at the same time as the rest?

setting the hot water to come on with the heating sounds suspicious; the whole idea of good controls is to be able to have it on separate times.

were these guys really qualified plumbers?

 

[munchkinsue01]

Just to clarify - heating and hot water can be set to come on differently, it's just that we have chosen for them to operate on same schedule.

Interested in this idea that not all back boilers are compatible with a sealed system. Might phone Baxi to find out. Could this be causing this strange pressure dropping / magically coming back up again? [I realise pressure does rise and fall as system heats up / cools down....the issue here is it dropping to zero, which everyone says it's not supposed to do.

Thanks

 

[bengasman]

call baxi and ask. without delay. make sure you have got every detail of the bbu at hand when you call them

 

[munchkinsue01]

It's an Inset 2 boiler 50/4 E and Baxi say it is compatible. Which makes sense, as for a couple of weeks it was working fine. Things only changed since extension up and running.

 

[bengasman]

so the boiler should be safe for the job.
get the boys back and show them the problem

 

[swbjackson]

What size is the expansion vessel? Sounds like it is too small. Initially the expansion of water has taken the pressure to over 3 bar and the safety valve has discharged. Over a short period this will drop the pressure to almost zero.

Have a look at http://www.ptsplumbing.co.uk/download/BOSS-Expansion.pdf for a guide.

Mike