Central Heating System Pressure

[julias dad]

My central heating is a sealed system which seems to lose pressure. There doesn't appear to be dripping from the pressure relief valve outlet. When I add more water to the system the pressure increases, but drops again after a while - a few hours. I have used some Fernox F4 superconcentrate sealer but the situation persists. The central heating appears to work normally, sometimes with a bit of vibration noise from the boiler, but otherwise normally. There are no obviouse leaks anywhere. When the gauge indicates zero pressure the system seems to work normally.

The system has worked like this for some years. It was new in 1999. I top it up every so often from the inlet pipe in the airing cupboard. It was looked at in 2003 by a plumber when the pressure relief valve, pressure gauge and pressure vessel were all replaced.

Maybe this is an indication fault? But then why would it indicate increased pressure when water is added? Maybe there is a small weep leak from a pipe behind the plaster wall? This would drip down to the ground in the cavity between the plaster walls. The Fernox F4 (2 tubes) was added over a week ago so it should have taken effect by now.

Should I be bothered?

 

[749s]

A heating engineer told me that some systems are like this for years. The biggest worry is the build up of scale from all the fresh water being continually added if there's no obvious damp anywhere.

 

[julias dad]

I am in a hard water area but fitted a water softener some years ago. That would prevent any scale build up.

 

[bellairaphon]

what boiler do you have

nath

 

[julias dad]

It's an Ideal Classic.

 

[julias dad]

So no real problem then? .....

 

[Burnerman]

The fact that your pressure is dropping obviously does indicate a leak somewhere...these can be absolutely dreadful to find, but common places are the outside PRV (tie a poly bag on the end to see if it ejects water when you aren't looking), the automatic air vent on the top of the boiler (which should only pass a tiny amount of water anyway) and the radiator valve stems.
The expansion vessel should be checked for air pressure when the system water pressure is at zero - and it should be pumped up to around 10 psi. With the system repressurised and CH on, expect the pressure to rise a few psi, but the pressure gauge needle shouldn't bounce around.
John

 

[julias dad]

Thanks for that John.