System pressure loss sigh

joe plumber · Sep 18, 2012

Hi I know this subject has been covered a few times over. But i would appreciate your thoughts. I have been alled to a fairly big house which runs underfloor heating and a 300l unvented cylinder. It's basically a primary loop which does the cylinder and the the manifolds one upstairs and 2 down. The system has recently been upgraded from a thermal store with a dedicated uf loop and ancient uf heating system. To the system I describe above. The customer is complaining every 5 days or so the system loses pressure. The prv has been changed and there is no sign of any discharge. All the primary pipework is either exposed or runningin ceilings so in theory should the leak be on the pipework it would show. This in my estimation leaves 3 options 1. There's a leak on one of the downstairs uf circuits. However they are Brand new installs and I havebeen told pressure tested at the time. 2. Theres a split in the coil on the cylinder again the cylinder is barely a year old Ithink it's unlikely. 3. Cracked heat exchanger on the boiler. It's a gloworm heat only condensing model I would say only a year or two old. Could anyone tell me if they think I'm missing ssomething obvious.

Thanks. Joe

Masood · Sep 19, 2012

I'm sceptical of pressure testing, personally. I've had a couple of instances where I've done a full pressure test (1.5 x system working pressure for an hour) then subsequently found a slow leak or a weep.I think pressurising the pipework causes any slight imperfections in the joint to seal by expansion, and under the lower pressure of normal usage, that seal is lost.

GasmanxxxR1 · Sep 19, 2012

Could it have anything to do with the expansion vessel, is it large enough to cope with the system?

joe plumber · Sep 19, 2012

GasmanxxxR1 said:
Could it have anything to do with the expansion vessel, is it large enough to cope with the system?

I did think this myself. Would that not mean the prv would let by all the time?

joe plumber · Sep 19, 2012

Also If it makes any odds the vessels are in the loft and the prv is on the ground floor with the boiler.

e/bourneplumber · Sep 19, 2012

on a big system i would usually add an external vessel on the system some where to cope with the large volume of water and to stop the pressure gauge from fluctuating too much, you can get them different sizes but a 12 or 18 litre in conjunction with the vessel already in the boiler is usually about right.

joe plumber · Sep 19, 2012

Thanks for that. There's no vessel in the boiler as its hey only. There's two vessels coupled together in the loft I can't remember the size I have a feeling they may be 10 or 12l sso maybe the vessel is undersized? But surely this would mean the prv would let by? The main pump is on the return and then there is a smaller bronze pump. In the middle o the house. Any thoughts? It's all coming out now 🙂

joe plumber · Sep 19, 2012

It's a very big house

GasmanxxxR1 · Sep 19, 2012

If the pressure vessel is faulty or under sized it doesn't always mean the prv will be going off

joe plumber · Sep 19, 2012

Ok so where does the pressure go? Or is it not there in the first place? My first thoughts were undersized vessel as the whole place runs on uf heating and it's a big old mansion so I figure a big volume of water.