pressure relief valve on gas boiler

[jfb]

i'm going to be getting a couple of plumbers to come and have a look at my boiler and i'd like to be a bit better informed when i talk to them!

water comes continually out of the pressure relief valve at the moment if I let any water into the boiler which is a Vailant Ecomax 828 converted to lpg (with UFH downstairs and rads upstairs).

i imagine that one would replace the pressure relief valve and/or expansion tank to see if that works. Can one try the pressure relief valve first (its cheaper) to see if that works and if not then change the expansion tank - or might that damage the new pressure relief valve?

pressure had been high on the boiler for a while and I had sort of ignored it - am i right in thinking bleeding the rads is important when trying to reduce pressure?

cheers john

[PeterW]

If it continually leaks then check the expansion vessel as that could have no pressure in it. 

[Nickfromwales]

The most common cause of that over pressurisation is when the filling loop has not been 100% fully shut and the system pressure creeps up and starts letting water out of the PRV. 

The only reason water will continuously discharge is if it's continuously being fed into the heating loop, other than that the water will come out a bit, then the boiler will shut off. 

If the expansion vessel had lost its back pressure ( pre-charge ) it can be topped up with a common bike pump. The PRV is probably now knackered so I'd change that first and check the pre-charge at the same time. 

[Temp]

+1

 

Can one try the pressure relief valve first (its cheaper) to see if that works and if not then change the expansion tank - or might that damage the new pressure relief valve?

 

As Nick said, you probably will need a new PRV but I wouldn't do it in that order. I wouldn't replace the PRV until after the bits have been checked and replaced if necessary. That's because for some reason PRV are prone to failing after they have been opened once or twice. I would start by checking the fill valves and the expansion vessel. If either are faulty replace those then change the PRV.   

[jfb]

thanks for the replies-  very helpful. I also managed to get through to valiant installer helpline and they were good.

I managed to get the expansion tank up to 0.75 bar pre charge and it seems to hold it and no water coming out so I am guessing that it is fine.

I think that i may indeed have had the filling loop open a bit that would explain the high pressure. so I have ordered a new PRV as well as the flexible metal hose that goes with it in case that has scaled up.