Central heating problem

[fine_feather]

The central heating at my church is run by two Potterton boilers controlled by one Potterton timer. One boiler for each side of the church.
The problem is that although both boilers come on (most of the time) one boiler sometimes doesn't come on and sometimes when it does, it goes off after a while, leaving the church getting colder. The other boiler comes on every time and stays on. A plumber has so far, fitted new expansion vessels to both boilers, replaced the cartridge in the pump of the unreliable boiler and now, after consulting Potterton has been told to replace the thermostat on the unreliable boiler. All radiators in the church have thermostatic valves. After months of this problem we are all at the end of our tether with this so please if anyone has any suggestions let us know.
Alan

 

[tamz]

Could be a thermostat problem but without seeing it i can't give a definitive answer i'm afraid.

Good luck in getting to the root of it.

 

[Rox]

Could be a controls problem. Presume there's a room thermostat associated with each boiler to provide boiler shutdown (a boiler interlock) when there's no call for heat (i.e. all the thermostatic radiator valves are closed) - could one be faulty or wired incorrectly?
Another thing is, if you have two boiler/heating systems that aren't linked, could system one be shutting down because heat from system two is being detected by set one's room thermostat, which thinks "I'm warm enough" and so keeps set one boiler switched off.

 

[fine_feather]

Could be a controls problem. Presume there's a room thermostat associated with each boiler to provide boiler shutdown (a boiler interlock) when there's no call for heat (i.e. all the thermostatic radiator valves are closed) - could one be faulty or wired incorrectly?
Another thing is, if you have two boiler/heating systems that aren't linked, could system one be shutting down because heat from system two is being detected by set one's room thermostat, which thinks "I'm warm enough" and so keeps set one boiler switched off.

Wow that's interesting and too technical for me. Ther are no room stats only rad stats if that helps.
I'll pass it on to the central heating plumber to see what he thinks. Many thanks