On a 20 year old conventional wet central heating system (oil fired boiler) all the rads except one have a TRV on them......that rad is always fully on. There is no wall or room thermostat, because the TRVs do the job fine. There is no bypass valve in the system. There is a thermostat in the boiler itself. The pumped system is done by a Grundfos Alpha Pump. Motorised valve means either central heating or hot water or both.....system programmer is a Honeywell unit. Is the rad without the TRV essential or can it have a TRV fitted, so all rads have TRVs? Thanks.....if anyone knows, because two plumbers I have talked to have different opinions.
Whilst not a “pro” my understanding when I installed a full replacement central heating system at home, the 1 permanently open radiator was a “mandatory” part of the installation. Andy
A quick google says not if there is no bypass valve in the system. I always leave the bathroom rad/towel rail without a TRV.
Essentially you can shut down and close all rads while heating is on. Due to the ring main type of system, it will never be blocked for flow. Another trv would be fine
You always have one rad that has no valve. When the pump is pumping (when boiler is going) the water needs to go somwhere. It goes to the non-valved rad and then comes back to the boiler. So the non-valved rad will always be warm when the heating is on.
You MUST have one radiator without a TRV or a Bypass, just look at the diagram above and ignore the T to the coil, as yours is via a motorised valve. If all the TRV's are shut you get no flow. The radiators are "wired in" parallel, so water can flow when the TRV is open. If the system was done as a ring the water wouldn't flow through the radiators properly as they effectively present resistance.... the water will flow through the easiest path. The bypass effectively allows a ring to be made ONLY IF all the radiators TRV's close. Your system (@Arquebus ) would normally have the main bathroom towel radiator as the manual bypass. If you were to shut it and all the TRV's were closed you'd hear the pump dead heading, also the boiler overpressure/temperature may operate to prevent a steam explosion (depending on the position of header tank connection). You may have other safety devices around the boiler, but the idea is that should all the options to reject heat absorbed into the water are closed, at least one radiator would be available to allow heat rejection. One of your plumbers knows his stuff and the other needs to have his number deleted out of your phone. My facepalm for the diagram is that we may have assumed what Al meant by conventional...
This all depends on the design of your system - but there are a myriad different heating systems out there so don’t make assumptions. Edit: What @AirCon said but beat me to it with a better answer!
That is if you have a hot water cylinder that is not controlled. Otherwise, if all of the TRVs are shut and the hot water cylines is closed, then the hot water has no path.
Fyi. Unless you know the system intimately. Ideally you want a bypass. If you cant fit a bypass put locksheilds on both ends of bathroom rads and dont ever close them. Ideally you also want a thermostat in the system to stop your boiler running more than necessary. As mentioned. These measures should stop overheating and lock outs.
And now I'm all confused again........... I don't have the 'open' rad in the bathroom, because it is too close to the pump and boiler......it would take a lot of heat away from the rest of the system, plus there is another rad in the bathroom which is a heat bleed for another appliance which isn't on the same circuit as the main one. (Got a twin coil cylinder). My house is about 30 metres long, so the 'open' rad is the one which is furthest away.....and at the North / coldest end of the house. Because it is the 'open' rad it also helps an adjoining room keep warm as the pipes in that room are exposed at low level and are roughly 9 metres long. Unfortunately owing to the nature of the house 22mm pipes run in the roof space for about 8 metres roughly halfway along the system. Although they are well insulated, because the 'open' rad is eventually being fed by those pipes, I was thinking of controlling the rad with a TRV just to reduce some of the potential heat loss. But it seems as though I shouldn't fit one. Thanks for all the thoughts and comments. PS...I think that the water doesn't always flow with the TRVs shut (if I fitted one to the open rad) because I can hear the Alpha pump adjusting itself as the TRVs are closing.
