Two Radiators on when Hot Water on but Heating off

budgetflyer

That is why a correctly fitted system will have at least one radiator without trv's.
hope that helps

Correct

Not realy true,as most systems should have a bypass fitted,either auto or manual.

Not quite right plumb 1.

If TRV's are fitted then it is a BS that at least one radiator is left to act as a bypass and should be normally at least 10% of system output. This room should have thermostatic control via a room thermostat

Regarding the 2 rads heating up, it could be a "passing" CH valve or as mentioned earlier the HW return piped up wrong.Another thing it could be is the pump set to high in which case if the rads are close by then the heat could just creap to these rads.

Regarding the system that only works on HW.
In this case if a "S" (2 valve system) I suspect the CH motor has packed in, the valve been opened and locked in open therefor boiler and pump only work when hw switched on. Get another price for replacing this should only be £100 or so if the "zone valve" is indeed the problem

plumb1_2

Correct


Not quite right plumb  1.

If TRV's are fitted then it is a BS that at least one radiator is left to act as a bypass and should be normally at least 10% of system output. This room should have thermastatic control via a room thermostat
quote;

In all my years in the plumbing trade I have never seen a system,with one Radiator acting as a bypass/heat sink controlled by a room stat.

MarkyMarkD

In our former house, the bathroom rad didn't have a TRV and acted as the bypass.

In our current house, all the rads have TRVs and there is some strange piping in the airing cupboard - is this the bypass? And is this a BS which matters or not? Everything seems fine since we had the TRVs installed over 5 years ago and the plumber never even mentioned any BS about one rad acting as a bypass.