Can I turn down the central heating temperature on combi boiler if I have TRVs?

[Harry Bloomfield]

The presence or lack of TRVs on the system makes absolutely no difference. For best efficiency, your boiler thermostat should be turned down as low as it can possibly go whilst still keeping your home at a comfortable temperature

Even better, is a compensated control system, where the system itself adjusts the flow temperature, to exactly match the requirements for heat.

TRV's do not set a room temperature, they limit the temperature in a room - once the temperature in a room approaches the TRV's setting, the TRV begins to close.

 

[Johntheo5]

A few numbers might help?

Assume a rad sized for a room temp of 20C.

If the boiler flow temp is set to 75C and 100% rad output is required on a particular day then the TRV will throttle the flow to give a return of 65C which gives the required rad output. (65C will give no condensing effect)
If the flow temp is reduced to 60C and if a rad dT of 5C is the practical minimum achievable with a very high circulation flow then the return will be 55C to give a rad output of 69%, not enough. (TRV will be fully open)

If only ~ 50% rad output is required then a flow temp of 75C will result in the TRV throttling the flow to give 25C return. (25C will give almost 100% condensing effect)
If the flow temp is reduced to 60C then the TRV will throttle the flow to give a return of 40C to give 50% output. (40C will still give excellent
condensing effect but not nearly as good as by keeping the flow temp at 75C, above)

 

[Mr Chibs]

A few numbers might help?

Assume a rad sized for a room temp of 20C.

If the boiler flow temp is set to 75C and 100% rad output is required on a particular day then the TRV will throttle the flow to give a return of 65C which gives the required rad output. (65C will give no condensing effect)
If the flow temp is reduced to 60C and if a rad dT of 5C is the practical minimum achievable with a very high circulation flow then the return will be 55C to give a rad output of 69%, not enough. (TRV will be fully open)

If only ~ 50% rad output is required then a flow temp of 75C will result in the TRV throttling the flow to give 25C return. (25C will give almost 100% condensing effect)
If the flow temp is reduced to 60C then the TRV will throttle the flow to give a return of 40C to give 50% output. (40C will still give excellent
condensing effect but not nearly as good as by keeping the flow temp at 75C, above)

If I read this correctly then, are you saying turn the boiler up, but turn the TRV's down?

 

[Johntheo5]

No, the TRV is sensing/controlling the room temperature and will modulate open/closed to try and maintain that room temperature that you have set on the TRV index, they work quite reasonably well, mine keep the room(s) temp within +/- 1C of the required room temperature as long as they are not closing off fully, in which case they don't give very tight room temperature control, maybe +/- 1.5/2C.
They work by throttling and reducing the flow of water through the rad to change the mean rad temperature to give the required room temp. In the two reduced output examples above, the mean rad temperature in both cases is the same, (75+25)/2 is the same as (60+40)/2, ie 50C.
So once your heating demand goes below 50% (with 75C flow temp) then the TRV will close off fully, the rad will cool down and eventually the TRV will reopen again but are a bit sluggish operating this way.
If, for example, you only require 25% rad output then, the flow temperature should ideally be reduced to 50C, this will result in a return temperature of 25C with the TRV almost, but not quite fully shut off.
In real life, of course, different rooms require different rad outputs so, in practice, you would be doing well to have a average boiler return temperature of 35/40C once all the rooms heat up. 65C might be a reasonable boiler flow temperature. Mine is set to 70C and I get a boiler return of 38/45C.