I've always had the boiler on a high setting, and the radiators on a low setting, thinking this was the most energy-efficient way of using it. However, the Father-In-Law, who believes that he Knows Everything, says that the boiler should be low, but the radiators on full blast. Anyone got any ideas?

My guess is that it makes no difference in a well insulated home. You're using gas to heat water and that water will then release it's heat into the house. Presumably your arguments are that your individual methods waste less heat, but as any leaked heat from boiler/pipes stays inside the house, it's not really wasted. Unless of course your boiler lives at the bottom of the garden 🙂

If you have TRV's (thermostatic rad valves) then you set them to the temp you want the room at and they shut off themselves, standard rad valves are just on or off (not the balancing end ones). If you have TRV's then run the boiler as low as you can else you're just pumping over hot water round the ring when the rad valves shut off.

Thats the way I see it! If you can follow that 😆

I always thought having the boiler on full and thermostatic valves on each radiator was the most efficient thing to do. Some rooms like the kids bedrooms can be turned off completely then while the important rooms like the one I work in can be up full.

lol

It takes a fair bit more energy to keep your boiler hot than it does to keep it just over room temp, so basically the most effiecient way is to run it as low as possible. Now obviously youll lose heat in the system so to get the room furthest from the boiler (in the hot water loop) to a comfortable temp youll have to run the boiler a little higher than what you actually want.

…and when its proper cold outside, is it best to keep the heating on for longer on a lower temp overall, or have it come on – say – once in the morning and once in the evening? Mine's a modern condensing boiler, but I suspect its a piece of shit.

Never been able to figure that out definitively – experimenting last winter didn't really show one method as better than the other (ie using less gas).

…and when its proper cold outside, is it best to keep the heating on for longer on a lower temp overall, or have it come on – say – once in the morning and once in the evening?

Depends how much heat your house loses over the day. If your house is very well insulated it wont take much energy to maintain a sensible temperature (and by sensible i mean a happy man temperature not happy woman temperature, but thats a different argument ;0). However if your house wastes a lot of heat then it could take more energy to maintain heat for 24hrs than it would to heat it up from cold twice a day for only a few hours (suppose it depends how long your in the house with the heating on for that one).

IMO it's best to have the heating on when you're in and off when you're out. Takes less energy to heat the house back up to the required temperature than to maintain that temperature. Think of a kettle – you boil water when you need it. You don't keep the kettle boiling for a few hours in between.

Not sure about the boiler temp, might depend on the hot water/central heating system. My boiler provides both hot water and heating. I have the boiler set to high, hot water tank thermostats high enough to just get the amount of hot water I need, TRV's on all radiators except lounge where the room thermostat (16 degrees) is. Timer set for 1 hour in the morning and 6 hours in the evening.

No figures to back this up but my theory is. With the boiler set to high then the hot water cyclinder heats up quicker and then the radiators heat up quicker than if the boiler was set to low. Therefore the boiler is on for a shorter period of time. In winter, the boiler will kick in when the timer turns on, heats up the water/house and then may come on late evening if it's very cold or if I run hot water off.

But, maybe there is a limit as to how high I should run the boiler? But the reality is to save money you should run the house at a "man's temperature" rather than "tropical hot house" 😯

Just reread the original post – radiators on full blast ??? TRV set to turn off when the room temperature is reached. Or if you only have a room thermostat then you have to balance all the radiators (ie limit flow thru them) so that when the room thermostat turns off each of the rooms is at the required temperature.