is it cheaper to reduce rads or boiler temp

[D_Hailsham]

the 21 figure is within the hour not a max temp with minus 1 outside,

What you are saying goes against everything I have read so far on heat loss calculations and radiator sizing. But if you can post a link justifying your position, I will read it.

 

[gas112]

Dhailsham central heating is not designed to only reach 21 degrees it is designed to reach it within an hour .
If its minus one outside and there is no heat on the fabric of the building will be a very low temp as well.
Heating comes on and the room temp gets to 21 within the hour and guess what by some miracle involving U values of the material used to construct the building the actual fabric of the building heats up considerably as well and the longer its on the more heat it absorbs masking the outside temp, so the correctly sized heating can then heat the house far higher than 21 degrees, its not rocket science.

 

[ajrobb]

Yes, you will make significant savings by turning off radiators and keeping doors shut. Turn up boiler temperature to cope with moderate heat loss to unheated rooms. However, cold rooms should be better ventilated to prevent condensation.

I have thought about running large radiators at low temperature but I don't think it is worth it:
//www.diynot.com/forums/viewtopic.php?t=252269

If your heating has to cope with -17°C, I'd expect the RH inside to drop very low (about 20%). This is typical in Sweden, where my sister lives, and those temperatures are common.

If you have any poorly lagged pipes running outside the heated envelope, a room thermostat will be more efficient than only TRVs as the pipes will be hot for less of the time.

I also live in a dormer bungalow and the heat lost through uninsulated skeilings is horrendous. You might consider fitting timer TRVs upstairs to set back bedroom temperatures during the day.

 

[D_Hailsham]

Dhailsham central heating is not designed to only reach 21 degrees it is designed to reach it within an hour .
If its minus one outside and there is no heat on the fabric of the building will be a very low temp as well.
Heating comes on and the room temp gets to 21 within the hour and guess what by some miracle involving U values of the material used to construct the building the actual fabric of the building heats up considerably as well and the longer its on the more heat it absorbs masking the outside temp, so the correctly sized heating can then heat the house far higher than 21 degrees, its not rocket science.

This goes against everything I have learnt about heat loss and sizing heating systems.

Can you point me to a link or a book which justifies your position.

In any case U-values have nothing to do with the heat absorbed by a material, but are a measure of the heat passing through the material.

 

[Ric2013]

OP: Try it and read your meters is all I can say. You seem to understand how to do so, so might as well run the experiment.

There are many variables and sometimes you don't get the results you would expect to get.

In one house I lived in, I used slightly more gas if I ran the heating 24h, as I would have expected. In another house, I ran the heating 24h and the gas consumption fell by a fair bit.