One of the most common pieces of advice we see for saving energy is to turn off heating in unused rooms. Now this advice could absolutely save you energy, but it could also cost you more money.
If you have an old boiler (pre 2005) you will likely save energy turning off unused rooms, but the majority of homes now have condensing boilers or heat pumps!
Condensing boilers are much more efficient by working at lower flow temperatures. So rather than having incredibly hot radiators, and importantly incredibly hot fumes pouring outside via your flue, the heat that's leaving via that flue can be put into your radiators, and the way we achieve that is low flow temperatures.
Now if you do have a condensing boiler (that's one with a white pipe underneath) or a heat pump, you want as low a flow temperature as physically possible, as the lower you go, the higher the efficiency.
There are even controllers or modulating thermostats that do this for you on an ongoing basis (check out our must watch before choosing a smart thermostat video for which ones).
Essentially, the way you get your flow temperature down to as low as physically possible with or without a modulating thermostat is to use as much radiator surface area as possible.
A larger radiator surface area in a property means that the radiator can be less hot, but still heat the rooms to the same temperature.
This in turn means your boiler will extract more heat from the combustion process and send less heat out your boiler flue.
For the scientific version on this, check out our condensing theory article over on heatgeek.com
And by the way, heat pumps have even more efficiency gains from lower flow temperatures.
Imagine boilers and heat pumps like a car’s engine, and the flow temperatures like a rev counter. The higher the temperature, the higher the revs, the lower the efficiency.
Turning off individual radiators is like applying the breaks. The heat source will have to work hotter to put the required energy into the property!
Yes, not as much energy (or speed) is needed, but the efficiency is lower which can cancel out any savings and potentially even cost more!
For more detailed information on this check out our video (why not to zone boilers or heat pumps) where we break down all the math.
So instead of turning off radiators in unused rooms, what we would suggest is setbacks.
If you're not using a room, you still want flow through that radiator or the other radiators will have to work harder. A setback temperature is a slightly lower temperature that will still allow flow to the radiator.
We would suggest that these unused rooms can be turned down by about half a digit on the TRV (Thermostatic Radiator Valves) if you have them.
It could be less, but does depend on your internal and external wall insulation level, and how often you leave internal doors open!
Keeping unused rooms heated will matter slightly less if you have traditional on/off thermostats, as opposed to the high efficiency modulating room thermostats.
We advise that if you don't have a modulating thermostat you should still turn down your boiler flow temperature and perhaps turn it down more in autumn and spring than winter.
Just know this does depend on how you generate hot water, for more information on this specifically please watch our 11 tips to save energy video.
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