Most people, most websites and most experts have finally agreed that it’s not cheaper to leave your heating on all day long when you’re out at work. But the explanation hasn’t been forthcoming, other than a generic “due to heat loss”. So let’s give a clear reason why, with some numbers for illustration and a lot of common sense and reasoning.
We need to start by making some assumptions to simplify the situation. Firstly, the property is insulated to modern standards. The source of heat is a gas boiler (condensing or not, it doesn’t matter here) with radiators and steps have already been taken to make the house heat efficient – such as using a room thermostat, TRVs, reducing the flow temperature where possible and so on. See some tips on a heat-efficient house here.
Let’s say the house inhabitants are out for 6 hours a day and they’re contemplating whether to leave the heating on during this time at a setback temperature of 15 deg and have it go up to 20 deg when they’re back.
If they turn the heating off for those 6 hours, let’s say the temperature in the house drops from 20 deg to 10 degrees. Probably impossible to lose that much heat in a well-insulated house but we want nice, easy numbers to work with here.
We’ve come up with all the numbers we need and can start making the comparisons.
2 scenarios
With the boiler running all day long at low temperature, it’s heating the house to 15 deg for 6 hours and needs an extra burst at the end to make it reach 20 deg.
If the boiler is left off during the day, it must supply enough heat after 6 hours to raise the temperature of the house from 10 deg to 20 deg. That’s a lot more heat it needs to deliver than the 5 deg gap with the heating on all day.
However, with the heating on all day, the boiler had to provide 5 deg worth of heat in those 6 hours, otherwise the temperature would have dropped from 15 to 10 deg. So overall, even with the boiler running all day, it still had to raise the temperature by 10 degrees overall.
If fact, with the boiler running all day, it would have burnt more gas to close the same 10 deg gap due to overcoming the heat loss to keep the temperature at 15 deg for 3 hours.
Heat loss curve
Perhaps the clearest explanation yet as to why it’s more expensive to heat an empty house is because the house loses heat faster when it is warmer, even when it is well-insulated. Think about it, a hot cup of tea will cool down fast in the beginning and then stay warm and lukewarm for longer. Or if you take an old-fashioned bulb thermometer and put it in cold water, the temperature drops quickly at first and then slows down. It’s the famous heat loss curve.
So, how does that prove that it’s more expensive to use a set-back temperature? The house temperature would have dropped to 10 deg but was maintained at 15 deg. At 15 degrees, the house will lose heat faster than at 10 deg. So the boiler keeps having to pump heat into the house to maintain this temperature. With the boiler off all day, the temperature is raised only at the end when needed, so less heat loss.
Time to heat
One argument for heating the house all day on a low temperature is that it’s faster for the house to reach the 20 deg target when it starts from 15 deg as opposed to 10 deg. If it takes 1 hour to close this 10 deg gap to reach 20 deg, then the inhabitants when they come in the evening stay in the cold for some time. The easy way around this is to simply switch on the heating earlier with a programmer, or a smart thermostat may do this automatically by switching on the heating early so as to hit this target on time. Some smart thermostats also incorporate geo-fencing, which is a fancy way of saying that it has the user’s location from the app on their phone and can tell when they are close enough to home to switch on the heating.
Some people may say, oh, I have an old cottage with stone walls and it takes 4 hours to heat up my house so I may as well leave the heating on all day. Certainly but the heat loss must be enormous and the heating bill just as big.