Is it cheaper to run your heating continuously or not? How does insulation change that?

The level of insulation can make a big difference to the “difference” in energy usage between running a system 24hours a day vs just when needed and it can certainly reduce the energy usage per hour of maintaining a particular temperature. It does not, however, change the fact that a building at a temperature above ambient will always be losing some energy to its surroundings and therefore some energy must be introduced to maintain that above ambient temperature.

When a building’s internal temperature is the same as it’s external temperature no energy is being wasted and therefore every hour that the building sits cold (and heat is not required) it is performing at 100% efficiency. When the building’s internal temperature is higher than it’s external temperature some energy will be being lost and the level of insulation only affects the rate at which that energy is lost. The point here is that a building’s efficiency only drops when there is some higher than ambient internal temperature, the rest of the time it’s performing at the maximum possible efficiency.

The more insulation a building has the less time the building spends with its internal temperature being the same as its external temperature. Therefore the difference in cost between running the heating system continuously or as needed decreases as insulation increases.

A building which is very well insulated and cools back towards ambient temperature so slowly that it doesn’t reach the ambient temperature in the time when heat is not required therefore is continuously losing energy at a constant albeit low rate. Therefore the heating system will be no more or less efficient if it is set to be on continuously or only as needed because the building never cools to ambient and so it always has some energy to lose every hour. It would not be possible for continuous operation to become cheaper than only heating when required because that would imply that you are actually creating energy, and as we know it is not possible to create energy.

It may certainly be more convenient to always have the heating turned on and I’ll acknowledge that with some really good insulation the financial cost of running your heating continuously may be very low especially if you have solar panels or some other “free to run” energy gathering systems but it still wouldn’t be cheaper (though it also wouldn’t be any more expensive) than running your heating on demand.

Here are three examples using some hypothetical numbers:

Assumptions:
1. The exterior temperature is 10C (degrees centigrade)
2. A comfortable interior temperature is 15C – 20C
3. The thermostat turns on the heating system when the temperature falls below 15C and turns it off when the temperature reaches 20C
4. We’ll assume no delay in heat being produced and that heat reaching the thermostat (i.e. no overshoot)
5. Numbers may be somewhat unrealistic in order to make the mathematics easier.
6. The temperature drop (energy loss) is linear over time.
7. The heating system increases the temperature by 1C for each kWh used.
8. The occupants get up at 7 am and leave the building at 8 am.
9. The occupants return to the building at 6 pm and go to bed at 10 pm.
10. The occupants only require the building to be at a comfortable temperature when they are awake and in the building.
11. A poorly insulated building’s internal temperature drops by 5C per hour
12. A medium insulated building’s internal temperature drops by 1C per hour
12. A well-insulated building’s internal temperature drops by 0.5C per hour
13. Energy (kWh) can come from multiple sources e.g. gas boiler, solar panel, sun trap, etc and some of these energy sources are cheaper than others.

 

If the system put on a time clock to be active only during hours of occupancy in a poorly insulated building:

At 7 AM the temperature of the building is 10C and the system uses 10kWh to bring the temperature to 20C
At 8 AM the building has dropped to 15C and the system is turned off
At 9 AM the building dropped back to 10C

At 6 PM the temperature of the building is 10C and the system uses 10kWh to bring the temperature to 20C
At 7 PM the temperature has dropped to 15C and the system uses 5kWh to bring the temperature back to 20C
At 8 PM the temperature has dropped to 15C and the system uses 5kWh to bring the temperature back to 20C
At 9 PM the temperature has dropped to 15C and the system uses 5kWh to bring the temperature back to 20C
At 10 PM the building has dropped to 15C and the system is turned off
At 11 PM the building has dropped back to 10C

Over a 24 hour period, the system would use 35kWh to maintain the temperature

 

If the system put on a time clock to be active only during hours of occupancy in a medium insulated building:

At 7 AM the temperature of the building is 10C and the system uses 10kWh to bring the temperature to 20C
At 8 AM the building has dropped to 19C and the system is turned off
At 5 PM the building dropped back to 10C

At 6 PM the temperature of the building is 10C and the system uses 10kWh to bring the temperature to 20C
At 10 PM the building has dropped to 16C and the system is turned off
At 4 AM the building has dropped back to 10C

Over a 24 hour period, the system would use 20kWh to maintain the temperature

 

If the system put on a time clock to be active only during hours of occupancy in a well-insulated building:

At 7 AM the temperature of the building is 13.5C and the system uses 6.5kWh to bring the temperature to 20C
At 8 AM the building has dropped to 19.5C and the system is turned off

At 6 PM the temperature of the building is 14.5C and the system uses 5.5kWh to bring the temperature to 20C
At 10 PM the building has dropped to 18C and the system is turned off

Over a 24 hour period, the system would use 12kWh to maintain the temperature

 

If the system is active 24 hours in a poorly insulated building:

At the beginning of each hour, the system will use 5kWh to bring the temperature from 15C to 20C and over the course of the hour, the building will drop back to 15C. On average each hour the system will use 5kWh to maintain a comfortable temperature.

Over a 24 hour period, the system would use 120kWh to maintain the temperature

 

If the system is active 24 hours in a medium insulated building:

Every 5 hours the system will use 5kWh to bring the temperature from 15C to 20C and over the course of 5 hours the temperature will drop back down to 15C. On average each hour the system will use 1kWh to maintain a comfortable temperature.

Over a 24 hour period, the system would use 24kWh to maintain the temperature

 

If the system is active 24 hours in a well-insulated building:

Every 10 hours the system will use 5kWh to bring the temperature from 15C to 20C and over the course of 10 hours the temperature will drop back down to 15C. On average each hour the system will use 0.5kWh to maintain a comfortable temperature.

Over a 24 hour period, the system would use 12kWh to maintain the temperature

 

Now I’ll agree that these examples are somewhat simplistic but the principle still holds.