The old fashioned way of controlling central heating, by having it timed to come on when needed and turn off when not, is often described as inefficient when it comes to modern heating systems, especially heat pumps.
This article is an analysis of the physics behind the issue, as we feel this is a typical example of oversimplifying a problem and misleading.
Heat Loss Analysis
A property looses heat at a rate proportional to the temperature difference between the internal walls and the external walls.
As such, the losses of a property are completely linked to the internal temperature, so its a simple fact that heat loss will be lower if the internal temperatures drop for a period. In other words, if you let the inside go cold, there is no more heat loss.
Heat Input Analysis
Heat is transferred into rooms via radiators or underfloor heating. The radiator(or floor) needs to be hotter than the air, so that heat energy can move from the hotter body (the radiator) towards the cooler body (air). The rate of heat transfer is again directly proportional to the temperature difference, as well as the surface area. As radiators are fixed size, the rate of transfer is linked entirely to the radiator temperatures.
Thermal Mass
The property will have a certain thermal mass that needs to be heated up each time the heating comes on. Cold walls will absorb heat at startup, and only once the property has warmed up will the heat input stabilise, and equal the losses from the property.
This alone is sometimes used as a reason why its less efficient to time heating, but its not strictly the case. The amount of energy required to bring the property up to temperature will depend on the construction of the building, so it is too simple to call one way or the other without first examining the actual thermal mass, working out the energy absorbed, and comparing this to the savings that are to be gained by turning the heating off completely.
Efficiency of Heat Generation
The other issue that affects the overall efficiency is how the efficiency of the heat generator (gas boiler or heat pump for example) varies with what it is trying to achieve.
If we need to heat a property up from cold then because we need to put more heat into the rooms at the start, as described above, the temperature of heat emitters will need to be higher in order to achieve target temperatures within a reasonable time (again a loose definition), and this is where a greater inefficiency may lie. Certainly with heat pumps, the higher the target temperature, the lower the efficiency, so the initial warm-up period will result in less efficient operation of the heat source during startup.
By leaving heating on all the time, one never needs to switch into a higher temperature mode to drive the start-up, and the heat pump would be operating at optimum efficiency all the time.
There is also a need to consider the availability of alternative heat sources. Where a heat pump is combined with a gas boiler for example, it may be more efficient to use the boiler at startup, falling back to the heat pump once over a certain room temperature.
Summary
So what's the answer ? Well, as said at the beginning don't expect a black or white response.
Its a mathematical calculation that decides the length of time over which it is more efficient to simply leave a heating system on, and when to let it turn off.
Feeding into the equation is:
- The thermal mass of the property
- The size of heat emitters
- The target room temperature
- The external air temperature
- The efficiency curve of the heat generator
- The minimum accepted heat up time and whether optimum start functionality is implemented
- The availability of alternative heat sources
What will come out is a length of time where it becomes more efficient to turn off.
Given central heating loads are fairly predictable - we know the weather in advance as well as the use patterns - a decent controls system should be able to make the decisions automatically.
Data
Only real world data can take us further. As part of our analysis we intend to connect a heat pump to a real world system, and monitor the efficiency of it through an array of control strategies. Our hope is to be able to arrive at some software routines we can incorporate into our IHIU controls system to control a typical heat pump to the best possible efficiency, deciding itself when to turn off and when not to, and at what temperature to run the heat pump.