Setting up your central heating so it performs to the best of its ability is a skill in itself. We asked one of Dublin’s best local plumber for their best tips with regards to this.
So far we have looked at heat losses with a steady indoor temperature, maintained by continuous heating, and an outside temperature of – 1 °C. If we turn the heating off for any length of time, obviously the air temperature will fall but so also will the temperature of the building itself. When the heating is switched on again it must not only provide heat for the steady-state losses but also raise the temperature of the structure.
Fortunately, when the inside temperature is low, the heat losses through the structure are lower because they are proportional to the difference in temperature. There is some spare heat therefore to raise the temperature of the structure but it is going to take some time.
The behaviour of buildings
The longer a structure has been unheated, the longer will it take to get up to temperature. The time taken and the energy required for reheating depends on the duration of the unheated periods and on the mass of the structure and its specific heat capacity.
I referred to specific heat capacity in connection with air-change losses. Generally, heavy dense materials have higher specific heat values, which means that they need more heat input to lift their temperature. Because of this the idea of ‘weight’ is often applied to buildings.
If one building is constructed of lightweight materials such as timber walls with insulation between panels to give a Building Regulation U-value of 0.6 W/m2oC, and another is constructed of solid sandstone, the sandstone wall would be nearly 2 m thick for the same U-value.
It is often thought that it doesn’t matter what materials are used as long as the U-value is the same. However, if the heating were to go off in, say, a hospital, it could be very serious and immediate steps would be taken to rectify the fault. While doing so, a lightweight building would cool so fast that soon the temperature inside could be close to that outside. A heavy stone building, on the other hand, would give out its stored heat over a longer period. It is obviously a good thing to create the required U-value of a continuously-heated building such as a hospital using heavy materials.
If a building is used only occasionally, say once a week, a massive stone structure would take so long to heat up that it might be necessary to switch the heating on three days before to get up to working temperature. A lightweight building which can warm up in an hour would be more suitable. You can see that the structure of a building should be related to its heating pattern.
You may think that this is not applicable to the domestic scene but there are many different materials now being used for housing, and the placing of insulation can also affect the ‘weight’ of a building, as will be explained in the next chapter on insulation.
Because of the extra heat required after each ‘off’ period, the running costs are not directly proportional to running time. If you heat your house for only eight hours per day and your neighbour heats his for 16 hours your bill will be more than half his; because of this it is some- times recommended that it is more economical to run continuously. However, the economics depend on each individual case; on the pattern of use, the type of heating and the response of the particular house.