Seasonal heating

To decide on the best way to heat a house, it is necessary to know how much a whole season of heating will cost. There are two new questions here. What is the average temperature difference over the whole season? What is the length of a heating season?

The temperature difference will depend upon the temperature at which you like to keep the house (what is that?) and upon the average outside temperature. In London, for example, the latter may be assumed to be 6 °C during the colder months.

In England, the heating season is taken to be 33 weeks, that is 5544 hours. 5544 is far too precise a figure for such a variable quantity as an English winter. Take 5000 or 6000 according to the district in which you live. Calculations are based upon 6000 for this chapter. A kilojoule is too small a unit for convenient use in a whole season and 1 Megajoule (MJ) is used instead.

Below there are values representing approximate heat supplied by different fuels if fully burnt or used.

Coal or coke 1 tonne produces 28000 MJ

Anthracite 1 tonne produces 34000 MJ

Gas 1 m³ produces 20 MJ

Oil 1l produces 40 MJ

Electricity 1 MJ produces 1 MJ, or

1 kWh produces 3-6 MJ

A unit of electricity is equivalent to the electrical energy used when a 1 kilowatt fire is switched on for an hour; it is sometimes called a kilowatt-hour (kWh).

In the case of an electric fire, all the heat produced goes directly into the house, but, for example, when coal is burnt, part of the heat escapes uselessly up the chimney; the amount which escapes depends upon the efficiency of the appliance. The term house efficiency (h.e.) is used for the percentage of available heat that goes directly into the house.

Figure 1.2 is a nomogram from which the heat loss in MJ on the upright scale can be converted into the heat requirements for differing house efficiencies and then directly below to the actual quantity of fuel required. For example, suppose 90000 MJ is the heat loss; following the dotted line across, for a central heating system of 70 % efficiency, 128000 MJ must be supplied; this would require 3-7 tonnes of anthracite, or 4-6 tonnes of coke, or 3200 1 of oil, or 6400 m³ of gas, or 128000 MJ of electricity.

Fig. 1.2

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