Some people assume HVAC engineers simply put in radiators according to what fits in the room or whatever they had in their van, at times this may be true, but a well-designed heating system is all reliant on good mathematics.
Factors to throw in to the equation include the size of the room, how it is constructed and the difference between the cold outside temperature and the warm indoor required temperature. We all know insulation in the walls, floor and roof can cut heating bill, this has to be factored in, as does surface area given over to doors and windows and the general ventilation (air flow) through the room.
What Units of Measurement are Involved in Calculating Central Heating?
The heat output of any radiator, fire or other heater is given as units of energy, in the the UK and Europe the metric unit of Kilowatts (KWs) is used. The imperial unit used in the US and still understood in the UK is called the British Thermal Unit (BTU).
We can easily calculate the cubic size of a room in metres or in feet. We can decide that we would like the indoor temperature to be 21 degrees centigrade when it is freezing outside.
The warmth inside a room will slowly escape through all of the surfaces to the colder outside so we do not calculate the cubic area of the room as such, but the surface area of walls floor and ceiling taking into account the type of material involved. Glass, for example, will let the warm air escape far quicker than thick brick walls.
Each construction material has a transmittance coefficient, more commonly known as the "U-value", which measures the quantity of heat that will flow from air on one side of the material to air on the other per unit area and for unit air temperature difference in time. For example, a solid brick wall 115mm thick has a metric U value of 3.93 watts/m2 degree C, in metric units. The glass of a single glazed window has a U-value of 5.7.
How do we create a formula with measurements of length and of temperature and come up with an answer as a unit of energy, in KWs. The question of how big a radiator should be seems far from simple now.
What is the Formula For Calculating Heat Loss?
The formula for calculating the heat loss from a room or a whole building is as follows:
surface area in metres squared multiplied by the U-value in watts/m2 degrees C multiplied by the temperature difference required in degrees C = energy in watts.
Note, the square metres and the temperatures cancel each other out giving a result in energy. There are, of course, a thousand watts in a Kilowatt.
Fortunately, most for most purposes there are many online calculators that can generate results without difficulty, they are already programmed with a number of assumptions and it is worth trying a few calculators and checking whether the assumptions apply to a specific room heating project.
Deborah Percy - BSc. MA. Managing Director of a heating & plumbing business. Interest in maths, science & research. Mother of 3, including twins.
Join the Conversation