Calculating heat loss from a room

Before we can consider selecting radiators or heat emitters for a particular area it is necessary to be able to calculate the heat loss from that area. A radiator can then be selected which provides an output, (under a set of predetermined conditions), to match the heatloss from the area under the same conditions. This process can then be repeated for other areas eventually leading to a heat loss calculation for the whole building. With the addition of an allowance for domestic hot water and possibly intermittent usage a suitably sized boiler can then be selected. In order to establish the radiator output and hence size required we need to consider the following: -

• Fabric Heatloss
• Ventilation Heatloss

Fabric heatloss

Where a temperature difference occurs between the inside and outside of a property, heat will flow through the structure towards the lower of the temperatures. This heat flow (loss) will occur through walls, floors, roofs, and windows, and even between rooms of dissimilar temperatures. It is important to know how much heat is lost through each structure so that calculations can be made to heat a building to the desired temperature. The transfer of heat is calculated using 'U Values' together with the surface areas of the various structural components and the temperature difference between the two sides of the structure concerned. For ease of calculation it is assumed that heat is lost at a uniform rate through each surface.

The 'U Value' of a building element is the rate of loss of heat in watts per square meter of that element per degree centigrade temperature difference across that element. Thus the rate of loss of heat through a building element is give by: -

Complete tables of pre calculated "U" Values, are published by British Standards Institute.

Ventilation heatloss

The requirements for the provision of ventilation in dwellings detailed in the Building Control Regulations are to restrict the build up of moisture and pollutants, which would otherwise be a hazard to health. This ventilation air, flowing through the building, loses heat. Ventilation rates are usually quoted as 'air changes per hour' defined as the volume of ventilation air moving through the room per hour, divided by the volume of the room itself. The air will be heated by the heating system and the heat needed is calculated by multiplying the room volume, by the air change rate, by the temperature rise the air needs, and by the ventilation factor. Thus the rate of loss of heat through ventilation loss is given by: -

Ventilation Heat Loss

The ventilation factor is taken as the specific heat of air at 200C which is 0.33 W / m3 0C and is used to calculate the heat loss to the air changing within the rooms due to infiltration or mechanical ventilation.

Internal design temperatures and ventilation rates

The following internal design temperatures and air change rates are recommended for the design of full and part central heating systems and are based on providing the customer with acceptable comfort levels throughout. The minimum design temperature and air change rates required by BS 5449: 1990 are set out in the table below.

The air change rates above are for modern buildings. When designing for older properties, consideration should be given to increasing the air change rates to allow for ill fitting doors, windows etc

Where mechanical extraction ventilation is installed in a room it is possible for the minimum fan duty to exceed the minimum air change rate. In such cases it is advisable to allow for the increased air change in the heat loss calculation for both the room and the connecting rooms from which the air will be drawn.

Where a shower or bath is fitted into a bedroom or where an opening without doors exists between the bedroom and the en-suite facility, then the air change rate of the bedroom should be increased accordingly to allow for the movement of air caused by the extract fan.

Example : heatloss calculation

Consider a simple heatloss calculation for a typical bedroom shown in the drawing below.

NOTES :
1. Temperature of room above - 1 oC (Roof)
2. Temperature of room below + 23 oC

The total room heat loss from the room / area is given by: -
Total Room Heat Loss = Fabric Heat Loss + Ventilation Heat Loss

Heatloss calculation for the bedroom example

Notes :
1. All "U" values are take from British Standards
2. Heat Gains (#) from adjacent Warmer Spaces
3. Correction factors need to be added to the calculated

Note: Before a radiator can be selected the calculated heatloss figure needs to be adjusted to compensate for the difference between the tested radiator outputs advertised in the brochures and the actual output obtained from the radiator after considering the design criteria of the system.