A Guide to Heat-Losses
| Why are calculating heat-losses important? |
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It is important that the heating system is sized correctly. This can only be accurately confirmed with completing room by room heat-loss calculations. It is then possible to check the outputs of the UFH/Radiator system against the heat-losses to show any shortfalls. If the heating system isn’t sized correctly it can lead to inefficiencies, which is especially important when dealing with low temperature systems. |
| What affects the heat-losses within a building? |
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U Values – This is the measure of heat-loss. It is expressed in W/m2K, and shows the amount of heat lost in watts (W) per square metre of material (for example wall, roof, floor etc.) when the temperature (K) outside is at least one degree lower. The lower the U value, the better the insulation provided by the material. We can calculate the U value for the construction element using the thermal conductivity and the thickness of each material. Air tightness – Air infiltration and air change is the biggest heat-loss within the building. The more air tight a building is the less these factors contribute to the heat-loss. It is still necessary to have air movement and air changes within the building to provide fresh air, remove contaminants and prevent moisture buildup. If MVHR is used then we can apply a 15% reduction of the heat-losses due to intermittent heat no longer being a factor. Building form – The shape of the building affects its heat-loss, a compact square building with the same U values and air tightness will have a lower heat-loss then a longer narrow building which will have a larger proportion of exposed construction elements. Elevation – The altitude of the building affects the heat-loss; for every 100m above sea level, the external temperature reduces by – 0.60C. Site exposure – If properties are close to the coast, or in an elevated exposed position it is necessary to add 10% to the heat-loss. |
| Peak Load |
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The heating demand (peak load) is based on fabric and ventilation losses under the design conditions specific to the project. This is used to size the boiler or heat pump system. |
| Design Heat Load |
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The design heating load is the adjusted peak load, this takes into account the passive gains, and can include solar/ occupancy/ equipment. This is used to size the loads in and low energy buildings where gains are more likely to influence over heating potential. |
| Can Standard Assessment Procedure (SAP) Help? |
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SAP can be used to give an indication of the design heat load. SAP provides the overall fabric losses, not room by room losses. If you refer to line 39 on the SAP report the heat-loss coefficient W/K, it is then possible to use this figure to give an indication of the the peak load. Our calculations meet the requirements laid out by CIBSE in EN12831 and MCS in MIS3005. |
| What do we need to complete heat-loss calculations? |
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