Calculating heat loss in a building. A guide to what you need to know before selecting the appropriate heating system.

Written By

Martin Arden

10 June 2020

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Calculating heat loss in a building is often a challenging task and should always be calculated by a professional. Calculating heat loss in a building correctly will result in ensuring you select the right amount of heaters to ensure your building maintains the desired temperature. In this article, our Midlands Area Sales Manager Martin Arden explains the process and challenges in calculating building heat loss correctly.

So how many kilowatts per hour will I need to heat my warehouse/factory?

This is a common question, and is in fact not easily answered without going into detail regarding the building structure and use.

Although there are ‘rules of thumb’ methodologies, to get accurate results for a heat loss estimation, often a site survey is required to ascertain what the expectations are of the heating system.

First of all an internal temperature requirement must be known, based on an external ambient temperature constant of -5 deg C, from this start point, a design can be built, but further information will be required to ensure the accuracy of the calculation.

What makes up a heat loss?

Steady state heat losses are made up from two components, losses through the building structure, and losses through air change caused by natural or forced air movement.

Structural heat losses are governed by the building material ‘U’ Value, which is a value of thermal transmission through a given area (1m x 1m), per degree C differential between inside and outside. The lower the ‘U’ value the better the insulation.

The diagram shows heat loss through a typical structure.

‘U’ values can be calculated but is a complex equation and not all data would always be available, fortunately this data can be found for various building materials in the CIBSE guide, but a degree of caution is required to ensure the correct figures are used.

Air change heat losses are caused by air leaks in buildings and can be determined by the quantity of air that has to be heated and the temperature difference between the incoming air and the inside design temperature.

Fortunately again the CIBSE guide comes to our rescue, and gives empirical values for natural ventilation rates for types of construction and sizes of buildings.

Care should be taken if the building has any form of mechanical extract as this can have a large impact on the calculation, and often accurate data is unavailable. Heat losses due to ventilation rates can be considerably greater than the losses from the building structure.

What other information is needed in order to calculate heat loss in a building?

So we now have some information to help build the design, we know the ‘U’ values, the ventilation rate, and a design temperature, but more is needed.

The obvious piece of missing information is the size of the building, the length, width, and average height are required, the larger the building generally the more heat is required to lift the temperature.

What heater capacity will I need?

Another factor that needs to be considered is the use of the building. Will it be operated intermittently, in which case the heater will be required to lift the temperature at full capacity from the night set back temperature, in a reasonable preheat period, or continuously, in which case once the set point has been reached, the heater will only be required to maintain the temperature.

These different uses will have an impact on the capacity of the heater chosen.

Heat Loss App or a manual Calculation?

Whether the heat loss calculation is to be generated by a manual calculation or via a heat loss app, the information above will still be needed for an accurate result. It is not always the case that all the required information is available, in this instance educated assumptions can be made to give a guide to the required capacity.

What’s next?

With all the assembled information a heater capacity can be ascertained, and a product selected to suit the site requirements. Many product options are available, and choosing the correct type is another decision which requires a degree of care and knowledge.

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