Part L Explained – Heat Loss Calculations - Viva Training Centre

One of the major changes to Part L is the way in which heating engineers now have to do heat loss calculations, focussing on each individual room rather than the house as a whole.

Heat loss calculations – what’s changed and why?

Gas and oil boiler installers are used to doing heat loss calculations based on the type of house (three-bed semi, for example) and the number of radiators, often leading to oversized systems to cover the lack of accuracy.

Under Part L, heating systems must be as close to meeting the specific needs of an application as possible, with a very slim margin of error. To achieve this, heat loss calculations must be applied on a room by room basis, taking into account a number of factors in order to get a more accurate picture, including:

  • Building fabric – walls, floors, roof insulation, windows and doors
  • Room size
  • Air change rates
  • Expected comfort temperatures
  • Lowest temperature (on average) during winter

Systems specified in this way will be more energy efficient, cost less to run and ensure end-users are comfortable in every room.

How do you calculate heat loss?

The heat loss calculations focus on fabric and ventilation losses. For fabric losses you need to consider the following:

  • *U value = thermal conductivity of the fabric.
  • A = area of the room (m²)
  • ∆T = temperature difference between inside and outside

To calculate the fabric loss, these three factors should be multiplied together. (U x A x ∆T).

For ventilation losses, the following elements must be multiplied together:

  • V = volume of the room or building (m³)
  • N = air change rate per hour, dependent on the room type and age of the property
  • ∆T – temperature difference between inside and outside
  • 33 – the factor for the specific heat and density of air

*U Values Explained

A U-value is used to measure how well (or how badly) a component transmits heat from the inside to the outside. The slower it is for heat to transfer through a wall, door, window etc., the lower the U-value – which is what we want.

The U value is based on the rate of energy lost through each square metre of material broken down as: The rate of heat flow (in Watts) through 1m² of a structure when there is a temperature difference across the structure of 1 degree (K or ˚C).

Heat loss calculation support

The new heat loss calculations might seem daunting, but there is help at hand. Many of the boiler and heat pump manufacturers provide heat loss calculation tools, designed to take some of the headache away from installers.

Daikin, for example, which we partner with at our Sustainable Homes Centre, have a number of tools for installers. These nclude the ‘Heating Solutions Navigator’, which brings together everything required to complete the design and selection of a heat pump system, including heat loss calculations, product specifications and schematics, energy labels and sound pressure information.

Their ‘Althermo Selection Software’ is a web-based tool will help you to choose the correct capacity heat pump to provide 100% capacity at the MCS Design Temperature.

Click here to view Daikin’s full range of installer support software.

Want to train as a heat pump installer? View our BPEC Air Source Heat Pump Course.


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