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How to do a heat loss calculation for an oil boiler installation
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How to do a heat loss calculation for an oil boiler installation

The changes to Part L Building Regulations, effective from 15th June 2022, stipulate that a full room by room heat loss calculation must be carried out prior to installing a boiler. Grant UK has developed an Oil Heat Loss Calculator and in this blog, Steve Ellison, Grant UK’s Assistant Training Manager, shares a step-by-step guide to help installers understand how to use this calculator.

Effective from 15th June 2022, heating engineers who are installing a new boiler (either gas or oil) will be required to carry out a full room by room heat loss calculation prior to installation. The purpose of this change is to encourage the installation of correctly sized heating systems as the new Part L Building Regulations state that systems should not be significantly oversized. The additional step of completing a heat loss calculation as part of a boiler replacement is a major change so to help installers with this process, I will outline the key steps involved in the heat loss calculation process and explain how to use Grant UK’s Oil Boiler Heat Loss Calculator.

What is a heat loss calculation?
While this may feel like we are ‘going back to basics’, it is important to understand the purpose of a heat loss calculation. By accurately calculating the heat loss of each room within a property, installers can determine the required heat input that the building needs. This in turn helps to identify the correct size of heat generator needed to fulfil the property’s heat demand, whether that be a heat pump or a boiler.

A room-by-room calculation will determine the required heat input for each room, factoring in the use of existing and new heat emitters, and ensuring that each room is heated to its design temperature. The calculation will determine both the fabric losses and ventilation losses based on the difference between the indoor and outdoor temperatures. The greater the difference between these two temperatures, the greater the heat losses.

Fabric Losses
The rate of fabric heat loss is dependent on the following factors:

  • U – the Thermal conductivity of the building element or ‘U-value’ (W/M²/°K)
  • A – the Area of the building element (m²)
  • ∆T – the Temperature difference across the building element: ∆T = Inside temp – Outside temp (ºC)

The Fabric Loss (Watts) is calculated by multiplying these three factors together (U x A x ∆T).

Ventilation Losses
The rate of ventilation heat loss is dependent on the following four factors:

  • V – the Volume of the room or building (m³)
  • N – the Air change rate (air changes per hour)
  • ∆T – the Temperature difference between inside and outside building: where ∆T = Inside temp – Outside temp (ºC)
  • 33 – the factor for the specific heat and density of air

The Ventilation Loss (Watts) is calculated by multiplying these four factors together V x N x ∆T  x 0.33.

The air change rate (N) varies depending on the room type and the age of the property.

Grant UK’s Oil Boiler Heat Loss Calculator
The Oil Boiler Heat Loss Calculator that Grant UK has developed is designed to be a quick means of calculating the heat loss for a property. It allows the installer to select a suitable size of Grant oil-fired boiler to meet this heat demand.

To access this calculator, G1 Installers need to visit the G1 Portal and head to the ‘Downloads’ tab. They can then follow these steps to use the calculator tool – in summary, any cells highlighted in yellow require a value and the cells which are white (not highlighted) are locked because they will fill automatically based on the data entered elsewhere:

Tab 1 (Details)

  • Enter the customer’s details, site address and postcode.

Tab 2 (Heat Loss)

  • First enter the age of property and height above sea level. Select either ‘Constant’ for same temperature in all rooms, or ‘CIBSE’ for room temperatures based on the room type (from CIBSE Guide A).
  • Select the ‘Room Name’ from the drop-down menu (eg Living Room, Kitchen etc) – this will then automatically fill the values for air change rate and room temperature.
  • Enter the length and width measurements for the room in question.
  • Enter the ceiling height.
  • Confirm whether or not there is an open flue in the room.
  • The value for the ‘Outside Design Air Temperature’ will fill automatically based on the site postcode entered on the ‘Details’ tab.
  • The ventilation heat loss will now be automatically calculated for the room in question.
  • For the wall heat loss, enter the external wall length. Refer to Tab 3 (Tab 11) for the U-value based on the wall construction and enter this.
  • The window and door U-values will be automatically entered when the window type is selected from the drop-down menu.
  • The fabric heat loss through walls, windows and doors will now be automatically calculated for the room in question.
  • For the floor, the area will be automatically entered. The correct U-value will be entered when the floor type is selected from the drop-down menu. (This is only applicable for ground floor rooms only)
  • For the ceiling heat loss, enter the ceiling area. (This is only applicable for top floor rooms and single story properties.) For vaulted ceilings, Tab 4 is used to calculate the ceiling area. The correct U-value will be entered when the ceiling insulation is selected from the drop-down menu.
  • The fabric heat loss through the ceiling and floor will now be automatically calculated for the room in question.
  • Repeat this process for every room in the property.

Provided all the required data is entered on the ‘Heat Loss’ tab, the calculator will automatically work out the following:

  • Ventilation loss (of each room)
  • Fabric heat loss (of each room)
  • Total heat loss (of each room) by combining both the fabric and ventilation losses
  • Specific heat loss (of each room) by dividing the total loss by the floor area
  • Total heat loss of property (calculated by totaling the heat loss from every room)

Tab 3 (TAB 11)

  • Use this tab to select the correct U-value for the wall construction type – this value can then be inputted into the relevant cells on Tab 2 (Heat Loss).

Tab 4 (Vaulted Ceiling)

  • This tab is designed to help calculate the heat loss of a room with a vaulted ceiling. Again, the data here can be inputted into the relevant cells on Tab 2 (Heat Loss).

Tab 5 (1 HL Calculator)

  • This tab automatically provides a schedule giving the design temperature, ACH, floor area, heat loss and specific heat loss for each of the rooms in the property.

Tab 6 (DHW Calculations)

  • This tab determines the total annual hot water energy demand based on the number of persons in the property.

Tab 7 (Summary)

  • This tab provides the total heat loss for the property and the heat demand to be met by the boiler (including 3kW for domestic hot water).

To further assist installers with using this useful Oil Heat Loss Calculator, a free on-demand training module is available via the Grant eLearning Academy.

Steve Ellison
Assistant Training Manager at Grant UK