Update on Heat Loss calculations

2nd June 2025

Motive to update our software is to align with the MCS online tool calculator for MIS 3005-D that utilises EN 12831:2017 and to include the option for an air permeability value to be entered from a Pulse or blower door test result.

The main difference is that thermal bridging and ventilation heat loss is calculated and used differently.

Ventilation heat loss totals

The European and British standard BS EN 12831-1:2017 (Energy Performance of Buildings) outlines that when calculating the total building heat loss for fabric and ventilation, it is not simply the total ventilation from each room summed together. Since air movement between an internal room occurs between each other and are not lost through to outside. Therefore, the net total for the building ventilation heat loss will be less. In fact, in most cases it's half the ventilation (volume flow ratio fi-z)

Heat Engineer Software uses the general standard method for ventilation heat loss, but to help explain this, here is the simplified version.

In most cases for domestic dwellings there is only one zone but with multiple rooms.

Volume flow ratio between room and zone default values

Table B.5 from EN 12831:2017

Building ventilation heat loss total is the sum of zone totals for all rooms.

=0.33×Bedroom1 min ACH×Volume×0.5×(Internal Temp−Ext Temp)= 0.33 \times \text{Bedroom1 min ACH} \times \text{Volume} \times 0.5 \times (\text{Internal Temp} - \text{Ext Temp})

=0.33×Kitchen min ACH×Volume×0.5×(Internal Temp−Ext Temp)= 0.33 \times \text{Kitchen min ACH} \times \text{Volume} \times 0.5 \times (\text{Internal Temp} - \text{Ext Temp})

=0.33×Bathroom min ACH×Volume×0.5×(Internal Temp−Ext Temp)= 0.33 \times \text{Bathroom min ACH} \times \text{Volume} \times 0.5 \times (\text{Internal Temp} - \text{Ext Temp})

=0.33×Living room min ACH×Volume×0.5×(Internal Temp−Ext Temp)= 0.33 \times \text{Living room min ACH} \times \text{Volume} \times 0.5 \times (\text{Internal Temp} - \text{Ext Temp})

Note: 0.33 = Density x Heat capacity of air

Room heat loss total to size emitters don't factor in the 0.5, therefore:

Total room ventilation heat loss

Note: 0.33=Density×Heat capacity of air

Minimum Air changes per hour

What is important to note is that historically we have used ACH as an input, this is still the case for a minimum value, but is now compared to another calculation that calculates the room envelope volume flow rate at m³/h. This is used in equation 17 within the EN12831:2017. The overall calculation will use the maximum flow rate from these two numbers to produce the ventilation heat loss for that room.

Thermal Bridging

By default, thermal bridging will be added to all external and internal building elements and the values used will differ based on the age of construction. EN 12831:2017 do have generic values, however, to align with MCS we have used the following:

Modified U value = Thermal Bridging U value + Material U value

'This is typically where there is either a break in the insulation, less insulation or the insulation is penetrated by an element with a higher thermal conductivity, such as wall ties, junctions in materials, or lintels. This can lead to a higher heat loss at this point than the calculated value.' CIBSE Domestic Heating Design Guide. 3.5.2.6 Thermal Bridging

External Building Elements

By default, a thermal bridging allowance will be added to all external building element types to create an effective U-value for calculating fabric heat loss.

• External Walls (Types A and B)
• Roofs (room above = none)
• Windows (Types A and B)
• Doors
• Suspended floors (room below = Suspended floor)
• Ground floors (room below = None)

The thermal bridging factor will depend on the age band of the main property (not the element itself) and is sourced from RdSAP (9.94) Tables S13.

Internal Building Elements

By default, a thermal bridging allowance will be added to all internal building elements types to create an effective U-value for calculating fabric heat loss.

• Internal Walls
• Floors
• Ceilings

The thermal bridging factor will depend on the age band of the property and is derived from CIBSE Domestic Heating Design Guide (2021) Table 3.4.

The following table is from RdSAP S2 Age bands