Ventilation calculations in Heatpunk

When selecting which ventilation calculation to use, you will have the option between the Standard method, Measured air permeability (both based on BS EN 12831-1:2017) and the Legacy method based on (BS EN 12831:2003).

MCS requires ventilation heat loss be calculated according to BS EN 12831-1:2017 (Energy Performance of Buildings). This standard aims to provide a more accurate and realistic heat loss calculation, generally resulting in lower ventilation heat loss values.

How are ventilation losses calculated?

Under BS EN 12831-1:2017, the ventilation losses for a building are no longer a simple sum of the ventilation losses for each room. This is because some of the air flow into a room may be pre warmed from another room. Rooms facing the wind may have cold outside air pushed into them, but then the warm air from those rooms will flow into other internal rooms thereby reducing the amount of cold air entering a room not facing the wind. So not all warm air is lost to the outside. This reduces the overall ventilation heat loss for the building compared to the individual rooms, typically by half as the total zone ventilation loss is multiplied by volume flow ratio of 0.5.

What is a zone?
A zone is a group of rooms that are air-connected by design, such as by an internal door that complies with the requirements in Approved Document F: Ventilation, Volume 1 – Dwellings. Homes are commonly single ventilation zones and Heatpunk assumes a building represents a single ventilation zone.

Ventilation losses can be calculated using either the standard method or using a measured air permeability test input.

Air change rates

Whichever method is selected for specifying the air permeability, the BS EN 12831-1:2017 ventilation loss equations will yield the maximum of these two values:

The loss computed using the measured air permeability or assumed ventilation rate.
The loss computed using the room air change rates.

Historically, room air change rates were key inputs to the ventilation heat loss calculations. However, now the air change based loss value is compared to the loss computed using the measured air permeability or assumed ventilation rate based on expected occupancy. The ventilation loss is the greater of the two values.

Standard method

Where a measured air permeability is not known, the whole-building infiltration rate is assumed based on the minimum ventilation rate required for the number of bedrooms in the property. This data is derived from CIBSE Domestic Heating Design Guide (2021) Table 1.7. The ventilation rate (in L/s) is converted to a whole-house air change rate (ACH or 1/h) based on the combined internal air volume of all rooms.

When this standard method is used, Heatpunk will apply the default air change rate values from Table 3.8 of the CIBSE Domestic Heating Design Guide (2021). The BS EN 12831-1:2017 ventilation loss equations will then yield the maximum of these two values:

The loss computed using the assumed ventilation rate.
The loss computed using the room air change rates.

Using the CIBSE fixed default air change rates may result in higher ventilation heat loss figures than using the estimated ventilation rate based on expected occupancy, especially in older buildings. In some cases, you may choose to override the default air change rates.

Measured air permeability

The airtightness of the building can be tested using the methodology outlined in CIBSE TM23. The result is expressed in m³/m².hr at a specific pressure difference of 50Pa.

When the building air permeability has been measured, Heatpunk will apply a minimum air change rate of 0.5 ACH to all rooms. This minimum air change rate for habitable rooms is taken from Table B.7 of BS EN 12831-1:2017 and will replace the default values from Table 3.8 of the CIBSE Domestic Heating Design Guide (2021). This approach is intended to more accurately represent the actual air permeability of the building as measured. This may result in lower ventilation heat loss figures than using fixed default air change rates. Fixed default can lead to a significant over estimation of ventilation heatloss, especially in older buildings.

You can choose to override the minimum air change rate of 0.5 ACH. Increasing the ACH for a room will only affect its ventilation loss if the ACH yields a larger loss than the loss computed using the measured air permeability.
By using Heatpunk, you are acknowledging and accepting this methodology.

By using Heatpunk, you are acknowledging and accepting this methodology.

How does this impact my designs?

Heat pump sizing: Heat pumps are sized to meet the overall heat loss for the building. The ventilation losses for the building are generally half the sum of the individual room ventilation losses.
Heat emitter sizing: The emitter for an individual room must meet the worst case ventilation heat loss. The room ventilation losses are not discounted by the volume flow ratio that is applied to the building as a whole.

Legacy method

MCS certification in accordance with MIS 3005-D now requires heat loss calculations comply with BS EN 12831-1:2017. Please only select this method when viewing old projects.

This method is based on BS EN 12831:2003. This calculation is simply a sum of the individual ventilation losses of each room. For each room the following calculation is done:

Room ventilation losses = 0.33 * ACH * Volume * Temperature difference,

where the ACH was taken from Table 3.8 of the CIBSE Domestic Heating Design Guide (2021).