BS12831:2017 - Frequently asked questions

The total building heat loss in BS EN 12831:2017 is not simply the sum of the individual room heat losses because of the way ventilation losses are treated. While transmission losses (through walls, windows, floors, etc.) can be accurately summed across rooms, ventilation losses are handled differently at the room and building level to avoid overestimation.

At the room level, ventilation losses are calculated for each space using assumed air change rates or default ventilation flows. This can lead to double-counting because the same air may be considered as entering and exiting multiple rooms, especially in naturally ventilated or interconnected spaces. For example, if a hallway and an adjacent room both assume independent infiltration, shared airflow through internal doors may be counted twice.

At the building level, the ventilation calculation does take into account that some of the air entering a room will be from a neighbouring heated room. This gives a more realistic estimate for the overall building heat loss by accounting for internal air transfer and avoiding duplicated flows.

To illustrate:

Loss Type	Rooms Total (sum of rooms)	Total Building Heat Loss	Explanation
Transmission	Additive	Same	No overlap between rooms
Ventilation	Overestimated	Lower	Shared airflows double-counted in rooms total

 

For example, if room-level ventilation losses total 67 W and transmission losses total 105 W, the summed room heat loss would be 172 W. But if the building-level ventilation loss is correctly calculated as 20 W, the total building heat loss becomes 125 W. This corrected total is used to size the heat generator, avoiding an oversized system based on inflated room-level ventilation estimates.

Summary:

While room-level heat loss calculations are used to ensure adequate emitter sizing in each space, the total building heat loss corrects for inter-room airflows and is a more accurate basis for generator sizing.

BS EN 12831:2017 requires the heat generator to be sized using the total building heat loss, while individual emitters (such as radiators or underfloor loops) are sized using the room-level heat loss, because these components serve fundamentally different purposes within the heating system:

• The heat generator must be capable of meeting the combined heating demand of the entire building under the worst-case external conditions. This includes all transmission losses through the building envelope and the overall ventilation losses for the building. Importantly, the ventilation losses at building level are calculated in a way that avoids double-counting airflow between rooms, giving a more accurate estimate of the total heat demand. The generator sizing ensures that, even on the coldest design day, the system as a whole can provide sufficient energy.

 

• Emitters, on the other hand, are designed to maintain comfort in individual rooms. Each room has unique characteristics—such as size, insulation, orientation, window area, and usage—that determine its specific heat loss. Therefore, each emitter must be capable of offsetting the heat loss from the room it serves, regardless of what is happening elsewhere in the building. Using room-level losses ensures that every room can be kept at its target internal temperature.

 

Summary:

This approach means that the sum of the emitter outputs will typically exceed the generator capacity, which is expected. Not all rooms may experience peak demand at the same time, and the generator operates efficiently based on the building-wide, more realistic demand profile. This method also allows for diversity and ensures system efficiency without compromising comfort in individual spaces.

Component	Sizing Basis	Reason
Heat Generator	Total building heat loss	Needs to supply whole-building demand, including system losses
Emitters	Individual room loss	Must meet specific, local comfort needs based on each room’s conditions

In principle, you could calculate ventilation losses for each room using the building-level method (i.e. accounting for internal air transfer and avoiding double-counting), especially in an unzoned house where all radiators are active at the same time.

However, BS EN 12831:2017 still treats ventilation losses differently at room and building level, and recommends using room-specific ventilation calculations for emitter sizing. Here’s why:

1. Design Focus: Local Conditions for Comfort
   Radiators are sized to ensure comfort at the room level, not just energy balance across the whole building. Even if the house is unzoned, one room may be colder (e.g. north-facing, draughty, or more exposed) and experience higher infiltration than others. The standard accounts for worst-case conditions per room, so it errs on the side of slight oversizing to ensure local comfort.
2. Risk of Undersizing with Shared-Air Calculations
   If you spread the building’s total ventilation loss across rooms, rooms with higher exposure or worse airtightness might be under-served. Internal air transfer is not always balanced or predictable—doors may be closed, pressure differentials vary, and airflow paths are complex. Some rooms may not benefit from shared ventilation air in practice.
3. Room-Level Ventilation = Simpler and Conservative
   BS 12831 uses room-level methods partly for practical simplicity. Designers can assign default air change rates or ventilation flow per room type (e.g. 0.5 ACH for living rooms, 1 ACH for bathrooms). While this may overestimate total ventilation losses, it provides a safe margin—especially important with low-temperature systems such as heat pumps.
4. Zoning May Change in the Future
   Even if the system isn’t zoned now, it may be later—for example, with TRVs or smart heating controls. Sizing radiators conservatively ensures they still operate effectively even if run independently in the future.

 

Summary:
Room-level ventilation loss calculations are used for radiator sizing to ensure local comfort and robustness in a range of real-world scenarios, even if that slightly overestimates the total. Building-level ventilation calculations are reserved for generator sizing to avoid double-counting and reflect realistic system demand.