BS 12831:2017 introduced a new method for calculating ventilation heat losses—one that’s more accurate but also more complex. The GreenPro Installer Toolkit now fully supports this updated standard, automatically applying the correct calculation method for both individual rooms and the whole building. While there are a few extra questions you’ll need to answer about the property (such as ventilation type), we’ve kept this as simple as possible and provided sensible default values where appropriate. The main difference you’ll notice is that the total of the room-by-room heat losses no longer matches the building’s overall heat loss because they are now calculated in different ways.

Read below for a deeper explanation of the changes, or watch our YouTube video if you want to see what these changes mean for you as a Toolkit user.

What is Ventilation Heat Loss?

Ventilation heat loss refers to the energy required to heat up fresh air that enters a room – whether through vents, cracks, windows, or mechanical systems. This incoming air must be warmed to maintain the desired indoor temperature.

The Old Way (BS 12831:2003)

In the 2003 version of the standard, ventilation heat loss was calculated using a very simple and uniform approach. Each room’s heat loss was estimated based on its volume, how often the air inside it was replaced each hour (known as the air change rate), and the temperature difference between inside and outside.

Once each room’s ventilation loss was calculated, the results were simply added together to get a total for the building.

This method:

• Did not distinguish between different ventilation systems (e.g. natural ventilation vs mechanical systems)
• Assumed all air was fresh from outside, even if it had come from another room
• Was easy to apply, but often overestimated heat loss for the building overall, especially in modern, airtight homes

The New Way (BS 12831:2017)

The updated 2017 version introduces a more detailed and realistic approach that reflects how modern buildings are ventilated.

Key changes include:

• Different calculations for rooms and whole buildings:
  • Room-level calculations are used for emitter sizing (e.g. radiators or underfloor heating)
  • Building-level (or “zone-level”) calculations are used for sizing the heat source (e.g. heat pump)
• Ventilation systems are now taken into account:
  • Natural ventilation (like trickle vents)
  • Mechanical systems such as MEV, MVHR, and PIV
• Air transferred between rooms is now factored in, reducing double counting
• Zones are introduced: areas of a building that share the same ventilation system (most homes have just one)
• Wind exposure and external wall area now influence room-level calculations

Although this method is more complex, it gives a more accurate picture of how much heating is really needed.

Why the Change?

The previous approach often overestimated ventilation losses for the building, potentially leading to oversized systems, especially in well-insulated homes. Oversizing can cause:

• Higher upfront and running costs
• Poor system efficiency

The 2017 method gives:

• A more realistic estimate of heat needed
• Better alignment with actual building performance
• Smarter system sizing, especially for modern homes with mechanical ventilation

Summary: Key Differences

Feature	2003 Version	2017 Version
Ventilation type	Not considered	Clearly defined
Room vs building	Same method for both	Different methods used
Transferred air	Ignored	Included
Zones	Not used	Used to define systems
Wind/exposure	Ignored	Included in room calcs
Overall result	Often overestimated	More accurate

 

If you want to understand more about the standard or how we have applied it in Toolkit, please contact us by phone, email or online chat. We’re here to make it easy for you to stay compliant and get your calculations right, every time.