When designing a heat pump system, one of the most important figures is the property’s heat loss.

Heat loss tells us how much heat the home loses on a cold day. Once we know that, we can design the right size heat pump, radiators, pipework and flow temperatures.

Most people understand that homes lose heat through walls, roofs, windows and floors. This is called fabric heat loss.

But there is another big part of the calculation:

Ventilation heat loss.

This is the heat lost when warm air escapes from the home and cold air leaks in to replace it.

Why Ventilation Heat Loss Matters

Every home leaks air.

Some air leakage is obvious, like draughts around doors, windows, floors or old chimneys. Some of it is hidden, through gaps around pipework, loft hatches, service penetrations or poorly sealed floors.

The leakier the building is, the more cold air gets in.

That means the heating system has to work harder to keep the home warm.

For heat pumps, this matters a lot. Heat pumps work best when the system is accurately designed. If the heat loss is overestimated, the heat pump may be oversized and the radiators may be upgraded more than necessary. If it is underestimated, the home may struggle to stay warm on the coldest days.

The aim is simple:

Get the design as close to reality as possible.

The Old Way: Based Mainly on Property Age

Traditionally, ventilation heat loss was often estimated using broad assumptions based on the age and type of the property.

For example, an older property would usually be assumed to be leakier than a newer one.

That makes sense as a rough starting point, but it can also be very conservative.

The problem is that two older homes can perform very differently.

One might be full of draughts.
Another might have had new windows, sealed floors, blocked chimneys and better workmanship over the years.

So using age alone can sometimes overestimate the real heat loss.

The Newer Way: Whole-House Air Permeability

The newer method looks more closely at the air permeability of the whole building.

In simple terms, this means:

How much air leaks through the building fabric.

This gives a better picture of how the home actually performs, rather than just assuming it based on age.

Even better, this can be measured using an airtightness test, such as a Pulse test.

That gives us real data from the property.

Real Examples From Three Properties

We recently compared three properties using three different approaches:

1. 2015 CIBSE method — based mainly on property age
2. 2026 CIBSE method — based on whole-house air permeability
3. Measured Pulse airtightness test input — based on actual measured data from the property

Here are the results.

The difference is clear.

The older 2015 method produced the highest heat loss figure on all three properties.

Compared with the measured Pulse test input, the older method was higher by:

That is a significant difference.

On a heat pump design, 1.5–2.5 kW can affect the selected heat pump size, radiator upgrades, pipework design and overall project cost.

The 2026 Method Was Much Closer

What is really interesting is how close the newer 2026 method was to the measured Pulse test input.

Across all three examples, the newer method was within a few hundred watts of the measured result.

That is a big improvement.

It shows that using whole-house air permeability gives a much better estimate than relying only on property age.

Does Every Home Need an Airtightness Test?

Not always.

The newer calculation method gives a much better starting point, especially where a measured airtightness result is not available.

But where possible, a measured airtightness test gives the best data.

It removes guesswork and helps build a more accurate picture of how the home actually performs.

This is especially useful on retrofit projects, older homes and properties where the construction is unusual.

Why This Matters for Homeowners

This is not about making the heat pump smaller just for the sake of it.

It is about designing the system properly.

A more accurate heat loss calculation can help avoid:

• Oversized heat pumps
• Unnecessary radiator upgrades
• Higher installation costs
• Poor system efficiency
• Designs based on assumptions rather than real performance

With heat pumps, good design matters.

A gas boiler can often hide a poor design because it runs at high temperatures and usually has a lot of spare output.

A heat pump is different. It needs the building, the radiators, the pipework and the controls to work together.

That starts with an accurate heat loss calculation.

Better Data, Better Design

At VAYU, we use detailed room-by-room heat loss calculations when designing heat pump systems.

Where useful, we can also carry out airtightness testing to measure how leaky the building really is.

The comparison above shows why this matters.

The older age-based method overestimated heat loss on all three properties. The newer whole-house air permeability method was much closer to the measured airtightness result.

Better data leads to better design.

And with heat pumps, better design is everything.