The Truth about Heat Loss | Steph Wills from Spruce

The Truth about Heat Loss | Steph Willis

In this episode of Energy Unwrapped, I’m joined by Steph Willis from Spruce to talk about one of the most important parts of any heat pump installation: heat loss.

We break down what heat loss actually is, why it matters, and why many homeowners may have been given significantly inflated heat loss figures over the last few years. A huge part of the discussion focuses on air changes and ventilation losses, where outdated guidance often pushed installers towards extremely conservative assumptions that led to oversized systems, larger radiators and unnecessary disruption. 

The conversation also explores how the industry is improving, including updated guidance, whole-home infiltration approaches and better tools for installers. This is a really useful episode for homeowners trying to understand why heat loss matters, how it affects costs and efficiency, and why getting the numbers right can completely change the outcome of a heat pump installation.

What we cover in this episode

• what a heat loss survey actually is
• the difference between fabric losses and ventilation losses
• why heat loss matters for heat pump sizing
• how outdated guidance led to oversized systems
• why old air change assumptions were often far too high
• how ventilation losses are now being calculated differently
• why many homeowners were told to replace too many radiators
• the impact oversized heat pumps have on cost and disruption
• why accurate heat loss helps homeowners make better decisions
• balancing efficiency, upfront cost and disruption
• why lower flow temperatures are not always the right answer
• whole-home infiltration vs room-by-room assumptions
• blower door tests, pulse tests and CO2 decay testing
• how software like Spruce helps installers work faster
• why good installers still need judgment and experience
• how homeowners can have better conversations with installers

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Full Transcript

Below is the full transcript for this episode, lightly edited for readability.

Meet Steph Willis and the Story Behind Spruce

Steph starts by explaining her background in mechanical engineering, heat pumps and climate-focused work, including time spent designing ground source heat pump systems in Canada. That eventually led to co-founding Spruce, a software platform designed to help heat pump installers streamline surveys, heat loss calculations and compliance paperwork.

Why Heat Pump Installers Were Drowning in Admin

One of the first major topics is just how much time installers were spending on surveys, heat loss calculations and compliance paperwork. Steph explains how installers could easily spend 10 hours or more per potential project, even before knowing whether the homeowner would proceed.

What a Heat Loss Survey Actually Is

The episode then moves into the basics of heat loss itself. Steph explains that heat loss is simply how quickly a house loses heat, and therefore how much heat the heating system needs to replace in order to keep the home warm.

Fabric Losses vs Ventilation Losses

A key part of the discussion is the difference between fabric heat loss and ventilation heat loss. Fabric losses are heat escaping through walls, windows, roofs and floors, while ventilation losses come from warm air leaking out and colder air leaking back in through gaps and openings. 

Why Ventilation Losses Became Such a Big Problem

The biggest issue discussed in the episode is how ventilation losses were historically estimated. Older industry guidance used very conservative “air changes per hour” assumptions that often massively overstated how leaky homes really were.

The Problem With Old Air Change Defaults

Steph explains that some default values suggested older homes could have air change rates of 2 or more air changes per hour, which in practice would require extremely leaky buildings. Many installers simply followed the guidance they were given, which then inflated total heat loss calculations.

Real Example: A 1930s Semi Detached House

A really useful part of the episode is the example of your own 1930s semi-detached house. Using older defaults suggested an air change rate around 1.8 ACH, while real-world testing through CO2 decay tests, pulse testing and measurement showed the actual figure was closer to 0.6 ACH.

How Oversized Heat Loss Figures Affected Heat Pump Installs

The knock-on effect of inflated heat loss figures was huge. Systems were being oversized, homeowners were being told they needed far larger radiators, and in some cases installers were recommending larger heat pumps, more pipework upgrades and even three-phase electricity supplies unnecessarily.

Why Bigger Heat Pumps Are Not Always Better

Steph explains that the biggest issue with oversizing is often not efficiency, but cost and disruption. Larger units take up more space, need more expensive equipment and frequently lead to unnecessary radiator or pipework changes.

The New Whole-Home Infiltration Approach

The good news is that industry guidance is changing. Instead of trying to estimate air changes separately for every room, newer methods treat the home more holistically using a whole-home infiltration approach based on building type, exposure and overall construction.

Blower Door Tests, Pulse Tests and Better Measurements

We also discuss testing methods like blower door tests and pulse tests, which can give a far more realistic understanding of how airtight a property really is. While not every homeowner will pay for testing, these methods help validate more sensible assumptions.

Why Accurate Heat Loss Changes Everything

One of the biggest takeaways is that accurate heat loss gives homeowners proper choices. Rather than assuming every system must target ultra-low flow temperatures, homeowners can decide where they want to sit between upfront cost, disruption and long-term efficiency.

Basic, Better, Best: Balancing Cost and Efficiency

A really interesting discussion develops around “basic, better, best” approaches to heat pump design. Some homeowners may want the absolute lowest flow temperature possible, while others may prioritise lower upfront cost and less disruption. Accurate heat loss allows those decisions to be made properly.

Why Heat Pump Efficiency Is Not Everything

Steph makes an important point that efficiency alone is not always the main goal. For homeowners with solar, batteries or cheap overnight tariffs, chasing every last bit of COP may matter less than reducing install cost and disruption.

Room-by-Room Heat Loss and Radiator Sizing

The episode also explains the difference between whole-house heat loss and room-by-room calculations. The total heat loss helps size the heat pump itself, while room-by-room losses determine whether each radiator or emitter can provide enough heat.

Why Some Radiators Need Changing and Others Do Not

Rather than replacing every radiator, good design often focuses on the weakest rooms in the system. Sometimes changing just one or two poorly sized radiators can dramatically reduce the required flow temperature for the whole system.

Heat Pumps, Flow Temperatures and Real Home Constraints

The conversation also touches on the realities of retrofit homes. While very low flow temperatures may look great on paper, there are practical limits around radiator size, room layout and homeowner preferences.

What Homeowners Should Ask Their Installer

Towards the end of the episode, we discuss how homeowners can better evaluate installers. A good installer should be willing to explain trade-offs clearly, discuss disruption versus efficiency, and tailor the design around what the homeowner actually wants.

Final Thoughts: Why Getting Heat Loss Right Matters

The overall message is simple: getting heat loss right matters because it shapes almost every other decision in a heat pump installation. Accurate numbers can reduce cost, reduce disruption and help more homeowners adopt heat pumps successfully.

About the Energy Unwrapped Podcast

Energy Unwrapped is my podcast about renewables, home electrification and how energy technology works in the real world. You can browse all episodes, platform links and future updates on the main Energy Unwrapped Podcast page.

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