How Well Does Your Home Hold Its Heat? A Guide to SmartHTC
In a nutshell: SmartHTC measures how quickly your home loses heat, using real readings taken while you carry on living there as normal. We place a few small temperature sensors around the house, collect your energy use and the local weather for at least three weeks during the heating season, and then calculate a single score for your home’s heat loss. That score shows us what your home is really doing, rather than what a desktop estimate assumes, so we can recommend work that fits your house.
We offer SmartHTC in collaboration with Build Test Solutions, the company that developed it.
1. What is SmartHTC?
The Heat Transfer Coefficient (HTC)
Every home loses heat through its walls, roof, floor and windows, and through draughts as cold air gets in and warm air gets out. The Heat Transfer Coefficient, or HTC, adds all of that together into one number. It is the best single measure of how well a home keeps the heat in and the cold out.
The HTC is measured in watts per degree (W/K). Put simply, it tells you how much heating power your home needs for every degree it is warmer inside than outside. A lower number is better.
A worked example
Say your home has an HTC of 200 W/K. On a winter day that is 0°C outside with the house kept at 20°C, there is a 20-degree difference. 200 × 20 = 4,000 watts. Your heating needs to put in about 4 kW, continuously, just to keep up with the heat escaping. If insulation and draught-proofing brought the HTC down to 120 W/K, the same day would need only 2.4 kW.
How SmartHTC works
SmartHTC works out your home’s HTC from three things:
- Some basic facts about the building (size, location, windows, heating system), much of which is on your EPC.
- How much energy the home uses, from your smart meter or from meter readings at the start and end.
- The temperature inside, from small sensors we place in the house, alongside the local weather.
It works on an energy balance. We know how much energy goes into the house and how warm the house stays compared with outside, so we can work out how quickly heat is escaping.
Why measure it rather than estimate it?
Most home energy figures, including EPCs, are calculated from assumptions about how a house was built. Real homes often behave differently, especially older properties, solid-walled houses and homes that have been extended or altered over the years. Research has repeatedly found a “performance gap”, with homes often losing more heat than their calculations predict.
Measuring the HTC gives us evidence to work from. It helps us to:
- see how your home is actually performing before we recommend any work
- focus the budget on the improvements most likely to make a difference
- size a heat pump or other heating system on real data rather than assumptions
- check afterwards that the work delivered the improvement it was meant to
Until recently, testing a home’s heat loss meant moving out for a few days. SmartHTC is done while you live there, so it is far less disruptive.
2. How we measure it: step by step
- Survey visit. We visit to measure the rooms, windows and any party walls, note the glazing and frame types, and identify your heating system and its efficiency. We also check for any secondary heating, such as a log burner or gas fire, and anything else that uses energy, such as solar panels or an electric car charger. We take photos of the outside of the house, the sensor positions, the heating system and the meters.
- Placing the sensors. We place small temperature and humidity loggers, each about the size of a box of mints, around the house. A typical home has about five: in the living room, kitchen, main bedroom, bathroom and near the thermostat. Larger homes need more, up to ten. They sit at about mid-height, away from radiators, direct sunlight, appliances and cold draughts, and not on outside walls. We put them on shelves or use removable strips, so there are no marks.
- Energy data. With your permission, we collect half-hourly data from your smart meter. You own this data and can agree to share it through the free Bright app. If you don’t have a smart meter, we (or you) take gas and electricity meter readings on the first and last day instead.
- Monitoring period: you carry on as normal. Monitoring runs for at least 21 days while the heating is on and the house is at least 7°C warmer than outside. In practice that means autumn to spring. You don’t need to change how you heat or use your home.
- Collecting and calculating. We collect the sensors, download the readings and run the SmartHTC calculation, which uses local weather data too.
- Your results. We go through the results with you in plain language and explain what they mean for your home and for any work you are planning.
3. What the results tell you
| Result | What it means |
|---|---|
| HTC (W/K) | Your home’s total heat loss per degree of temperature difference. Lower is better. Because it depends on the size of the house, it is best compared with the same house over time, for example before and after works. |
| Confidence range | Every measurement comes with a margin of error, shown as a range either side of the result. More sensors and smart meter data make the range narrower. |
| HLP (W/m²K) | Heat Loss Parameter: the HTC divided by the floor area. Because it allows for size, it lets us compare your home with other homes and with benchmarks. |
| Peak heat demand | The heat output needed on a cold design day. This is useful when sizing a heat pump or boiler. |
| Predicted vs measured | If your home has an EPC or a design calculation, we can compare its predicted HTC with the measured one to see whether there is a “performance gap”. |
How does my home compare?
Build Test Solutions gives these example Heat Loss Parameter figures for a simple two-storey detached house built to different standards. They are illustrative figures from a model house, not measured homes, but they give a useful sense of scale.
| Example house | Heat Loss Parameter (W/m²K) |
|---|---|
| Existing solid-walled house (uninsulated, draughty) | 4.16 |
| 2016 Building Regulations minimum standards | 1.82 |
| Same solid-walled house after retrofit (wall, floor and loft insulation, better airtightness) | 1.57 |
| Future Homes Standard consultation limits | 1.56 |
| Passivhaus-standard example | 0.93 |
4. What information is needed
Some information is essential. The rest is optional, but each extra item makes the result more accurate and narrows the confidence range. Much of the building information can be taken from your Energy Performance Certificate (EPC).
| About the building: essential | About the building: optional (improves accuracy) |
|---|---|
| Total floor area Built form and attachment (detached, semi, terrace, flat) Heating system type and efficiency (e.g. boiler make and model) | Location (postcode) Party wall area Window sizes and which way they face Glazing type, frame type and how shaded the windows are Number of people living in the home |
| During monitoring: essential | During monitoring: optional (improves accuracy) |
|---|---|
| Energy use at your gas and electricity meters (smart meter data, or start and end readings) Inside temperature in at least one central location (more for larger homes) | Temperature and humidity in up to 10 rooms Half-hourly smart meter data Solar panel generation and export Electric vehicle charging Heat pump heat output and electricity use Separate heating and hot water figures Local outside temperature and sunshine When people are at home |
What we need from you
- Send us a copy of your EPC if you have one, or let us know if there isn’t one.
- Agree to share your smart meter data through the Bright app, or let us know if you don’t have a smart meter.
- Tell us how many people live in the home.
- Tell us about all heating and energy sources, including log burners, open fires, gas fires, portable heaters, solar panels, batteries and any heat pump. The calculation needs to know about all the energy going into the house.
- Tell us about anything that uses a lot of electricity or gas outside the house but on the same meter, such as a hot tub, heated outbuilding, garden office or electric car charger.
- Leave the sensors where we put them, and don’t cover them or move them near a heat source.
- Tell us if the house will be empty for a long spell (for example, a holiday) or if anything unusual happens during monitoring, such as a heating breakdown or building work.
- Otherwise, live as normal. There’s no need to change how you use your heating.
5. Common questions
Do we need to move out?
No. SmartHTC is designed for occupied homes. You carry on as normal throughout.
How long does it take?
A survey and set-up visit, at least three weeks of monitoring, then a collection visit. Your results follow once the data has been processed.
When can it be done?
Only during the heating season, when it is cold enough outside for there to be a clear temperature difference, typically from autumn to spring. We can’t get a reliable result in summer.
Is my data private?
Your smart meter data belongs to you and is only shared with your consent. We use your readings only to calculate your home’s results.
How accurate is it?
Build Test Solutions’ testing has found SmartHTC to be comparable with the established tests that require an empty house. Smart-meter methods of this kind have also been assessed independently in the government’s SMETER project. Every result comes with a confidence range, and more sensors and smart meter data make that range smaller.
Will it tell me exactly what to do?
Not on its own. SmartHTC measures how your home performs as a whole. We combine it with our survey and our experience of older and heritage homes to recommend the right next steps, and we can repeat the measurement after the work to check the improvement.
Thinking about SmartHTC for your home? It is most useful for older, solid-walled or altered homes, and before major retrofit or heating work. Get in touch and we’ll talk through whether it makes sense for your project.
Thank you to Build Test Solutions, the team behind SmartHTC, for developing the technology that makes this kind of measured, evidence-based approach possible.