Heat pump sizing is more nuanced than a rule of thumb or a quick survey visit. OpenHeatLoss gives you access to the same calculation professional engineers use — so you can understand the numbers for yourself, in your own time, before any commercial conversation begins.
These aren't abstract technical points. They're the things that change how you approach a heat pump project — and how well-prepared you are when you sit down with a contractor or make a decision about your home.
A rule of thumb — watts per square metre, or a quick look at the boiler size — doesn't account for how well your home is insulated, how exposed it is, how warm each room needs to be, or how much heat is lost through the floor and roof relative to the walls. Two houses of identical size can have very different heat loss figures.
A room-by-room heat loss calculation to BS EN 12831-1:2017 considers all of these factors, element by element, room by room. It's what MCS-accredited engineers are required to produce before specifying a heat pump — and it's what OpenHeatLoss is built around. The same methodology, the same standards, available to anyone.
The heat loss figure isn't just the number you need to size the heat pump. It tells you which rooms are losing the most heat and why, whether your existing radiators can deliver enough output at lower flow temperatures, and what effect any planned insulation improvements would actually have on the system size.
These aren't things you can intuit or estimate. They come from doing the calculation. OpenHeatLoss keeps the calculation chain transparent — so you're not just getting an answer, you're understanding what's driving it. If a figure surprises you, you can see exactly where it comes from and decide whether the input assumptions are right for your home.
Most homeowners go into an installer visit without any independent reference point. The installer surveys the property, produces a specification, and the homeowner has little basis for evaluating it. That's not a criticism of installers — it's just the information asymmetry that exists in most home improvement projects.
Running your own heat loss calculation — even a rough one using RdSAP10 default U-values for your construction type and age — gives you a basis for comparison. If an installer's specification is significantly different from what your calculation suggests, that's a conversation worth having before any contract is signed.
We won't pretend the tool is as simple as entering your postcode and getting an answer. A room-by-room heat loss calculation requires real inputs — room dimensions, construction types, window areas. It takes time, and some of the concepts take a little getting used to.
But it's not inaccessible. The RdSAP10 library means you don't need certified U-values for every element — you can select your property's age band and construction type and get defensible defaults drawn from the same dataset that underpins Energy Performance Certificates. The design temperatures for your region are pre-populated from your postcode.
The YouTube channel walks through the concepts and the tool in parallel — building envelope heat loss, what U-values actually mean, how to read an emitter schedule, what SCOP tells you about running costs. The videos are designed to be useful whether you use the tool or not.
If you're a technically minded homeowner, a self-builder, or someone who wants to understand their home's energy performance properly rather than taking a number on trust — this is built for you.
Width, length, height. Windows and doors as elements. You can do one room at a time — there's no requirement to complete the whole house in one session.
Select your property's age band and wall/roof/floor type. The RdSAP10 defaults populate automatically. Override with certified values if you have them.
Outdoor design temperature and annual mean for your region are pre-populated. Internal temperatures are set per room — living rooms warmer, bedrooms typically cooler.
Design load and typical load update as you enter data. You can see immediately what effect changing an insulation level or window specification has on the room heat loss.
Check whether existing radiators are adequate at lower flow temperatures. Run a SCOP estimate to get a sense of seasonal efficiency — and what that means for running costs.
A heat loss report in the same format a professional engineer would produce. Useful to share with an installer, or to keep as a record of the design basis for your system.
The calculation isn't an end in itself. Here's what it actually tells you — and how that changes the decisions you're making about your home.
Room by room, element by element. Not a per-square-metre estimate — a figure grounded in your construction, your insulation, your orientation.
The generator load your calculation produces is the input to heat pump selection. Too large and the unit will short-cycle and run inefficiently. Too small and it won't meet demand on cold days.
Heat pumps run at lower flow temperatures than gas boilers. The emitter sizing check tells you which rooms have radiators that can deliver enough output at 45°C or 50°C — and which need attention.
Planning loft insulation or wall insulation before the heat pump goes in? The calculation shows you the direct effect on heat loss — and whether it meaningfully changes the system size.
The SCOP estimate uses your building's W/K coefficient and regional climate data — not a generic figure. It gives you a realistic starting point for thinking about annual running costs.
Going into a survey visit with your own heat loss figure — even a rough one — changes the nature of the conversation. You're evaluating a proposal, not accepting one.
You don't need to complete the whole house in one go. Try the tool with one room — get a feel for how the inputs work and what the calculation produces. No account required to start.
Questions? heatloss@openheatloss.com