A heat loss calculation gives the heating power a house needs in the coldest hour of the year, worked out room by room. HeatAlgo calculates it to EN 12831-1, a European standard. The 1,100 ft² one-story house in the example below needs 15,651 Btu/h, or 14.2 Btu/h·ft².
The design heat loss is the first number in a whole heating project. Everything after it inherits its error: the heat pump capacity, the supply water temperature, the PEX tubing spacing in a radiant floor, the buffer tank, the heating bill. Get this number wrong and the rest of the work is perfect arithmetic on bad input.
This article covers what the calculation is, what depends on it, and why a rule of thumb is not enough - whatever software you use.
What HeatAlgo's heat loss calculation is - and what it is not
The design heat loss is the heating power a building needs during the coldest hour of the year to hold its target indoor temperature. EN 12831-1 itself calls the figure the design heat load (Φ_HL): the loss through the building shell and by ventilation, plus any heating-up power, less internal gains. The method is EN 12831-1:2017, and it works room by room rather than applying one figure to the whole house.
Be clear about what it is for. HeatAlgo's calculation is a European method, for heating only - there is no cooling, and there is no US climate data. It is not the load calculation a US building department or a rebate program asks for, and it is not accepted for permits or rebates. What it is good for: planning radiant floor heating room by room, and having a clear, checkable conversation with your installer about what each room needs.
Three streams make up one room's heat loss:
What a room's heat loss is made of
- Transmission through building elements
- Heat escaping through walls, windows, roof and floor. U-values from EN ISO 6946, heat transfer through the ground from EN ISO 13370.
- Ventilation
- The heat needed to warm incoming air. Depends on the ventilation type, the building's airtightness and any heat recovery.
- Thermal bridges
- The junctions where elements meet: corners, headers, balconies. Their share grows in a well-insulated building.
Heating-up power
An allowance for systems that run intermittently, so the building can recover its temperature after a setback.
The output is a figure for each room, plus a total for the building. Those are not the same number, as the example below shows.
A worked example: one house calculated in full
Numbers make it easier. HeatAlgo's reference house is a fictional one-story European house in Warsaw, Poland - not a US house: 1,100 ft² of heated floor area, eight rooms, mechanical ventilation with heat recovery at 85% efficiency, airtightness n50 = 1.5 1/h. The design outdoor temperature comes from the Polish climate table: -4 °F (-20 °C). Every input is published, so you can recompute the result yourself.
| Component | Result |
|---|---|
| Transmission through building elements | 11,721 Btu/h |
| Ventilation, after heat recovery | 1,822 Btu/h |
| Thermal bridges (ΔU_TB = 0.009 Btu/h·ft²·°F (0.05 W/(m²·K)), EN 12831-1, annex B.2.1) | 2,109 Btu/h |
| Design heat loss of the building | 15,651 Btu/h (4,587 W) |
| Per square foot of heated floor area | 14.2 Btu/h·ft² (44.9 W/m²) |
| Sum of the eight room heat losses | 16,580 Btu/h (4,859 W) |
The last two figures differ, and they should. Each room is calculated with its own worst-case share of air leakage, because that is what the radiator or radiant floor loop in that room has to cover. For the whole building, the standard counts ventilation once, for the whole zone. So the 16,580 Btu/h split across the rooms sizes the emitters, and the 15,651 Btu/h sizes the heat source.
The 14.2 Btu/h·ft² describes this house only: well insulated, airtight, with heat recovery, in a Warsaw climate. Do not carry it over to another building - the section on rules of thumb shows why.
What depends on it
This is the real answer to "why do I need this".
Heat source capacity. The obvious use, and the most expensive to get wrong. A heat pump sized against an inflated heat loss short-cycles at part load, which costs both seasonal efficiency and compressor life. Sized against an understated heat loss, it cannot keep up in a cold spell. More on that in the oversized heat pump.
Radiators and supply water temperature. This use gets lost behind the talk about capacity, and it matters just as much. Know one room's heat loss and the output of its radiator, and you can work out the supply temperature that radiator needs. One room with an undersized radiator raises the required temperature for the whole system - and every extra degree costs the heat pump efficiency all winter. Without a per-room split you never find that bottleneck.
Radiant floor heating. Radiant floor loops have a hard limit: the floor surface temperature. To check whether a loop can carry a room at all, you need that room's heat loss, not the building's. How many Btu/h per square foot a floor gives at a given water temperature and floor covering is in the guide to radiant floor heating water temperature; what that means for the whole design is in the radiant floor heating design guide.
Buffer tanks and hydronics. The buffer volume and the flow rates follow from the power the system has to move.
The heating bill. Annual energy use is estimated from the design heat loss. That is what tells you whether a heat pump pays off, and on which electricity rate.
Talking to your installer. A room-by-room heat loss with its method and assumptions stated gives you and your installer the same numbers to discuss. It does not replace any calculation your building department or a rebate program asks for.
Why a Btu per square foot rule of thumb is not enough
The common shortcut is a coefficient: so many Btu/h per square foot, depending on how well the house is insulated. The trouble is that the spread of that coefficient is wider than the difference between a good and a bad equipment choice.
We measured this on our own module. For one building, where a full calculation to EN 12831-1 gives 28,662 Btu/h, estimating the same building from a coefficient gives anything from 27,365 Btu/h to 80,254 Btu/h, depending only on which insulation class the person estimating picks. That is close to a threefold spread, and the choice of class is subjective.
| Method | Heat loss |
|---|---|
| Coefficient estimate, best insulation class | 27,365 Btu/h (8.02 kW) |
| Coefficient estimate, worst insulation class | 80,254 Btu/h (23.52 kW) |
| Calculated to EN 12831-1 | 28,662 Btu/h (8.4 kW) |
Room by room - where radiators get chosen - it looks worse. The mean error per room was 17%, the worst case 37%, and 4 of 14 rooms changed verdict: from "the existing radiator is enough" to "too small", or the reverse. Four rooms where the estimate leads to a different purchase than the calculation.
A rule of thumb is fine for a first conversation. It is not fine as the basis for ordering equipment.
Design temperature: one number, the whole chain
The calculation uses the design outdoor temperature for the building's location. HeatAlgo has no US climate data, so for a US house you type your own design temperature. For our European reference house, the Polish climate table gives a wide range:
| Cities | Design outdoor temperature | Difference at 68 °F indoors |
|---|---|---|
| Szczecin | 3 °F (-16 °C) | 65 °F |
| Gdańsk, Wrocław, Poznań | 0 °F (-18 °C) | 68 °F |
| Warsaw, Kraków, Łódź, Katowice | -4 °F (-20 °C) | 72 °F |
At 68 °F indoors, the difference between 3 °F and -4 °F is 65 °F against 72 °F, or roughly 11% of the heat loss. A few degrees of design temperature can move the selection by a whole equipment size.
More important still, the same temperature has to hold across the whole chain. If the calculation used one design temperature and the equipment selection assumed another, two documents for one house state different assumptions, and someone will ask about it. Manufacturers publish heat pump output at specific outdoor temperatures, so the design temperature also changes how much the unit actually delivers.
Where the method comes from
EN 12831-1:2017 is a European standard. Poland adopted it by recognition, without publishing a national annex. In practice, the default values in the standard's informative Annex B become the Polish defaults, unless the designer sets their own. That matters when you compare two programs: if they assume different defaults, they give different results, even though both "follow the standard". It also matters for a US reader: HeatAlgo's defaults are European defaults, so check every assumption against your house.
What a usable heat loss calculation contains
Whatever tool produced it, a result worth handing on includes:
- A per-room breakdown, not just a building total.
- The heat loss split into transmission, ventilation and thermal bridges, so you can see what dominates.
- The design temperature used, and where it came from.
- The U-value of each element, with the layers it was derived from.
- Ventilation assumptions - type, air change rate, heat recovery.
- The method and standard, stated plainly, so the result can be checked.
The last point is the one most often skipped, and it is the one that lets someone else check your numbers.
Common mistakes
- Calculating only the building total. You lose what you need to size radiators and radiant floor loops - the usual cause of a system that "does not heat one room".
- Taking the design temperature from the wrong place. Usually the software's default rather than the building's location. With HeatAlgo and a US house, always type your own.
- Ignoring thermal bridges. Their share grows in a well-insulated building, precisely because transmission through the elements shrinks.
- Confusing design heat loss with annual energy use. Two different quantities; neither replaces the other.
- Rescaling a finished result. If the heat loss was calculated to the standard, do not "correct" it afterwards with a coefficient or with fuel bills.
What a heat loss calculation costs in HeatAlgo
You enter the building data yourself, and you pay only for the report.
- Yourself, in the app. A HeatAlgo account is free. Once the house is entered, you see the summary and one fully calculated room. The full PDF report for one project - every room and every module, including heat pump sizing and radiant floor heating - costs $32, tax included. You enter the data from the floor plans and the layers of each element.
- An energy audit is a different document. It estimates annual energy use, not design power, and does not replace a heat loss calculation when sizing a heat pump.
How HeatAlgo does it
HeatAlgo's heat loss calculator works room by room to EN 12831-1:2017, with EN ISO 6946 for building elements and EN ISO 13370 for heat transfer through the ground, and shows the heat loss breakdown for every room. A US account sees it in feet, °F and Btu/h - the calculation itself stays in SI units, to the standard. How to use it, step by step, is in the heat loss calculation guide.
The heat pump sizing module brings in the rooms and their heat loss from that calculation, so the heat pump is sized at the same design temperature the house was calculated at. When the calculation changes later, the module shows what changed and asks before it overwrites anything. The radiant floor heating module brings in the rooms the same way. How the data moves between modules is in data flows between modules.
To see what that looks like on a real building, we have published one house calculated in full: the complete input set - dimensions, design temperatures, U-values, airtightness - and the results from four modules, in a form you can recompute by hand. It is a European house in Warsaw, in metric units.
More guides live in the help center.
Create a free accountA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.