The heat pump sizing module gives a preliminary heat pump sizing for a conversation with your client: the recommended heat pump class, the supply water temperature, SCOP, annual electricity use, installation cost and a check of whether the existing radiators are enough. It sizes for heating only - HeatAlgo calculates no cooling. It is not an official sizing and does not replace an installation design or an energy audit: a full design needs a site visit and measurements. This article walks through the wizard and explains how to read the result. Why a capacity margin is a mistake is explained in the guide on the oversized heat pump.
What to know before you start in the US
- Units. An account created from the US starts in imperial units: square feet, °F and Btu/h. You can switch in Settings (the "Profile" tab, "Units") or in the module settings. The calculation itself stays in SI units, and the PDF report prints metric units for now.
- Currency. The module's prices - electricity, fuels, installation cost - are Polish prices, calculated in Polish zloty. A US account can show them in US dollars, converted at the daily rate of the National Bank of Poland (NBP). You change this in the module settings, under "Units and currency" ("Currency of the heat pump's costs"), or in Settings.
- Climate and market. The locations, the electricity rates and the heat pump catalog in this module are Polish. For a US house, start from the heat loss calculation, where you type your own design temperature, and bring its result into this module. Use the result as a first sizing.
Two modes: from the heat loss calculation, or by hand
An empty module opens on the start screen "Size a heat pump with Algo" with the "Start with Algo", "I'll fill it in myself" and "Guide" buttons. The module has two modes and suggests the right one:
- From the heat loss calculation. When the project has a heat loss result, a "Ready to bring in - Heat loss calculation" line with the number of rooms, the area and the heat loss sits below the buttons. The "Bring it into this module" button on that line enters the heated area and the rooms (name, area, temperature) and stores each room's heat loss from the calculation. The sizing then runs from that heat loss, and the insulation class becomes optional. If the heat loss calculation is incomplete or blocked, the line says so plainly and leads you to it.
- By hand. "I'll fill it in myself" skips the calculation and estimates the heat loss from the insulation class and the year built - not recommended, but useful without data.
In both modes, the wizard has four steps: "Building", "System", "Rooms", "Result", and every field stays editable. Bringing data in only adds: rooms you added by hand stay, and in the rooms from the calculation, the radiators and radiant floor heating you already entered stay as they were - only the name, area and temperature are overwritten.
When the heat loss calculation changes after the import, the module shows the "Heat loss calculation has changed" dialog with the list of changes the next time you open it: you check what to overwrite and click "Bring in the ticked values" or "Not now". In the module settings, under "Data sources", the "Heat loss calculation" row then reads "newer data available", with a "Bring in again (newer data available)" button. Details in data flows between modules.
Older projects linked to the calculation switch to this way of working by themselves the first time you open them. If an older project has a different heat loss stored than the current calculation, the module shows the "The calculated heat loss has changed" dialog with the old and new values and the "Use the current load" and "Leave it as it is" buttons. A heat loss above 341,214 Btu/h (100 kW) is outside the module's range, and up to 50 rooms come in from the heat loss calculation.
The Building step
In manual mode, you enter the location (a Polish design temperature, from 3 °F (-16 °C) for Szczecin to -4 °F (-20 °C) for Warsaw, Kraków, Łódź and Katowice), the usable floor area, the year built or the insulation level (before 1980, 1980-2000, 2000-2015, 2017+, 2021+, the periods of Polish building codes), the ventilation type, the indoor temperature, the ceiling height and the number of residents. After an import from the heat loss calculation, the area is already entered and the insulation class is optional, because the heat loss comes from the calculation - including the design temperature you typed there. You add what the calculation does not know, for example the number of residents.
The "Sizing and tariff" section is in both modes:
- the bivalent point - the outdoor temperature down to which the heat pump works alone; monovalent sizing (no backup heater) by default,
- the electricity price - an empty field means the Polish national price. Enter your client's own price, and the cost comparison becomes theirs. The tariff choice (G11/G12) is a Polish one: a flat rate or a day/night rate.
- domestic hot water - check "The pump also heats hot water" and enter the tank reheat power. Without the number of residents, the hot water demand is not estimated, and a remark in the result says so.
Bills and consumption (optional): the current heat source (gas, coal, pellets, wood, oil, electricity), the annual consumption in the fuel's unit and the boiler efficiency. With these data, the algorithm corrects the physical model (70%) with the bill history (30%); without them, it works only from the building data. You enter the bills on the "System" step.
The System step
The heating distribution type: radiators (a higher supply temperature), radiant floor heating (a low temperature, ideal for a heat pump) or mixed.
The Rooms and Emitters step
Optional, but without it you get the heat pump sizing alone, with no radiator check. The rooms are open sections, one below another; on a phone you see one at a time, with a sticky "2 of 7" row and the room list sliding up from the bottom. For each room (up to 50) you enter a name, area, temperature, ceiling height, optionally "Unheated", and then the emitters:
- radiators - the panel type (11, 22, 33 or a bathroom towel warmer), height, length and quantity. These are European steel panel radiators: type 11 is single panel, single convector; 22 double-double; 33 triple-triple.
- radiant floor heating - the tubing spacing (one of five values: 3.9" / 100 mm, 5.9" / 150 mm, 7.9" / 200 mm, 9.8" / 250 mm or 11.8" / 300 mm; usually 7.9" / 200 mm, in bathrooms 5.9" / 150 mm - any other spacing you type only in the radiant floor heating module), the floor covering and the room type (standard, up to a 84 °F floor surface; bathroom, up to 91 °F),
- both at once in a mixed room.
You add a radiator with "Add radiator", and the pencil beside a radiator opens it for editing. Both open the "New radiator" dialog (named after the radiator when you edit): "Save" writes the radiator and "Cancel" discards the changes. The "Also save in My library" switch saves the radiator on your account too, and you pick radiators from My library in the "Type" field - the pick copies the type and dimensions. Choosing a towel warmer sets the room temperature to 75 °F - the dialog says so in a note, and the change applies only on "Save". When you remove such a radiator, the "Remove radiator?" dialog restores the previous temperature.
The "Emitters in every room" section sets radiant floor heating or radiators for the whole building at once; then you correct the exceptions. Changing the distribution type when rooms already have radiant floor heating set up clears that data - the "Change distribution type?" dialog asks you to confirm with "Change and drop UFH data". Check "This is the complete list of all heated rooms" only when every room has been entered - the algorithm then spreads the whole heat loss over the emitters you entered. It is unchecked by default, which is safer. You edit rooms brought in from the heat loss calculation the same way as rooms added by hand; their calculated heat loss is the basis of the radiator check.
The Result step
The header shows the preliminary sizing: the recommended heat pump, the supply temperature, SCOP, the annual electricity use in kWh and the installation cost. Below:
- "How this capacity was reached" - the sizing steps: the design heat loss at the outdoor temperature, the design margin, the output required from the heat pump, the output of the chosen heat pump at that supply temperature and that outdoor temperature (the nominal output drops in cold weather), the headroom above the requirement, the bivalent point and the backup heater output,
- "Variants" - V1 "No radiator changes", V2 "Minimal replacements", V3 "Best efficiency", each with a heat pump, supply temperature, electricity use and cost; the recommended one is the feasible variant with the lowest installation cost,
- "Radiators & feasibility" - for each room the status OK, marginal or infeasible, with the missing output, the required supply temperature and the main cause of the heat loss (transmission, ventilation, thermal bridges),
- "Operating point" - the heat loss against the heat pump's output for the variants,
- "Annual cost" - a comparison with the current fuel at the electricity price you entered; the cost leaves out the backup heater,
- "Warnings" and "Calculation notes" - among others, warnings about a small output margin, about towel warmers, about rooms that cover only part of the area, about oversizing ("Nx more than needed"), and explanations of why a variant is infeasible,
- "Method and result confidence" - the heat loss basis (the heat loss calculation or an estimate from the insulation class) and the model confidence: high, medium, low.
The remark "The radiator sizing solver is temporarily unavailable - the result is incomplete" means the solver is down on our side: the heat pump sizing is computed, the radiator check is not. Come back to the result later.
The count icon on the wizard's bottom bar, beside the step number, collects what blocks the calculation (for example no area, no radiator dimensions); its "Gaps and warnings" window has "Fix" buttons. On the Result step, two square icon buttons take the place of "Next": report notes and the report download. Their tooltips give their names ("Report notes", "Download PDF report"). Both are gray until there is a result - the Result step itself shows what is still missing. See the article on reports.
Where the heat pump data comes from
The solver picks one of 10 generalized output classes, HP-4kW to HP-16kW (13,649 Btu/h to 54,594 Btu/h). Their characteristics - output against outdoor temperature and supply temperature, COP, costs - are averaged from the catalog data of 239 units by 28 manufacturers on the Polish market. It is a reference point for preliminary sizing, not a database of models to pick from. The result says "Catalogue: ... (N brands)", and a remark in the report reminds you to confirm the specifications of a specific unit with the distributor. The module is built for single-family houses: the output classes go up to 54,594 Btu/h, and when none reaches the required output, the result suggests a cascade or lowering the building's heat loss.
Free account and report
On a free account, you see the recommended heat pump with its data. A report unlocks the full list of matched heat pumps and the variant comparison (SCOP, annual use, installation cost), and it covers the whole project - see what is free and what a report unlocks. You can also buy a report for heat pump sizing alone, for that project.
Module settings
The gear on the wizard's bottom bar opens the "Module settings" panel. The "Data sources" section has the source rows ("Heat loss calculation", "Client form") with a "Bring in" button, and an "Algo AI" row with a "Fill in" button, which opens Algo with the start question. The "Units and currency" section has two rows: "Units across the app" (metric or imperial) and "Currency of the heat pump's costs" (Polish zloty, euro or US dollar). The "Module data" section has a "Clear data" row with a red "Clear" button. Until the heat loss has come in through "Data sources", the panel also has a "Heat loss source" section (from the calculation or your own data, with the "Use the heat loss calculation" and "Unlink from the calculation" buttons). At the bottom, a gray line states the module's scope: "Preliminary sizing for a client discussion".
A HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.