What the tool does
Rules of thumb per square metre oversize equipment, and heat pumps run badly when they are oversized. The tool adds up each room from its surfaces and air, and shows every figure.
- Heat loss per room: U × A × ΔT for every wall, window, roof, floor and door, to outside, to a space at a set temperature or with a factor b, plus air changes and mechanical ventilation with heat recovery, and an allowance you choose.
- Constructions: U-values from layers, U = 1 / (Rsi + Σ d/λ + Rse), or entered directly.
- Cooling estimate: fabric with your outdoor or equivalent temperature difference, solar gain on glass by orientation with SHGC and shading, people, lighting, equipment, and ventilation, sensible and latent.
- Totals and equipment: per room and in total, per floor area, in W, kW or Btu/h, and the smallest unit from your list that covers the load with its oversizing.
- Report: a PDF with the design conditions and your sources, each room's elements and results, and a workbook in which every heat loss line is a formula.
How to use it
- Choose SI or US units and enter the outdoor design temperatures for heating and cooling with their source.
- Build the constructions you use from layers, or keep U-values from the product data.
- Add the rooms with floor area, height, indoor temperatures, air changes and gains, then their elements.
- Enter the units you can offer, read the loads and export the report.
What is built in
Only the formulas. Design temperatures, material conductivities, surface resistances, solar gains and internal gains come from national rules, climate data and product sheets that differ by country, and some are in paid standards, so you enter them and note the source.
How it was checked
Before every release the engine is run on published worked examples: a 20 m² living room with 24 m² of wall at U 0.24, 30 K, 0.5 air changes and 0.34 loses 172.8 + 255 = 427.8 W, 470.6 W with 10 % (autarc); a 12 × 14 ft corner bedroom at 70 °F ΔT loses 2 816 Btu/h through its surfaces and 847 Btu/h by air, 3 663 Btu/h in total (Arctic Heat Pumps). U-values from layers, unheated neighbours, heat recovery and the cooling sum are tested against hand calculations.
Limits
This is a preliminary estimate, not a calculation to EN 12831, ACCA Manual J or ASHRAE, and not a document for permits or heat pump grants. The cooling estimate is steady-state: it ignores thermal storage and the time of the peak, so it can overstate the peak. Thermal bridges, ground losses in detail, reheat after setback and duct design are not covered.
Last checked 2026-10-20