Heat pump sizing for a dwelling
How many kW of heat pump a dwelling needs: design heat loss to EN 12831 and the cooling peak, with ΤΟΤΕΕ 20701-3 climate and stated operating conditions.
Heat loss, cooling load and required capacity — always with the operating conditions stated, so quotations become comparable.
Heat pump sizing for a dwelling — method and FAQ
What the calculation answers
The question usually arrives as "I have a 100 m² house, how many kW of heat pump do I need?". It is too general to be answered seriously, but it gets answered every day — by someone. The tool takes it as it stands, asks only for what is strictly necessary, and gives the best number it can while stating explicitly what it assumed.
The winter heat loss and the summer cooling load are calculated together, because the same unit is asked to cover both and the two rarely come out equal.
Why the operating conditions are always stated
Every result carries the pair of temperatures it was calculated at — outdoor air temperature and water temperature. The nominal output at 7 °C with 35 °C water that is printed in the brochures is a different quantity, and does not compare with it.
A pump at −4.5 °C with 55 °C water — Ioannina with radiators — delivers appreciably less than at the nominal condition. Anyone comparing two quotations given at different operating points is comparing nothing. That is why the tool also produces the request you send to your supplier as it stands, so that they answer at your own point.
Why we give SCOP and not COP
Beside every model proposed a seasonal efficiency is printed — SCOP to EN 14825, or the ηs of the ErP fiche — and never a point COP. The reason is practical: COP is systematically misread. The nominal figure is one thing and the operational figure another, and it is rarely stated whether it includes the defrost cycle. Two quotations reading "COP 4.5" may be talking about two entirely different things.
The standard certifies two applications and nothing in between: low temperature at 35 °C and medium at 55 °C. A fan-coil system designed for 45 °C is a medium-temperature system, so it is compared against the 55 °C SCOP — judge it by the 35 °C SCOP and you would flatter it by close to a whole point.
Greece falls under the warm climate, but very few manufacturers publish the warm-climate SCOP. Where it is missing, the AVERAGE climate figure is shown and stated as such; the warm-climate one, where it exists, is marked separately. Ask your supplier for it: for a Greek climate it is the number that concerns you, and it is the more favourable one.
The CHOICE of model, however, is not made on the seasonal efficiency. It is made on the output at your own operating point, from the manufacturer's curves: the seasonal figure tells you what you will be paying, not whether the unit is big enough.
Which data are used
Climate from ΤΟΤΕΕ 20701-3/2010: the winter design temperature from Table 2.1 and the summer conditions from Table 2.2. Where Table 2.1 does not cover the area, the explicit rule of §2.1 is applied — the mean of the mean minimum and the mean absolute minimum monthly temperature — and it is stated that this is the standard's approximation and not a measured percentile.
Fabric from ΤΟΤΕΕ 20701-1/2017: the U values are read by the period of the building permit, which is the only realistic indication of insulation when there is no design file.
Solar radiation from PVGIS of the European Commission: real hourly data on vertical surfaces at four orientations and on the horizontal. The orientations' own peaks are not added — east and west never peak at the same hour.
What it does not do
It does not replace the supplier's quotation. It proposes models that cover the load at your own operating point, from the manufacturer's certified curves, but availability, price and the installation design are not the work of a calculator.
It does not count domestic hot water: in a dwelling the same pump covers it comfortably, and it does not coincide with the heating peak.
It does not replace an engineer's design. It is a preliminary calculation for pre-sizing and for checking quotations.
Frequently asked questions
How many kW of heat pump does a 100 m² (100 sq m) house need?
It depends mainly on three things: the location, the period of construction and the position of the dwelling in the building. The same uninsulated 100 m² house asks for about 16 kW in Athens and 23 kW in Ioannina, because the design temperature changes. A middle-floor flat in the same city asks for a fraction of that, because it loses only through its façades.
Why do two quotations for the same house give different kW?
Usually because they are stated at different conditions. The nominal output is measured at 7 °C outdoor air with 35 °C water. If your house requires 55 °C water and your area reaches −4.5 °C, the same unit delivers significantly less. Ask for the output at your own operating point to EN 14511 and the quotations become comparable.
Do I need a design file to use the calculator?
No. The simple mode asks for six items an owner knows: location, floor area, storeys, period of construction, position in the building and type of emitters. If you do have a design file, the "design data" panel replaces the geometry and fabric assumptions with real figures.
Does an underfloor system cool?
Partly. A chilled slab gives about 40 W per square metre before it starts to condense, and it removes no humidity. When the calculated load exceeds that limit the tool says so, and suggests fan coils at least in the rooms with the most sun. Radiators do not cool at all.
Related tools
- Radiator output at low flow temperatures — Checks existing radiators to EN 442 for a heat-pump retrofit — real output and the lowest flow temperature that suffices.
- Buffer tank sizing — Minimum volume from calorimetry — run time, defrost and hydraulic separation, not a litres-per-kW rule.
- Climate data for Greek locations — Design temperatures, degree days and monthly climate data to ΤΟΤΕΕ 20701-3, for 62 ΕΜΥ stations — with a two-location comparison.
- Solar collector sizing for domestic hot water — How many m² of collectors and how much storage for domestic hot water, and the share the sun covers each month — with your own certificate coefficients.