Better to choose a larger heat pump? Why the heating load is what matters

The old boiler had significantly higher output than the heat pump in the quote. Will that be enough in January? Or should I choose a larger unit straight away just to be safe? This question keeps coming up, especially during the heating season. Often followed by the comment that modern units can throttle their output anyway, so a few extra kilowatts won't hurt.

The short answer

Bigger doesn't mean safer. The benchmark is the calculated heating load of your house, not the output of the old boiler and certainly not a gut feeling. The klimaaktiv guide So läuft Ihre Wärmepumpe rund ("How to keep your heat pump running smoothly") notes that heating systems in practice are frequently oversized. That costs you twice: at purchase through the higher price and later during operation because efficiency and service life suffer.

What the heating load is

The heating load tells you how much heat output your house needs on a very cold day to stay warm inside. It depends on insulation, windows, location and individual rooms. Calculations are made according to standards: the klimaaktiv guide to installing heat pumps cites two sets of rules for this, ÖNORM EN 12831 and alternatively ÖNORM H 7500. The guide is even clearer. Estimates and rule-of-thumb figures are not enough; you should explicitly ask which standard your planner uses.

The old boiler is of little use as a comparison. It was often chosen with plenty of headroom, and if windows have been replaced or walls insulated since then, the house needs less heat today. klimaaktiv therefore advises looking at the building envelope first: anyone who renovates beforehand can get by with a smaller and cheaper unit.

And what about modulation?

The objection has a core of truth. Modulating heat pumps alter the speed of their compressor, delivering exactly as much heat as is currently needed. According to the klimaaktiv guide, they work significantly more efficiently than units that can only switch fully on or fully off.

However, modulation also has a lower limit. If a unit is far too large, the house needs less heat on mild autumn and spring days than the heat pump delivers at its lowest setting. Then it continuously switches on and off anyway — it cycles. Good planning is meant to avoid precisely that. The guide states it plainly: oversizing must be avoided.

Safety margin yes, but calculated

This does not mean that every safety margin is wrong. According to the guide, justified allowances are added to the standard heating load, for instance for shutdown periods when the electricity provider may switch off the heat pump and the house has to be heated up again afterwards. The difference is that such an allowance comes from a calculation and not from a blanket better go one size larger.

Why it's worth the effort

With a heat pump, efficiency means hard cash, year after year. The klimaaktiv guide demonstrates: if a system achieves a seasonal performance factor of 4 instead of 3, electricity costs fall by a quarter for the same initial investment. A correctly sized unit is one of the key factors for this, alongside a low flow temperature and proper balancing.

What you can do specifically

• Insist on a heating load calculation. It belongs in the quote, calculated according to one of the two standards, ideally with values for each room. A planner who charges for this is more likely to work thoroughly.
• Provide energy consumption figures from recent years. Litres of oil, cubic metres of gas or kilowatt-hours help with cross-checking. They do not replace the calculation.
• Ask for the expected seasonal performance factor. klimaaktiv provides the calculation tool JAZcalc for this, which installers can use to estimate efficiency right at the planning stage.
• Query quotes with very different capacities. If two quotes are far apart, ask both installers what their figure is based on.
• Consider renovation first. If insulation is planned anyway, factor it in before buying the equipment.

Our first assessment

The worry about freezing in winter is understandable, but a unit that is too large doesn't solve it — it merely shifts the problem to the shoulder seasons and onto your electricity bill. A properly calculated heating load is the best insurance, and it costs very little compared to the system itself.

Our heating load calculator provides an initial estimate for your home, while our heat pump calculator shows what that means in annual electricity use. If you want to compare quotes from installers who calculate the heating load properly, send us your request for quotes.