A Passivhaus has tiny heating and cooling loads because the fabric, the airtightness and the ventilation were designed together. A standard domestic split, oversized because it is the size a fitter usually carries, will short-cycle and overshoot. Opening windows while it runs throws away the airtight layer the house relies on. Survey the Passivhaus designer before anyone orders equipment or cuts a hole. The hole is an airtightness detail. F-gas rules still apply to the refrigerant circuit.
A 5 kW lounge unit short-cycling in a calculated house
These houses were calculated to need very little heat, and often very little cooling if shading and the ventilation strategy are doing their job. A 3.5 kW or 5 kW indoor unit that is normal in an ordinary lounge is large here. It cools or heats in a rush, reaches the sensor, stops, and starts again. That is short cycling. It is noisy, less comfortable, and hard on the compressor.
The load already written on the drawings
Ask the designer for the room load they already calculated. Size from that figure. Do not import a rule of thumb from an uninsulated Victorian room. The installer then matches a unit that can modulate down to that load.
- Use the designer's load, not a default kW.
- Expect a small capacity, sometimes the smallest in a range.
- Treat the wall penetration as an airtightness detail.
- Do not open windows to help a running split.
Purge windows and a split on the same afternoon
Passivhaus summer comfort often includes shading, a ventilation strategy, and sometimes purposeful opening for purge. That is the designer's sequence. Running a split with windows flung open at the same time undoes it: the unit tries to condition the garden, the airtightness you paid for is bypassed, and the MVHR balance is disturbed.
Agree when the split is allowed to run and when windows are the tool. The two are not a casual mix on the hottest hour without that conversation.
Minimum capacity and the membrane
The Passivhaus designer, or the certifier's documentation, knows the loads, the airtight layer and where a penetration is least harmful. An air conditioning survey that ignores those drawings will put a hole through the wrong membrane and pick a unit that cannot modulate low enough. Inverter units that can turn down a long way are the relevant machines. The brochure name matters less than the minimum capacity.
Stop before kit is ordered or the layer is cut
Survey the designer on the load and on the airtight penetration before equipment is ordered. Outdoor siting in England still sits under the Planning Portal conditions, even on a high-performance house. F-gas rules still apply to the refrigerant circuit. The pipe hole is not a small detail.
Frequently asked questions
Can I fit a normal lounge split in a Passivhaus?
Only if the load says so. Most of these rooms need a much smaller capacity. An oversized unit short-cycles and the open-window habit undoes the fabric.
Who decides the size?
Start with the Passivhaus designer and the calculated load. The installer then matches a unit that can modulate down to that load.
Core buying guides
Start with these high-intent guides, then dig into the rest of the library.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.