A west-facing room peaks late. Sun on that glass in the afternoon is a different load from a cloudy morning visit. If you occupy the room after work, the survey has to be told that. There is no honest rule that west-facing means one fixed extra kilowatt.
The peak is when you are home
East-facing glass does its worst in the morning and has often eased by the time people finish work. West-facing glass does the opposite. The wall and the floor keep some of that heat after the sun has moved, so the room can still be climbing at supper. A calculation that uses floor area alone, or a visit at 10am under cloud, misses the hour you care about. Tell the surveyor the hours you actually sit there, and whether the door to a hotter landing or kitchen stays open.
Blinds and curtains change the gain only if you will use them before the room is unbearable. A blind you refuse to close because it makes the room dark is not a blind in the calculation. Internal blinds help less than stopping the sun outside the glass, and outside shading is a planning and a fixing question of its own. Energy Saving Trust treats a proper survey as the basis for an air-to-air system, not a guess from the postcode.
- The hours you use the room, not the hours the surveyor prefers to travel
- How much of the west wall is glass
- Which blinds you will really close
- Whether the door to a hotter room stays open
The outdoor unit on the same hot wall
A condenser in full afternoon sun can be noisier and less effective than the catalogue figure taken in milder air. That does not mean you add a mystery percentage indoors. It means the outdoor position is part of the same survey. Shade from a deep soffit is useful. A box, a tight fence or ivy around the coil is not shade. It is a blocked fan. Leave the service gaps the manufacturer prints. In England the Planning Portal conditions for an air source heat pump still apply to that outdoor unit, including where it may go and the rule that it is not used solely for cooling. A west wall that faces a neighbour's window is a noise problem as well as a sun problem.
Frequently asked questions
Are west-facing rooms always one size larger?
No. Glass area, shading you will actually use, insulation and the hours you occupy the room all move the load. A fixed extra kilowatt is a sales shortcut.
If I promise to close the blinds, can I buy the smaller unit?
Only if those blinds are in the survey and you will close them before the peak, not after the room has already stored the heat. A promise you will not keep should not be in the calculation.
The outdoor unit is on the west wall. Does that change the indoor size by itself?
It changes how the outdoor unit works in the sun, and it may be a bad place for noise. It is not an automatic indoor upgrade. Deal with the position on the drawing.
Core buying guides
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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.