Which rooms benefit most

In most UK homes the heat isn't spread evenly. One or two rooms get noticeably hotter than the rest, and they're often the ones you most need to be comfortable in. Starting with those rooms usually gives the most comfort from the fewest units, and leaves room to add more later.

Start with the rooms that overheat

A room that's uncomfortable on a warm afternoon but fine by evening may manage with blinds, shading and ventilation. A room that stays hot into the night, or that you can't use on warm days, is the stronger case.

Bedrooms

Bedrooms are the most common choice. Heat rises and collects upstairs during the day, bedroom windows often face the afternoon sun, and a room that's still warm at bedtime makes it hard to sleep. Sleeping in hot weather covers what helps with or without air conditioning.

For a bedroom, the indoor unit's figure on its lowest fan speed matters more than its maximum output, and so does placement. A unit that blows straight at the bed is uncomfortable however quiet it is. If you'll also use it to take the chill off on winter mornings, heating performance counts too.

Rooms you use during the day

Top floors and loft conversions. Rooms under the roof take heat through the roof itself and through roof windows, and they often run the hottest in the house. They can be harder to fit, because sloping ceilings leave less wall high enough for a unit. Loft rooms and top floors goes into the options.

Home offices. A computer, screens and a person working all add heat, and the room is in use through the warmest part of the day. Keeping a home office comfortable covers the rest of the picture.

Living rooms and kitchen-diners. Large windows, glazed doors and roof lights let in a lot of sun, and open-plan spaces need a unit sized for the whole area rather than the part you sit in. In a kitchen, keep the indoor unit away from the hob, where grease and steam would coat it.

Conservatories and glass extensions. These gain heat very quickly in sun and lose it quickly in cold weather. Air conditioning can make them usable for more of the year, but they need careful sizing, and the result depends heavily on the glazing and any shading.

Rooms that usually benefit less

• North-facing ground-floor rooms often stay cool enough without help.
• Hallways and landings are places you pass through, and cooling them does little for the rooms around them.
• Bathrooms usually aren't suitable places for a standard indoor unit, as installation manuals often warn against places with a lot of steam or humidity.
• Rooms you rarely use are hard to justify, though a spare room that doubles as an office may be different.

A unit in one room mainly looks after that room. Leaving doors open lets some cool air drift elsewhere, but it won't cool a whole floor evenly, and the unit has to work harder trying.

Cooling more than one room

If several rooms need it, you can fit separate systems or connect more than one indoor unit to a single outdoor unit as a multi-split system. Each room still gets its own indoor unit, sized for that room.

Planning rules can make the choice for you. In England, permitted development usually allows one heat pump on a semi-detached or terraced house and two on a detached house, so a multi-split is often how a house cools several rooms without applying for planning permission. Flats follow different rules, and Scotland, Wales and Northern Ireland have their own.

If you're starting with one room but may add others later, say so at the start. It can affect the choice of outdoor unit, where it goes and how the pipes are run.