Bedroom and loft air conditioning in London

Heat rises inside a house and the roof takes the full sun all day. A loft conversion under a slate or tiled roof, often with big rear dormer windows and roof lights, can be several degrees hotter than the ground floor. On the hottest nights in London, the heat island effect means the outside air stays warm too, so opening the window does little. The Met Office explains that after sunset, temperatures stay higher in urban areas than in the countryside around them.

Loft conversions are everywhere in London's Victorian and Edwardian terraces, from Wandsworth and Fulham to Walthamstow and Ealing. They make great bedrooms and offices in winter and very hard ones in a heatwave.

The sound level that matters in a bedroom is the lowest fan speed, often called quiet or night mode. Manufacturers publish it in decibels for the indoor unit. Look for the lowest figure you can find for your size of room and ignore the headline number at full power.

Position matters too. Air should flow across the ceiling and down, not straight onto the pillow. In a loft with sloping ceilings, the unit usually goes on the gable wall or a dwarf wall under the dormer. A sleep setting that lets the temperature drift up slightly through the night saves energy and avoids waking up cold.

Floor area is only part of it. A loft room with a big west facing dormer and thin roof insulation needs more cooling than a ground floor room of the same size. We look at:

• floor area and ceiling height, including sloping sections
• glazing size and direction, and any roof lights
• roof insulation, which you can often see from the eaves cupboard
• whether it is used during the day as an office

Getting this right is what stops the unit running flat out all afternoon.

In a loft, pipes usually go through the eaves and down the rear wall in a neat cover, or through a ceiling void to the back of the house. Condensate from the indoor unit has to drain away, either by gravity to a gutter, soil stack or waste pipe, or with a small pump where there is no fall.

The outdoor unit normally goes on the rear wall at ground floor level or on the flat roof of a rear extension, at least 1 metre from the edge. Putting it high on a rear wall is possible but makes servicing harder. Planning rules and the noise at your neighbour's window apply just as they do for any other room.

A bedroom unit follows the same planning rules as any other. On a house in England, a reversible unit that also heats can be permitted development if it passes the 37 dB(A) noise calculation at the neighbour's nearest bedroom or living room window and meets the size and siting limits. Neighbours' bedroom windows are often the closest point on a terrace, so the outdoor unit position is chosen with that in mind. Flats, listed houses and cooling only units need consent.