Multi split air conditioning in London

Under the permitted development rules for heat pumps, a terraced or semi detached house can have only one outdoor unit, and a detached house two. A block of flats gets one in total. If you want to cool three bedrooms in a Fulham terrace with separate split systems, you would need three outdoor units, which would mean a planning application and a crowded back wall. A multi split solves that with one outdoor unit serving all three rooms. Remember that the unit must also be used for heating, not only cooling, to fall under those rules. The full list is on the Planning Portal.

Each outdoor unit has a maximum number of indoor units and a maximum total capacity, set by the manufacturer. Domestic multi splits commonly serve two to five rooms. The outdoor unit is sized for the realistic peak load of the house, which is usually less than the sum of every room at full power, because the sun does not hit every room at once.

There is a trade off. If the outdoor unit fails, every room on it loses cooling. And a multi split with one tiny room and one big room can run less efficiently than two well matched single splits. We explain the options plainly at the survey.

Every indoor unit needs its own pair of refrigerant pipes, a cable and a condensate drain back to the outdoor unit. In a London terrace these usually run:

• through the loft and down an internal wall
• under floorboards to the rear of the house
• in neat covers down the rear wall

If you are refurbishing, it is far easier to run pipes before plastering and floors go down. We can first fix the pipework early and come back to hang the units at the end.

With most multi split ranges, you can mix wall units in bedrooms with a ceiling cassette in a kitchen or a hidden ducted unit in a living room, all on the same outdoor unit, as long as the manufacturer allows that combination. Each room has its own remote or app control, so a home office can cool during the day while bedrooms are only cooled at night.

Because one outdoor unit serves the whole system, its position matters more than on a single split. It is larger than a single room unit, so it needs space, airflow and a spot that passes the noise calculation at every neighbouring window. On a London terrace that is usually the flat roof of a rear extension, set at least 1 metre back from the edge, or a low position at the end of the garden. On a semi, the side passage often works if there is room to service it.