One 3.5 kW head does not cool two closed rooms. It cools the room where it is mounted. A shut door leaves the second room outside the airflow. Two rooms need two indoor units, either on a multi-split or as two systems, after a survey. 3.5 kW is a common size for one living room or one large bedroom. It is not a two-room machine.
The back room still hot with the door shut
People buy one head to save an outdoor unit and then find the back room still hot in July. The fan throws air into one space. A bedroom door that is closed for sleep does not. An open arch into a kitchen may share some of that air.
A 3.5 kW head measured for one room
The 3.5 kW figure is a cooling capacity for the room the survey measured. Glass, loft heat and a west-facing wall can use all of it in a single room. Splitting it in half on a sketch does not split the air. It is a common size for one living room or one large bedroom, and it is still one room.
Two indoor units, or a ducted design that is real
A multi-split uses one outdoor unit and an indoor unit in each room, each with its own control. Two separate splits do the same with two outdoor units, which is sometimes right when pipe runs cannot get home to one condenser. Both are surveyed jobs for an F-gas certified company. A ducted indoor hidden in a cupboard can serve more than one grille only when the ducts, the fan and the zones are designed that way.
- One indoor unit serves one room properly.
- Closed doors stop the cooling.
- Open-plan is one space only while the air can move.
- Two rooms mean two indoor units on the survey.
Hundreds of pounds and where the outdoor unit stands
A second indoor unit adds equipment, pipe, a drain and commissioning. It is often cheaper than a second full outdoor unit, and it is more than a single 3.5 kW pair. As a 2026 illustration, the jump from one simple room to two rooms is hundreds of pounds and can be more with pumps and access. That is an illustration, not a price list. In England, the outdoor position still has to meet Planning Portal heat pump conditions. One outdoor unit is usually easier to place than two.
A hall head is not a two-room system
A wall head in the hall is not a ducted design. Stop there if the rooms sleep with the doors shut. The quote has to show an indoor unit in each room you actually need cool.
Frequently asked questions
Will a 3.5 kW unit cool two bedrooms if I leave the doors open?
Some air will drift through open doors, and the far room will lag. If you sleep with the doors shut, fit an indoor unit in each room.
Can one outdoor unit feed both rooms?
Yes, on a multi-split, with an indoor unit in each room, if pipe lengths and the combined load fit that outdoor unit's manual.
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.