A domestic system is sized from the room’s heat gain – glazing, orientation, occupancy and equipment – not from its floor area. Siting the outdoor unit matters as much as the indoor one, because noise and condensate are what cause problems after installation.
Sizing from heat gain, not floor area
Every online BTU calculator asks for room dimensions and produces a number. It is a starting point and it is frequently wrong, because two rooms of identical size can have completely different cooling loads.
What actually determines the load:
- Glazing area and orientation. A west-facing room with a large window takes an enormous solar gain in the afternoon. The same room facing north takes a fraction of it.
- Whether the glazing has solar control or external shading. This changes the number substantially.
- Occupancy. People are heat sources. A home office with one person is a different load from a living room with six.
- Equipment. Servers, computers, kitchen appliances and televisions all put their entire electrical consumption into the room as heat.
- Construction. A well-insulated modern room holds cool air; a solid-wall room with a south-facing roof does not.
- Ventilation and infiltration. Every cubic metre of warm outside air entering has to be cooled.
- What is above. A room directly under an uninsulated roof is a much harder proposition than the same room on a middle floor.
An oversized unit cools the room to setpoint quickly, then shuts down. Because it ran briefly, it has not removed much moisture, so the room is cool and clammy. It then short-cycles – on, off, on, off – which is both inefficient and hard on the compressor. A correctly sized unit runs longer at lower output, dehumidifies properly and is more comfortable.
The types, and which suits what
| Type | Description | Best for |
|---|---|---|
| Single split | One indoor unit, one outdoor unit | A single room – bedroom, home office, conservatory |
| Multi-split | Several indoor units on one outdoor unit | Two to five rooms, where only one outdoor position is available |
| Ducted | Concealed unit in a ceiling or loft, ducted to grilles | Where wall units would spoil a room, or several rooms from one unit |
| Floor / console | Low-level indoor unit | Rooms with no usable high wall, and better for heating mode |
| Cassette | Recessed into a ceiling, four-way discharge | Larger rooms with a suspended or accessible ceiling |
For most Norfolk houses the practical answer is a single split in the room that actually needs it – typically a top-floor bedroom or a west-facing room – rather than whole-house cooling. Multi-splits are efficient when several rooms genuinely need it and there is only one sensible outdoor position; they are less flexible than separate systems because all the indoor units share one outdoor unit’s capacity.
Siting, which causes more problems than sizing
The outdoor unit
This is where installations go wrong socially rather than technically. A condenser needs clear airflow in and out – crowding it against a wall or fence, or boxing it in decoratively, makes it work harder and run louder. It also needs to be somewhere that its noise does not become a problem.
Noise is the thing to think hardest about. A modern inverter condenser is not loud, but it is a continuous mechanical sound outdoors, often running in the evening when everything else is quiet, and sound reflects off hard surfaces. A unit on a wall facing a neighbour’s bedroom window across a narrow gap is a dispute waiting to happen. Under a window, in a narrow passage between houses, or in an enclosed courtyard are the three positions most likely to cause trouble.
Planning: a domestic air conditioning unit is generally permitted development within limits on size, position and noise, with exceptions for listed buildings and conservation areas. The permitted development rules for air source heat pumps are the relevant comparison, and the conditions are worth checking against your specific property rather than assumed.
The indoor unit
High on a wall for a wall-mounted unit, with clear air distribution and not blowing directly at where people sit or sleep. It also needs a route for the pipework and, critically, for the condensate.
Condensate
An air conditioner in cooling mode pulls a substantial amount of water out of the air, and that water has to go somewhere. Gravity drainage to an appropriate point is the reliable answer. Where gravity is not possible, a condensate pump is used – and a condensate pump is a moving part in a ceiling void that will eventually fail. A failed condensate pump means water in a ceiling. Where a gravity run is achievable it is always the better answer.
What the installation involves
- Survey and load calculationRoom by room: glazing, orientation, occupancy, equipment, construction. Plus the practical questions of where units can go and how pipework will be routed.
- Agree positionsIndoor and outdoor unit positions, pipework route, condensate route and electrical supply. Walked through on site, not decided from a plan.
- Core drilling and pipeworkRefrigerant pipework in insulated copper, condensate drain, interconnecting cable and power. Everything routed and sleeved properly.
- Mount the unitsIndoor bracket fixed to something solid, outdoor unit on wall brackets or a ground stand with anti-vibration mounts.
- Pressure test and evacuateThe step that separates a good installation from a bad one. The system is pressure tested with nitrogen, then evacuated with a vacuum pump to remove air and moisture before refrigerant is released.
- CommissionRefrigerant charge verified, operating pressures and temperatures checked, controls set up, and the system run in both modes.
- Handover and recordsDemonstration of the controls, filter cleaning shown, and the refrigerant charge recorded.
Air and moisture left in a refrigeration circuit react with the oil and form acid, which destroys the compressor over time. A proper vacuum, held and verified, takes time. It is invisible in the finished installation and it is the single biggest determinant of how long the system lasts.
Heating as well as cooling
Almost every modern split system is a heat pump and will run in reverse to heat the room, often very efficiently. In a Norfolk shoulder season – October, April – a split system heating one room is frequently cheaper than firing the whole central heating system for it.
Two caveats worth knowing. Efficiency in heating mode falls as the outside temperature drops, so on the coldest days it is less impressive. And a wall-mounted unit blows warm air from high on the wall, which is thermally the opposite of where you want it – floor or console units are considerably better for heating and worth considering if heating is a real part of the intention rather than an afterthought.
This is also why the comparison with a proper air source heat pump matters – see air conditioning vs a heat pump.
Questions we get asked about this
Do I need planning permission?
Domestic units are generally permitted development within limits on size, siting and noise, but there are exceptions – listed buildings, conservation areas, and positions on or near a principal elevation. We will flag anything that looks like an exception, and the local planning authority is the definitive answer.
How noisy is it?
Indoor units are quiet – a low fan noise. The outdoor unit is the consideration, particularly at night and particularly in a narrow gap between houses where sound reflects. Siting is what determines whether it is a problem, and it is worth thinking about properly before drilling.
Will it heat as well as cool?
Nearly all modern splits do, and efficiently in mild weather. Efficiency falls as it gets colder outside. A floor-mounted unit heats a room considerably better than a high wall unit, because warm air rises.
How much does it cost to run?
It depends on the unit’s efficiency, how hard it is working and how much you use it. A correctly sized inverter unit cooling one room for a few hours on a hot evening is not expensive. An oversized unit short-cycling, or a system cooling a room that is taking a large uncontrolled solar gain, is a different matter.
Can one outdoor unit serve several rooms?
Yes – that is a multi-split. The trade-off is that the indoor units share the outdoor unit’s capacity, so simultaneous demand has to be designed for. Separate single splits are less elegant and more flexible.
Where does the water go?
Cooling produces condensate, which drains to an appropriate discharge point. Gravity is far preferable to a pump – pumps are moving parts in ceiling voids and they eventually fail, which means water where you do not want it.
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
- Norwich
- Wymondham
- Dereham
- Attleborough
- Aylsham
- North Walsham
- Cromer
- Diss
- Loddon
- Long Stratton
- Watton
- King's Lynn
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.