The short answer: which system
One or two rooms
The short answer: which system
- Cheapest option and usually fitted in a day
- Cheapest option and usually fitted in a day
- Needs space outside for an outdoor unit per room
Three to five rooms, little outside space
- Up to five indoor units on one outdoor unit
- Every room still has its own control
- If the outdoor unit fails, every room goes off, so if you have the space, separate splits are often better
More than five rooms, or a large house
- One main pipe run that branches to each room, so much longer runs are possible
- Finer control room by room
- Bigger, floor-standing outdoor unit and more design work
Nothing on show
The short answer: which system
- All you see is a grille or slot
- Best planned during a renovation, new build or loft conversion
- By far the most expensive, and it must have return air and service access
Loft room, conservatory, or under a window
Floor console
- Sits low on the wall like a radiator, so it fits under sloping ceilings
- Good for heating as well as cooling
- Costs a little more than a wall-mounted unit
Large open-plan room
Ceiling cassette, or a larger wall unit placed mid-wall
- A cassette throws air four ways from the middle of the ceiling
- Needs enough ceiling void to take it
- A wall unit works too, as long as it's positioned for air throw and not stuck in a corner
The short answer: which system
Where the outdoor unit goes. Room to breathe, away from neighbours’ windows and walkways, reachable for servicing. Flats and leaseholds need the freeholder’s written consent first.
Pipework, drains and power. An outside wall drains by gravity; an inside wall needs a condensate pump. Pipe runs have limits, bigger systems need their own circuit, and in heating mode the outdoor unit drips in winter.
Not a DIY job. Refrigerant work legally needs an F-Gas certified engineer, and the warranty depends on it.
What a proper installation looks like. Site survey, design, written quote, deposit, install with pressure test, vacuum and commissioning, handover, then an installation report with the final invoice. Allow for lead times in summer.
Comparing quotes. Check they’re like-for-like: exact models, pipe allowance, drain method, electrics, commissioning and who registers the warranty. On a home install, VAT should be 0% until 31 March 2027, then 5%.
After it’s fitted. Clean the filters, service it once a year, keep the outdoor unit clear.
Most people who call us already know they want air conditioning. What they don’t know is which type, what size, where it should go, or what a good installation actually looks like.
Those decisions matter more than the brand. Get them right and the system will run quietly and cheaply for 10 years or more. Get them wrong and you end up with a unit that’s noisy, can’t keep up on the hottest days or doesn’t keep up at all, and fails years earlier than it should. This guide walks through the questions we work through on every survey, so you can go into your quotes knowing what to ask.
Start with how you'll use it, not the unit
Before looking at models, be clear on three things.
- Which rooms actually need it. Bedrooms and loft rooms usually get hottest. A home office with a computer running all day can overheat even on mild days. Living areas are often second priority.
- Cooling only, or heating too. Almost every modern split system is a heat pump. It heats as well as it cools, and in spring and autumn it's often a cheap way to top up one room. If heating matters to you, say so at the survey, because it can change the size we'd recommend.
- How much you want to see. Are you happy with a wall-mounted unit on show, or do you want nothing visible but a grille? That one answer narrows the options more than anything else.
The main types of home air conditioning
Indoor units
Panasonic Etherea, garden studio in Richmond. Fitted by Thermacool.
Wall-mounted unit
Fixed high on the wall. The most common choice and usually the best value.
Best for: bedrooms, home offices, most single rooms
Pros: the best air throw of the four, the cheapest to buy and fit, and the easiest to service; many models have Wi-Fi built in
Cons: it’s on show, so where it sits on the wall matters
Floor console under a sloping loft ceiling, Weybridge. Fitted by Thermacool.
Floor console unit
Sits low on the wall, about the size of a small radiator. Warm air comes out at low level, which makes it good for heating as well as cooling.
Best for: loft rooms with sloping ceilings, conservatories, under windows
Pros: good air throw at low level, which suits heating; fits under sloping ceilings and windows; easy to service
Cons: takes up floor-level wall space; costs more than a wall-mounted unit
Four-way ceiling cassette, Barwell Business Park. Fitted by Thermacool.
Ceiling cassette
Set into the ceiling with only a flat panel showing, blowing air out in several directions. Spreads air evenly across a large room.
Best for: large open-plan kitchens and living spaces, home offices and studios
Pros: four-way air throw covers a large room evenly; sits flush in the ceiling; serviced through its own panel
Cons: needs a ceiling void deep enough to take it, so check before ceilings are boarded; mainly suits larger rooms
Cost: ££ an uplift on a wall-mounted unit
Concealed ducted system, Chertsey: two linear grilles in the orangery ceiling are all you see. Fitted by Thermacool.
Concealed ducted unit
Hidden completely. A horizontal unit sits in a ceiling void, loft or bulkhead; a vertical unit stands in a cupboard or boxing. Either feeds air through ductwork to discreet grilles, so all you see is a slot or grille.
Best for: renovations, new builds, loft conversions, anyone who wants nothing on show
Pros: invisible, quiet, and one unit can serve more than one room
Cons: air throw isn’t as strong as the other three, because the air is pushed through ductwork and grilles; harder to service and repair because the unit is hidden, so it needs a proper access panel; must be planned before ceilings and walls are finished
Where the indoor unit goes matters: air throw
Positioning of the indoor unit matters as much as the unit itself. We call it air throw: how far, and how evenly, the unit moves the air across the room.
A wall-mounted unit tucked in the corner of a large room may not work as well as the same unit in the middle of the wall. From a corner, the air runs along one side and the far end of the room stays warm, so the unit works harder for a worse result. Mid-wall and facing down the length of the room, it covers the whole space. It’s one of the first things we settle on the survey.
Plan view of the same large living room, roughly 7 m by 5 m. Same unit, same settings, different wall.
Ducted units: two things that are non-negotiable
1. A return air path. The unit has to draw air back in as well as push it out. Without a proper return air plenum and grille it can’t perform and gets noisy.
2. Service access. Every ducted unit needs an access panel big enough to reach the unit, filter and drain. Boxed in with no access, it can’t be serviced, and a blocked drain becomes a ceiling leak.
Outdoor units: how the system is put together
One indoor unit, one outdoor unit, a short run straight through an outside wall.
Single split system
One indoor unit connected to one outdoor unit by refrigerant pipework, a condensate drain and a cable. Any of the indoor styles above can be a single split.
Best for: one or two rooms, or any home with space outside for an outdoor unit per room
Pros: most affordable, efficient, often fitted in a day, each room completely independent
Cons: each room needs its own outdoor unit
In practice: the Richmond garden studio job from further back. A Panasonic single split outdoor unit on rubber feet, with the pipework in trunking up the wall.
Five bedrooms on one outdoor unit, the most a multi-split can take, each with its own pipe run. Drains not shown.
In practice: a multi-split outdoor unit at Weybridge with the cover off. Three pipe runs, one per room, labelled for the loft, front bedroom and study, with the isolator alongside.
Multi-split system
One outdoor unit runs up to five indoor units, each with its own pair of pipes back to a port on the outdoor unit. Useful when there isn’t room on the outside of the house for several outdoor units.
Best for: two to five rooms where outside space is tight
Pros: one outdoor unit instead of several, tidier outside
Cons: if the outdoor unit fails, every room goes off. Each indoor unit has a maximum pipe run back to the outdoor unit, so a far-away room may not reach (more on this in Step 6).
What is the diversity factor? On a multi-split, the indoor units added together can be rated higher than the outdoor unit. That works because rooms are rarely all running flat out at once: bedrooms work hardest at night, living areas during the day. The difference between the two is the diversity. The upside is a smaller, cheaper outdoor unit. The catch is that on the hottest afternoon with every room on full, they share what the outdoor unit can give, and each room gets a little less. The connectable limit varies by model, and we size it around how you actually use the rooms.
Our honest view: if you have the outside space, two or three separate splits are often the better choice. Full independence between rooms, no single point of failure.
Seven bedrooms over three floors on one outdoor unit: one main pipe run, with branch joints feeding each room. Drains not shown.
Mini VRF (larger homes)
For larger houses, a mini VRF (variable refrigerant flow) system runs many indoor units from one outdoor unit, with every room controlled on its own. Mini VRF outdoor units are bigger, and we recommend floor-mounting them rather than hanging them on the side of the house.
How VRF differs from a multi-split
- More rooms. Not limited to five indoor units; the number depends on the model.
- Different pipework. One main pair of pipes runs through the house and branches off to each room, instead of a separate pair from every room back to the outdoor unit. That allows much longer runs.
- Finer control. Each indoor unit meters exactly the refrigerant it needs, so rooms hold temperature more precisely.
- More design work. Larger refrigerant charges come with rules on minimum room sizes, and some designs need leak detection in small rooms. That's all part of the design.
- Commercial VRF goes further again, with heat recovery systems that heat some rooms while cooling others. It's rarely needed in a home.
See it in practice: our Chertsey Property case study, with concealed ducted units throughout and pipework run underground in twin-wall duct.
Can't have an outdoor unit?
In some flats and listed buildings an outdoor unit isn’t allowed. A through-the-wall unit with no outdoor unit (often called a monoblock) is an option, but it’s noisier, less efficient and needs two large holes in the wall. Worth it only when a split system genuinely can’t be fitted.
What about portable units?
Cheap to buy, but noisy, inefficient, and they struggle to cool a room properly in a heatwave. Fine as a stopgap, not a substitute for a fixed system.
What size air conditioner do I need?
Air conditioning is sized in kilowatts (kW) of cooling. Whichever type you choose, sizing starts with the square metres of each room: measure the length and width and multiply them. Then the room itself adjusts the figure up or down. As a rough guide for a typical UK home:
| Room | Floor area | Typical size |
|---|---|---|
| Small bedroom or study | up to about 15 m² | 2.0 – 2.5 kW |
| Double bedroom or medium living room | about 15 – 25 m² | 2.5 – 3.5 kW |
| Large living room or kitchen-diner | about 25 – 35 m² | 3.5 – 5.0 kW |
| Open-plan space or large loft room | 35 m² and over | 5.0 – 7.1 kW+ |
Treat that table as a starting point only. Floor area is only part of the picture. A proper sizing looks at:
- Glazing and orientation. A south- or west-facing room with bi-folds can need far more capacity than a north-facing room of the same size.
- Loft rooms. Rooms under the roof take on heat all day and usually need more than their floor area suggests.
- Insulation and ceiling height.
- People and equipment. Home offices, gyms and kitchens all add heat.
Why size matters both ways
Too small and it runs flat out and still can’t hold temperature on the hottest days. Too big and it switches on and off constantly, wasting energy, wearing the compressor and leaving the room feeling clammy because it never removes the humidity. A good installer works out the heat load room by room rather than guessing from square metres.
Step 4
Choosing the system: brand, refrigerant and efficiency
Brand and parts support: why it's worth paying for a better brand
You’re not just buying the unit, you’re buying the backup behind it. If a unit goes down in five or ten years, what matters is whether the part is still available and how fast it arrives. Reputable manufacturers keep spare parts available for years after a model is discontinued and have proper UK technical support. Lesser-known brands often don’t, and a unit that can’t be repaired has to be replaced.
We work with Panasonic and Daikin. Both have strong UK parts and technical support, and both are brands we’d put in our own homes. A cheaper unit is a false economy if a part takes weeks to arrive in July.
Warranty: with Panasonic and Daikin you can get up to 7 years’ parts and labour manufacturer warranty when the system is fitted by an accredited installer and registered. We register every install for you.
Refrigerant: why it matters, and why costs are rising
Refrigerants are being phased down under the F-Gas rules to cut their climate impact. R410A is being phased out and can no longer be used in new single split systems. R32 is the current standard: it has a much lower impact and better efficiency, but it will be phased down in time too, and lower-impact refrigerants are starting to appear.
As supply is cut, refrigerant gets more expensive, which pushes up the cost of repairs and top-ups on older systems. Buy R32 now and avoid anything on R410A, however good the price. We don’t install new R410A systems.
Efficiency ratings
Look at SEER (cooling) and SCOP (heating); higher is better. As a rule, the more premium ranges are more efficient, and over 10 years that adds up. If you’ll rely on it for heating, check its heating output in cold weather too. Our running costs guide shows what it means in pounds.
Inverter technology
The compressor speeds up and slows down instead of switching on and off. Quieter and cheaper to run. Almost every modern unit has it; avoid one that doesn’t.
Noise
Controls and Wi-Fi
Air quality features: what's worth having
Dry mode takes the humidity out on muggy days without over-cooling. A self-clean or anti-mould cycle dries the inside of the unit after cooling, which keeps it fresher between services.
Where the outdoor unit goes
- Noise to neighbours. Keep it away from their windows and bedrooms where possible.
- Airflow. It needs clear space to breathe. Boxed into a narrow alley or behind a tight fence, it'll struggle and get noisy.
- Mounting. Wall brackets keep it clear of leaves and kids; ground mounts suit larger units. Either way, on anti-vibration mounts.
- Access. It must be reachable for servicing without a scaffold tower every year.
- Planning. Most homes in England don't need permission for a standard installation, but it depends on the property and position. See our planning permission guide. Listed buildings and conservation areas need checking first.
- Flats and leasehold properties. If you don't own the freehold, you'll almost always need written consent from the freeholder or management company before an outdoor unit or pipework goes on the outside of the building. Check your lease and get consent in writing before booking the install. It's the most common reason flat installs get held up.
Pipework, drains and electrics: where installs go right or wrong
- Pipe route. The shorter and simpler, the better. On the outside wall, pipes belong in neat trunking, not taped bundles.
- Concealing pipes in a renovation. We can first-fix pipework into walls and ceilings before plastering, then come back to fit the units. Far neater, and usually cheaper than hiding pipes afterwards. Bring your installer in early so your builder leaves the right openings.
- Pipe length. Every system has a maximum distance between indoor and outdoor unit. On a multi-split, each indoor unit is typically limited to around 25 m from the outdoor unit, and there's a total limit across all the ports too, so on a five-port unit a far bedroom can push the total over. Runs beyond the pre-charged length need extra refrigerant added and recorded. A quote should allow for the actual route, not the minimum.
- Power supply. A small unit can sometimes be fed from a fused spur off an existing circuit. Larger units, multi-splits and VRF need their own dedicated circuit from the consumer unit. Either way, we check the property can take it: the size of the main supply, a spare way in the consumer unit, and the right protection. A new circuit is notifiable work and must be certified by a qualified electrician. Check your quote includes it.
Why air conditioning produces water, and why it matters
Think of a cold glass of water on a hot summer’s day. Within a minute the outside of the glass is wet. The air around it carries moisture, and when that air touches something cold enough, the moisture condenses out as water. The coil inside an air conditioning unit does exactly the same thing. Room air passes over a cold coil, the moisture in it condenses on the metal and drips into a tray underneath. That’s the condensate, and it has to be drained away.
In cooling
The indoor unit is the cold side, so the water forms indoors and the indoor unit needs a drain. The outdoor unit is the warm side and stays dry.
In heating
The system runs in reverse. Now the outdoor unit is the cold side, so the water forms on the outdoor coil instead. In cold weather it freezes there, and every so often the unit runs a defrost cycle to melt it, so water drips from the base of the outdoor unit. The indoor unit stays dry.
Winter icing. If you’ll use the system for heating, think about what’s underneath the outdoor unit. Defrost water dripping onto a path, step or driveway can freeze into a sheet of ice on a cold morning, and on a route people use every day that’s a slip hazard and a nuisance. We keep outdoor units away from walkways where we can, or fit a drain tray and pipe the water away.
Indoors, where the water can drain to decides where the unit can go.
Why an external wall is the easiest spot
Not on an external wall? You'll need a condensate pump
Can you install air conditioning yourself?
For a standard split system, no. Any work that opens, pressure tests, evacuates or charges the refrigerant circuit must be done by an F-Gas certified engineer. That’s a legal requirement, not a sales line. Manufacturers usually require an accredited installer for the warranty to apply, and the electrics need certifying.
What happens on a proper installation
1
Site survey
We visit your home and look at the rooms, the pipe and drain routes, the electrics and where the outdoor unit could go. We then give you our free advice on what we think is best for you, and answer any questions you have at that point.
2
Design and calculations
Back at the office we work out the heat load room by room, size the units and plan the pipe runs. This is where the system is designed rather than guessed.
3
Written quote and your questions
An itemised quote with exactly what’s included. We answer any questions before you decide, and nothing goes ahead until you’re happy with it.
4
Acceptance and deposit
Once you accept the quote, a deposit invoice secures your install date and we order the kit. We confirm lead times at this point.
5
Installation
Units fixed, pipework run and insulated, drain installed and tested. Three things happen inside this step that a good installer never skips:
-
a
Pressure test. The pipework is pressurised with nitrogen and held to prove it's leak-tight before any refrigerant goes in.
-
b
Evacuation. A vacuum pump removes air and moisture from the pipework. Rushing this is one of the most common causes of early failure.
-
c
Commissioning. The system is run in cooling and heating, and temperatures, pressures and running current are recorded.
6
Handover
We show you the controls, the app and how to clean the filters, and register the manufacturer warranty for you.
7
Installation report and final invoice
A written report with the commissioning readings and photos, for your records and the warranty, sent with the final invoice.
Most single-room installations take one day. Multi-room and ducted systems take longer depending on the layout.
Planning for summer? Allow for lead times
When a heatwave hits, everyone wants air conditioning at once. Popular units can sell out at suppliers, and installers’ diaries fill weeks ahead. If you’re hoping for a quick install in July, it isn’t always possible. Ducted and VRF systems often need kit ordered in, which adds time again. Our advice: get your survey and quote done in spring, and you’ll be cool before the first hot spell. We’ll always confirm kit availability and a realistic install date with your quote.
How to compare air conditioning quotes
Quotes vary widely, and the cheapest is rarely like-for-like. Check each one covers:
- Exact make and model of every unit
- Pipe length allowance, and the cost of any extra
- How the condensate drains (gravity or pump)
- External trunking or concealed pipework
- Electrical supply and certification, included or extra
- Brackets or ground mounts with anti-vibration feet
- Pressure test, evacuation and commissioning
- Warranty length, and who registers it
- F-Gas certified engineers doing the work
- Making good and removal of waste
VAT: 0% on home installations until 31 March 2027
A fixed air conditioning unit that heats as well as cools counts as an air source heat pump under HMRC’s energy-saving materials rules. So when we supply and install one in a home, the job is zero-rated for VAT, pipework and electrics included. From 1 April 2027 the rate is due to go back to the 5% reduced rate, so if you’re planning an install, sooner is cheaper. The zero rate only applies to supply and fit: buying a unit on its own is charged at 20%, and so is work in commercial premises. Check your quote states the VAT rate it’s using.
For typical prices, see our air conditioning cost guide.
Step 10
After it's fitted
- Clean the filters every few weeks during heavy use. It takes two minutes.
- Have it serviced once a year. Coils and drain cleaned, performance checked, small problems caught early. Many warranties require it. See our servicing page.
- Keep the outdoor unit clear of leaves, plants and garden furniture.
Frequently asked questions
What size air conditioner do I need for a bedroom?
Is a multi-split better than separate split systems?
Can I have air conditioning with no visible indoor unit?
Yes. A concealed ducted system hides the unit in the ceiling or loft and leaves only a grille. It’s best planned during a renovation and needs a return air path and service access.
Does air conditioning heat as well as cool?
Yes. Almost all modern split systems are heat pumps and provide efficient heating as well as cooling.
Can I install air conditioning myself?
How long does installation take?
Which refrigerant should a new system use?
R32 is the current standard. R410A is being phased out and will keep getting harder and more expensive to service.
Is air conditioning a heat pump?
Do I need a condensate pump?
Only if the indoor unit can’t drain by gravity, usually because it’s on an internal wall. Units on an external wall normally drain straight outside with no pump.
Why is my outdoor unit dripping water in winter?
How many rooms can one outdoor unit run?
Is there VAT on air conditioning installation?
James Cretella
Not sure which system suits you?
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