The air conditioning installation process

A single-room installation is usually a day's work, and most of it is unremarkable — brackets, a core hole, some copper and a tidy run of trunking. But there is one stage in the middle that separates a system which lasts fifteen years from one that fails in three, and it is the stage most homeowners walk away from because it looks like nothing is happening.

Knowing what that stage is, and watching for it, is the most useful thing on this page.

Before the day

The survey. A proper survey produces a heat-load calculation for each room, a decided position for the indoor and outdoor units, a pipe route, and a condensate solution. If you have not been given a capacity figure and a reason for it, the design is not finished.

Permissions. For most houses in England a reversible system within the Class G limits is permitted development [3]. Flats, listed buildings and cooling-only units are the exceptions — see our planning permission guide. Confirm the position before equipment is ordered, not after.

Preparation. Clear access to both the indoor position and the outdoor position, move furniture away from the walls involved, and agree parking and materials access. On a shared drive or a terraced street this is worth a conversation with neighbours the day before.

What happens on the day

1. Setting out. The installer confirms the agreed positions, checks for cables and pipes in the wall before drilling, and marks the core hole. A good installer re-checks the position with you rather than assuming the survey note is still what you want.

2. Mounting. The indoor bracket goes up — level, and fixed into something structural rather than plasterboard alone. The outdoor unit goes onto wall brackets or a ground stand, on anti-vibration mounts.

3. Core drilling. A single hole through the external wall, usually 60–80mm, angled slightly downward to the outside so condensate and any rain drain away from the building rather than into it. That slight fall is a detail worth knowing about, because getting it wrong causes damp patches years later.

4. Pipework and cabling. Refrigerant lines, insulated along their full length, plus the interconnecting cable and the condensate drain. Insulation gaps are where condensation forms and drips inside walls.

5. Pressure test and evacuation. This is the stage that matters.

6. Electrical connection. Either to an existing suitable circuit or to a new dedicated one. A new circuit typically adds £400–£700 and should have been priced in the quote.

7. Commissioning. Refrigerant released or topped up as required, the system run in both cooling and heating, temperatures and pressures checked, and the controls demonstrated to you.

8. Making good and clean-up. The core hole sealed inside and out, trunking finished neatly, and the site left clean.

How long it takes

Add time for scaffolding where the outdoor unit is out of ladder reach, and for chasing pipework into walls rather than surface trunking.

What you should be handed at the end

Do not let the van leave without this:

• A commissioning record stating the refrigerant type and the charge in kilograms and in tonnes of CO₂ equivalent. You need this to know whether any leak-check duty applies [1], and records must be kept where it does [2].
• The F-Gas certification details of the engineer and company who did the refrigerant work.
• Manufacturer documentation and warranty registration, which some manufacturers make conditional on accredited installation.
• The controls explained, including night mode and the heating changeover, in person rather than by handing you a manual.
• A service interval recommendation — see how often air conditioning should be serviced.

The three things that go wrong

Condensate. By a wide margin the most common post-installation complaint. Either the drain falls the wrong way, or the pipe is too small, or a pump was needed and not fitted. Symptoms appear as water marks below the indoor unit weeks later.

Vibration. An outdoor unit hard-mounted to a lightweight wall or a hollow structure transmits a low hum into the house that no amount of settings will fix. Anti-vibration mounts and a solid fixing point prevent it.

Undersized or oversized capacity. Undersized never reaches temperature on hot days. Oversized short-cycles, controls humidity poorly and is noisier. Both come from skipping the heat-load calculation, and neither is fixable without new equipment.

Frequently asked questions

How long does air conditioning installation take? A single-room installation is usually a day. Multi-room systems take two to three days. Ducted retrofits take considerably longer because of the builder's work.

How messy is it? Less than people expect. The main dust event is the core drill, which a good installer manages with a vacuum at the drill point. Chasing pipework into walls is a different order of disruption and should be discussed in advance.

Do I need to be home? Yes, ideally for the whole day, and particularly for the commissioning handover. It is also the only way to see the vacuum stage happen.

Can I install it myself? No. Work on the refrigerant circuit is legally restricted to F-Gas certified engineers, and self-installation voids most manufacturer warranties.

What if the installer wants to skip the vacuum? Do not accept it. It is the difference between a system that lasts and one that fails early, and the failure will arrive long after any goodwill has expired.

Will they make good the hole? Yes — sealing inside and out is part of a competent installation. Confirm it is included rather than assumed.

Get an installer who does it properly

The difference between installers is rarely the equipment. It is whether the calculation was done, the vacuum was held, and the paperwork exists. Tell us about the property and we will match you with one vetted installer — one enquiry, one installer, and your details stay private until they accept: get an installer match.