A lineset is the pair of copper pipes that carries refrigerant between the indoor unit and the outdoor unit, plus the insulation on those pipes. On a UK install the F-gas engineer makes the joints, checks the run against the manufacturer limits and connects the interconnecting cable. The homeowner's decision is the route: trunking, a cupboard, or a chase the builder finishes before plaster. Length and height difference belong on the quote.
Liquid line, gas line, insulation and cable
One pipe is the smaller liquid line. The other is the larger gas line. Both are insulated so the cold pipe does not sweat and so heating mode does not waste energy into a loft. A communication cable usually follows the same route. Condensate is a separate water pipe and should not be confused with the lineset. Joints are part of the refrigerant circuit. They are made by a certified installer, then the system is commissioned as a whole. There is no safe householder version of that work, and this page will not describe it. GOV.UK publishes certification for companies working on equipment containing F-gas. Buy the installation as one commissioned system. Loose copper and a separate indoor are how warranty and refrigerant rules get lost. The certified installer supplies the pipe to the surveyed length.
Trunking, lofts and the manual's length
White plastic trunking on an external wall is the ordinary UK finish. A longer run through a loft, a boxed-in soil pipe or a builder's chase looks tidier and costs more to do properly. Every extra metre can add refrigerant allowance, clips and making good. The installation manual sets a maximum, a minimum and sometimes a maximum height difference for that model. There is no single UK domestic maximum. The installer does not stretch those limits to suit a prettier outdoor wall. The quote should state the measured run. Bends must be formed, not kinked. Crushed insulation is a condensation stain waiting for next summer. When you walk the route with the surveyor, look at gutters, fascia, neighbour boundaries and where a ladder can stand. Ask for pipe length, any extra refrigerant the manual requires for that length, trunking colour, and who patches the wall. As a 2026 illustration, once the run leaves a short back-to-back route, labour and materials climb in steps of hundreds of pounds including VAT rather than tens. A first-floor indoor with a ground-floor condenser is a different job from a hole straight through behind the unit. Keep the commissioning record with the model numbers and the F-gas company details. Energy Saving Trust's air-to-air pages describe the equipment those pipes serve. Outdoor position for heating plant is a separate Planning Portal question.
Frequently asked questions
Can I buy the lineset and have someone connect it later?
Buy the installation as one commissioned system. Loose copper and a separate indoor are how warranty and refrigerant rules get lost. The certified installer supplies the pipe to the surveyed length.
How long can a lineset be?
As long as that manufacturer's manual allows for that model, including height difference. There is no single UK domestic maximum. The quote should state the measured run.
Does the lineset include the water drain?
No. The lineset is the refrigerant pipework. Condensate is a separate drain, by gravity or pump. Ask for both on the drawing so one is not forgotten.
Is a pre-charged DIY hose kit a lineset?
A pre-charged DIY hose kit from an overseas listing is not the lineset a UK warranty expects. Domestic jobs here are piped to length and commissioned by an F-gas company. The indoor unit alone, sold without a matched outdoor unit, is not a system.
Do decorative covers change the pipe limits?
Vacuum-formed decorative covers hide pipes. They do not change diameter, length limits or the need for insulation that stays continuous through the cover. Ask the quote to include the cover if you want it, so it is not a later extra.
What should the commissioning record hold?
Keep the commissioning record with the model numbers and the F-gas company details. A later engineer needs that history more than they need the brochure. The quote should also state pipe length and any extra refrigerant the manual requires for that length.
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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.