Copper sizes on a split are a pair: a smaller liquid line and a larger gas line. The installer selects them from the manufacturer's limits for that model, that capacity and that length. A homeowner should see the diameters written on the quote, next to the model codes, and should not substitute a forum chart. Common small wall units are often written up with a narrow liquid line and a wider gas line. The manual still wins if the run is long or the indoor is larger.
Inch sizes on the quote are a selection, not a shopping list
The liquid line carries dense liquid. The gas line carries vapour and is larger. Insulation thickness sits on top of those diameters and is part of whether the pair fits a wall opening. Quoting a copper size without the insulation is how a neat drawing fails on site. You will see inch sizes on UK paperwork because the trade still speaks that way, sometimes beside a metric equivalent. Treat those figures as the installer's selection for this system. They are not a shopping list. Either inches or millimetres is fine, as long as both pipes are identified and they match the manual for the model being installed. Ambiguous notes such as standard pipe are not a specification. There is no one standard pipe size for UK splits. Small wall units often use a familiar small pair, and larger indoors step up.
Length, height, old copper and the chase already drilled
A small bedroom pair and a large commercial pair do not share copper. Extra length, a big height difference, or a multi-split leg into a branch box can move the size or add a refnet the manual names. Upsizing on a hunch, or reusing a fatter old R410A pair because it is already in the wall, is an engineering decision for the new manufacturer's rules. It is not automatically allowed. Only if the new manufacturer's instructions allow that diameter, condition and preparation for the new refrigerant may the old copper stay. The survey decides. The certified installer owns that choice and the joints. Nothing here is a guide to flaring, brazing or charging. If the existing pipes might be reused, the survey says yes or no in writing, with the cleaning and sizing rules that apply. Check that the quote names diameters, insulation, length and the outdoor model those numbers belong to. Check that a long run's extra refrigerant, if the manual requires an additional charge for length, is mentioned as the installer's responsibility at commissioning. You are checking completeness, not recalculating the pair. Check the route can take the insulated size: a narrow chase, a small conduit left by the builder, or a 50 mm hole already drilled can be too tight. Say so before plasterboard. Silence here becomes a surface-mounted trunking surprise. GOV.UK certification is what allows an engineer to open the circuit later.
As-installed sizes on the commissioning sheet
Copper diameter itself is rarely the headline cost on a house. Access, making good, and whether the run is three metres or twenty metres are the headline. As a 2026 illustration, extending a simple back-to-back domestic job into a full-height external run with covers often moves the quote by hundreds of pounds including VAT. A commercial pair into a ceiling void moves it further. Keep the as-installed sizes with the commissioning sheet. The next engineer should not have to guess which manual applied. Energy Saving Trust's air-to-air pages describe the split those pipes serve. Where the outdoor unit is heating plant, the Planning Portal page is the permission starting point, separate from the millimetres.
Frequently asked questions
Is there one standard pipe size for UK splits?
No. Small wall units often use a familiar small pair, and larger indoors step up. The manufacturer limit for that model and length is the rule. Read the sizes on your quote.
Can I reuse the old copper with a new unit?
Only if the new manufacturer's instructions allow that diameter, condition and preparation for the new refrigerant. The survey decides. It is not a default yes.
Should the quote show inches or millimetres?
Either, as long as both pipes are identified and they match the manual for the model being installed. Ambiguous notes such as standard pipe are not a specification.
What if a 50 mm hole is already drilled?
A narrow chase, a small conduit left by the builder, or a 50 mm hole already drilled can be too tight for the insulated pair. Say so before plasterboard. Silence becomes a surface-mounted trunking surprise.
Core buying guides
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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.