A combi and a split can live in the same house if the outdoor coil is not sitting in the boiler flue plume and both appliances can still be serviced. Flue gas is wet and corrosive. It will wreck a coil that faces the terminal. The indoor unit does not belong in the boiler cupboard. Condensate pipes stay separate. The split is F-gas work. The boiler flue clearances stay as the boiler installation left them.
Wet flue gas on the outdoor fins
A combi terminal throws a wet plume. Pointed at an outdoor coil, that plume corrodes fins, upsets defrost, and can trip the unit in cold weather. Site the condenser away from the terminal, not only a token gap that still sits in the plume on a still day. Leave boiler service access and flue clearances as the boiler installation left them.
The cupboard the gas engineer still has to enter
Combis are usually room-sealed, but the cupboard is still hot, tight, and full of the boiler's own clearances. An indoor unit in that cupboard cannot be filtered, cannot throw air into the room, and blocks the gas engineer's route. Put the indoor unit in the room you want to treat.
Two condensates, two isolators
Boiler condensate is acidic and already has a route. Air conditioning condensate is a different pipe, trapped and taken to a proper drain or a pumped line. Refrigerant lines should not be clipped across the boiler's flue or across the gas pipe in a way that blocks spanners. Each appliance needs isolation a competent person can reach. In England, the outdoor unit still has to meet Planning Portal conditions. F-gas certification is for the refrigerant circuit, not a substitute for gas-safe work on the boiler.
Frequently asked questions
Can the outdoor unit sit under the boiler flue?
No. The plume is wet and corrosive and will damage the coil. Choose a spot the plume does not wash.
Can the indoor unit go in the boiler cupboard?
No. The cupboard is for the boiler, its clearances and its service. Put the indoor unit in the room you want to cool or heat.
Can both condensates share one pipe?
Only if someone competent designs a proper connection. Do not tee them on site as a shortcut. Keep access to both.
Can the air conditioning installer move the boiler flue to clear the coil?
Do not move the flue to suit the condenser unless a gas engineer is doing that as its own job, with the clearances the boiler needs. The air conditioning installer should not just adjust a flue.
What if an open-flued appliance shares that cupboard?
Its air supply is not yours to box in. Say what else is in the cupboard. A split does not provide combustion air.
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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.