The alcoves beside a chimney breast look like a neat place for an indoor unit. They are only neat if the breast is not a live flue, the alcove is wide enough for the unit to breathe, and nobody cores a hole through the flue to shorten the pipes. A boarded fireplace is not proof that the flue is dead.
Do not drill the flue to save pipe
A chimney breast can still be a flue. It can be shared with a neighbour, lined for a stove, or only boarded over in the room you are standing in. Approved Document J is England's guidance on combustion appliances and fuel storage, and it is the reason a flue is not a cable route or a pipe sleeve. Coring through it can break the lining, join your room to a neighbour's flue, or put refrigerant pipes where heat and soot belong. If the breast is decorative and a competent person has confirmed there is no flue, the wall may be ordinary masonry. That confirmation comes before the drill, not after the dust.
A tight alcove recirculates the unit's own air
Installers like alcoves because the unit sits off the main wall and the pipes can drop in a corner. The unit still needs the space the manufacturer shows for air to enter and leave. A joiner who then builds a cupboard front across it, leaving a slot, turns the alcove into a loop. The unit cools its own exhaust and the room never catches up. Sit in the alcove at the survey and look at both sides. A gas fire, a log burner or a television already in that alcove may leave no lawful, breathable space. The other alcove, or a side wall, is then the honest answer.
The terminal outside is a separate problem
The outdoor unit does not belong in front of a flue terminal, a stove outlet or a boiler flue. Discharge air and flue gases should not be aimed at each other. This page will not invent a universal gap in millimetres, because the appliance maker and Approved Document J are the documents that set clearances for that flue. Ask the survey to mark the terminal and the proposed condenser on the same sketch. If the only spare wall is the chimney stack on the gable, treat that as a flue question first and a bracket question second. Refrigerant connections are F-gas work even when the difficult part of the job is the breast.
Frequently asked questions
The fireplace is boarded and we never light it. Is the flue dead?
Boarding is not a test. The flue may still be open at the pot, shared, or used by a stove in another room. Find out before anyone drills the breast.
Can the pipes go up inside the chimney to reach a roof unit?
Not as a casual route. A flue is for combustion products. Using it as a pipe duct needs someone who understands that flue, not a shorter copper run.
The outdoor unit would sit on the same wall as the chimney pot. Is that fine if it looks tidy?
Tidy is not the test. Flue gases and condenser air need the clearances for that appliance. Put both on one drawing before the bracket goes up.
Will the split heat the room if we still use the wood burner?
The split heats or cools the air it can reach. An open fire or a stove is a separate heat source with its own air supply and flue. Do not ask the split to correct a smoky stove, and do not site it so the stove's heat is blown straight into the indoor coil.
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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.