Running refrigerant lines and condensate beside an internal soil stack can hide them in a cupboard that already exists. The stack is not a handy drain to pierce, and the boxing is not a place to lose the joints. Access for leaks has to remain: a panel at the connections, continuous insulation, and a condensate route that is a proper drain detail if it joins a waste at all. F-gas commissioning still has to reach those joints. In England the outdoor unit is a Planning Portal question even when the pipes are indoors.
A boxed stack used as a chase
A boxed soil and vent pipe is a vertical chase through the house. It is tempting because the damage to finishes is already done. Pipes can run in that zone if they do not crush the stack, block its rodding access, or force the soil vent closed. Keep the stack's own access and its vent.
Do not drill the soil stack and push the air conditioning condensate into it. A waste connection, if one is even appropriate, needs a trap and an arrangement a competent installer will own, with the stack still venting as designed. A random hole is a smell path and a leak path. Not through a hole you make in the pipe. Any waste connection has to be a proper trapped detail that leaves the stack venting. Many jobs are better with a separate drain or a pump to a lawful point.
It can be a good place to hide pipes if panels remain at the joints and the stack can still be maintained. A sealed box hides leaks until the ceiling stains.
Panels, sweating copper and a fire-stopped floor
Oil at a flare and water from a split drain both need a person to see them. A panel screwed over the indoor connections and over any joint in the cupboard does that. A stack boxed tight to the ceiling with no opening means the leak announces itself in the room below. Insulation must be continuous in the cupboard or the cold pipes sweat onto the soil pipe and the plaster. That wet patch looks like a plumbing leak and is often the missing lagging. Mark the panel so the next engineer does not have to hunt. Leave enough room to see a weep.
Soil cupboards can be tight, and they can be part of how fire and sound are separated between floors. Cutting a large new slot through a floor beside the stack may need more than an air conditioning fitter's usual hole. Say if the cupboard is also a fire-stopped zone so the installer does not treat it as an empty duct. The refrigerant circuit is F-gas work wherever it is hidden. Commissioning that cannot reach the joints is not commissioning. Hiding the pipes does not change that. Outdoor siting still follows the Planning Portal in England.
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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.