Putting an outdoor unit under a deck hides it and, more often, starves it. Decking voids are tight, damp and full of leaves. Airflow and service access fail more often than the look of the unit. The fan needs the manual's free gaps, the service panel has to come off without lifting half the garden, and defrost water needs a way out. If those are not true with the boards down, the deck is the wrong site. A hatch you must unscrew with a full board set is not service access.
Warm air trapped once the boards are down
Warm discharge in cooling collects under the boards and is drawn back through the coil. In heating the same void restricts the air the heat pump needs, and frost melts into a damp box. Leaves drift in and stay, because nobody rakes under a deck. A unit that fitted on the day the deck was open may fail the same manual once the boards are down.
Joists, trellis and the gaps in the manual
The manual's clearances are measured to the nearest solid surface. Joists, boards and a skirt of trellis all count. Airflow fails first, appearance second.
- Service access has to exist with the deck in use.
- Defrost water and rain must drain away.
- Leaves will collect. Leave a way to clear them.
Defrost water and a hatch that ices shut
Do not bag or cover the unit for winter and then enclose it as well. It has to breathe, including during defrost. Water dripping from the base is normal in heating. It should not pool around the electrics or ice the only hatch. An oil sheen on that water is not normal. That wants an F-gas engineer, not another board over the top.
A stand beside the deck, with a screen that breathes
A proper stand beside the deck, with a screen that does not choke the coil, is usually the better design. Under the deck, the price should state the gaps, the drain path and a hatch large enough for the panel and a person's hands. Pipe length to the indoor unit still has to fit the manual. A few air bricks make it acceptable only if the free area and the service hatch match the manual. A token vent in a tight void usually does not.
Refuse the void if the boards kill the gaps
If the installer cannot write the gaps, the drain and a real hatch, they should refuse the spot. If those are not true with the boards down, the deck is the wrong site. A hatch you must unscrew with a full board set is not service access. Planning Portal conditions in England still apply to a unit you cannot see from the lawn. Hidden is not the same as permitted.
Frequently asked questions
Can I hide the outdoor unit under the decking?
Only if airflow, drainage and service access meet the manual with the boards down. Most under-deck voids fail those, which matters more than hiding the cabinet.
Does a cover under the deck protect it in winter?
No. Do not bag or cover a heat pump outdoor unit. Under a deck it still has to breathe, including during defrost.
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.