External and Weatherproof AHUs: What’s Different

Not every building has a plant room. Plenty of units end up on a roof, in a yard or bolted to the side of a building, and an air handling unit built for a dry internal plant room will not last outside.

Here’s what changes when the unit lives in the weather.

Casing and weatherproofing

The casing is doing a different job outdoors. It has to shed water, resist wind loading and keep driving rain out of every joint, not just the obvious ones.

That means:

• Weatherproof roof sections with falls and overhangs rather than a flat top
• Sealed panel joints designed for external exposure, with the right gaskets
• Weather louvres and cowls on intake and discharge, sized so rain isn’t drawn in
• Upstands and curbs where the unit meets the roof, so water doesn’t track underneath
• Drainage that works when the unit is wet on the outside as well as the inside

Internal insulation has to stay dry. Once water gets into a panel, the insulation becomes waterlogged, loses its value and starts a corrosion problem you won’t see until it’s advanced. Our guide to AHU casing and frame corrosion covers what happens next.

Materials and corrosion

Outside, the materials decision gets more important. Coastal and industrial sites are harder on metal than an inland office roof, and the wet sections of the unit are harder on it still.

Stainless drain pans, appropriate coatings and the right grade of fixings all extend the life of an external unit considerably. Specifying them at the start costs far less than dealing with corrosion in year six.

Frost and freezing

An internal unit has the plant room around it. An external unit has nothing. Frost protection matters a great deal more:

• Coil frost protection and correct control strategy
• Trace heating on drains and condensate lines, which freeze first
• Protection for any water services feeding the unit
• A control sequence that reacts before the coil freezes rather than after

Our guide to AHU frost protection sets out how this is done.

Access and safety

This is the part most often underestimated. Maintenance on a rooftop unit means working at height, and that affects everything:

• Safe access to the roof, and a route to the unit
• Guarding, edge protection and anchor points
• Door positions that work when there’s a parapet or another unit next to you
• Enough clearance to pull a filter bank or a coil out, not just open the door
• Whether components could be lifted off the roof when they need replacing

A unit that’s technically serviceable but awkward to reach ends up serviced less often, which is how small problems become expensive ones. Our guide to AHU maintenance contracts covers what a schedule should include.

Noise

External units are often closer to neighbours than internal ones, with no building fabric in between. Breakout noise, discharge noise and vibration through the structure all need consideration at design stage rather than after a complaint. Our guide to AHU noise reduction techniques covers the options.

Getting the unit up there

Rooftop replacement is a logistics exercise as much as an engineering one. Craneage, road closures, out of hours working and the sequencing of old unit out and new unit in all shape the programme and the price.

Where craneage isn’t viable, flat-pack construction solves it. We’ve built units in place on sites where nothing else would work, including restricted plant rooms at Dalston Cross. Our flat-pack AHU guide explains the approach, and our guide to replacing AHUs in rooftop and high-rise plant rooms goes into the logistics.

Specifying one

If you’re putting a specification together for an external unit, say so explicitly and set out the exposure, the access arrangements and the frost protection requirements. Our guide to specifying a replacement AHU covers the rest of the document.

How we could help

We design and build external and weatherproof air handling units, and we replace rooftop plant on live buildings. See our air handling units page or get in touch.

Frequently asked questions