Most air handling units arrive on a lorry in one or two pieces, get craned into position, and are connected up. It’s the quickest way to do it and the quality is consistent, because the unit was built and tested in a factory.
Then there are the buildings where that simply isn’t possible.
A plant room in a basement, reached through a doorway and down a stair. A rooftop with no crane access. A retail unit trading seven days a week where the delivery route runs through the shop floor. In those buildings the question isn’t which unit performs best — it’s which unit can physically get to where it needs to go.
That’s what flat-pack air handling units are for.
What a flat-pack AHU actually is
A flat-pack, or knock-down, AHU is designed and manufactured as a complete unit, then broken down into components small enough to pass through the available access, and rebuilt in position.
The frame comes in sections. The panels come separately. Coils, fans, filter sections and dampers are handled individually. Everything is designed, prepared and marked up before it leaves the works, so the assembly on site is a build rather than an improvisation.
The important distinction is that this is still a manufactured unit. It’s designed as one thing and assembled where it will live. That’s different from cobbling something together on site, and the performance and casing integrity should be the same as a factory-assembled equivalent — provided the assembly is done properly.
When you need one
The access is smaller than the unit. Standard doorways, tight corridors, existing lift shafts. If the smallest opening on the route is smaller than the unit’s smallest dimension, a factory-built unit isn’t going in.
There’s no crane access. Rooftop plant on a constrained site, or a plant room in the middle of a building with nothing above it.
The route is long or awkward. Multiple level changes, tight turns, load restrictions on floors or lifts.
The building can’t be disrupted. Where taking out a window or a section of wall isn’t acceptable, or where the delivery route runs through occupied space.
We built and delivered a flat-pack unit on a poultry farm operating 24/7, where the facility could not stop and the access constraints ruled out a conventional delivery.
We took a similar approach on a retail scheme in Epsom, where the constraints of a live trading environment shaped how the plant was delivered and assembled.
What it costs you
It would be dishonest to present flat-pack as a free alternative, so here’s the trade-off plainly.
It takes longer on site. A factory unit is craned in and connected. A flat-pack unit is assembled piece by piece, and that’s days rather than hours.
It costs more in labour. The manufacturing cost is broadly comparable; the site time is not.
Assembly quality matters more. A factory build happens in controlled conditions with the same team every time. On-site assembly has to achieve the same casing integrity — air leakage, thermal bridging, panel sealing — in a plant room. It’s entirely achievable, but it depends on who’s doing it.
Access still has to be planned. Flat-pack solves the size problem, not the route. Someone still has to carry every component to where it’s going.
Against that, the alternative is often no unit at all, or major building work to create access. Set against taking out a wall or hiring a crane and closing a road, the additional site time usually looks reasonable.
Planning one properly
Survey the route before anything is designed. Every doorway, every turn, every level change, every lift. The narrowest point on the route sets the maximum component size, and finding it after manufacture is an expensive discovery.
Check weights as well as dimensions. A coil section that fits through the door may still be too heavy to carry up a stair without mechanical assistance.
Plan where assembly happens. You need working space around the unit, not just space for the unit. Assembling in a plant room with 300mm clearance on all sides is a great deal harder than it sounds.
Agree the phasing with the client early, particularly in a live building. Where the work has to fit around trading hours or a continuous process, that shapes the programme more than the technical work does.
Consider removal of the old unit too. If the new unit has to come in piece by piece, the old one probably has to go out the same way. That’s often overlooked in the programme.
Is it the right answer?
Not always. Where there’s decent access, a factory-assembled unit is quicker, cheaper on site and simpler. We’d recommend that every time it’s possible.
But where the building won’t allow it, flat-pack turns an impossible job into a manageable one — and it does so without compromising on the specification of the unit itself. You still get the AHU the building needs, rather than whatever happened to fit through the door.
If you’re looking at a plant room and wondering how anything is going to get into it, that’s exactly the conversation to have early. Our air handling units page covers what we build, and our installation guide sets out what a typical project involves.
Get in touch and we’ll take a look at the access before anything else is decided.
