Ductwork: the half of the system that decides whether the rest of it works

Flexible has a place: a short, fully extended length as the final connection to a terminal, where it takes up a small misalignment and stops vibration travelling. What it must not be is the run. Flexible is far more resistant to airflow than rigid of the same diameter even when it is pulled straight, and nobody pulls it straight — it gets left concertinaed, bent round a joist and squashed under insulation, and each of those takes another slice off what reaches the room. If a system is underperforming and there are metres of flexible above the ceiling, that is usually most of the answer.

Because of what happens to warm, wet air inside a cold pipe. Any run crossing a cold loft, a void or an unheated space is a cold surface with moist air travelling through it, and the water comes out of the air onto the inside of the duct and runs back down. That is why a new extract fan sometimes drips, and why an uninsulated run in a roof space eventually wets the ceiling under it. Insulating the run keeps its inside surface above the temperature where that happens. It is not about heat loss; it is about condensation.

Yes, and in a gym, a bar, a workshop or a warehouse it usually should be — boxing it in costs money, takes headroom and makes everything inside it unreachable. Exposed simply means the run has to be set out properly: one line, matched brackets at even centres, seams turned consistently, and drops landing where the lighting and the ceiling design expect them. That is a decision to make before the first bracket goes in rather than afterwards.

Both. On commercial work we routinely install to a consultant’s design and coordinate it on site with the other trades, which is often where the real work is — a duct zone on a drawing and the space actually left after the sprinklers and the structure are not always the same thing. On domestic work and smaller commercial jobs we set the system out ourselves from a survey. Either way you get the commissioning figures at the end, because that is the only evidence the route does what the design said it would.

Often it can be worked around, and how much of it has to come down is the single biggest thing separating a cheap job from an expensive one — so it is the first thing we look at. Existing boxing, a cupboard, a bulkhead, a loft, a riser or the void over a hall will frequently carry a run with very little disturbance. Sometimes there is genuinely no route without opening something up, and you should be told that before you commit rather than on the second morning.