Six air changes an hour, and a building nobody can hear it in

Consulting rooms need the air changed and need to be quiet while it happens. Those two requirements pull in opposite directions and the usual casualty is the second one.

We oversized the ductwork, ran the terminal velocities under 3 m/s, and put attenuators where the drawings had none. The plant is a single MVHR unit with a plate heat exchanger — no wheel, because a wheel carries a small amount of air across the seal and this is a health centre.

Measured on handover: six air changes an hour in every room and NR 29 in the worst of them.