We have a new ducted heating system. When we turn it on the house starts to smell like the roof space, which has a distinctive smell especially when it's hot. It's the smell of old wood, three types of insulation, borer poison (which is apparently harmless to humans), etc, etc, in an old house. Last time I smelled it I went into the ceiling space and found a rip in the ventilation system duct, which I fixed and the smell went away.

The heat pump supplier said they have checked but cannot find the source of the smell ingress. I suspect they think I'm making it up.

Any suggestions how we might find the leak? It must be between the air return and the heat pump input, not on the output between the heat pump and diffusers. I wondered if fake smoke from the hardware store meant for testing smoke alarms might help, spray it in particular locations and see if it comes out the vents, but it might disperse through the motor and not be visible.