Renovating the bathroom and moving the location of the toilet. The waste pipe for the toilet will run under the floor between two joists, however, part way down the joist run there is a cold air return that boxes in the joists. Can I run the waste pipe through the return or do I have to go under it?
You can pipe within an air plenum as long as you meet the fire codes- basically noncombustible materials only and the penetrations should be sealed with fire caulk.
A lot of buildings use the space above a suspended ceiling as plenums. It's one less duct system to install. But then fire rated assemblies are required.
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