Would you under any circumstances do the following?: You've temporarily removed some floor or ceiling material (including a joist or two) as part of a renovation. During this process you install a water heater in the attic, and an air handler for the HP in the crawl space, taking advantage of the temporary access you have created. When the ceiling and floors are reconstructed there really will be no way to remove and/or replace said water heater or air handler without opening up the floor or ceiling again. You tell your client, the homeowner that the water heater will last 20 years so it's not a problem. To me, this scenario is preposterous, outrageous, ridiculous, and maybe criminal. Or at least a code violation. But that's me. What do you think?