hmmm, well, my duct fan specifically instructs to use it ONLY for 'pushing' cool air, and not 'pulling' hot air into the fan motor ... but, since it's intended use is for assisting furnace air to be moved, I can't see the heat bothering the motor (but, I'm a truckdriver, not a HVAC guy ) ... not knowing shit about HVAC, I would think you would want the 175cfm at the Inlet, and the 275cfm at the Outlet ... I'm thinking if that was reversed, the first fan would be fighting against some back-pressure, when it encounters the slower fan ... well, that's the theory for exhaust pipes, anyway