Will this adapt a 21" furnace to a 17.5" coil?

Furnaces in any system pairing are not allowed to be wider than the coil. They are required to either be the same width or smaller than the coil. Running an HVAC system where the furnace is wider than the coil will cause issues with the static pressure of your ductwork and will ultimately void the warranties of both units. We strongly do not recommend attempting to make the pairing work by installing a transition in reverse.

If you have any further questions, please call us and we will be happy to assist you.