look n for some advise i am install n a new rheem grk 120000 btu furnace but i want to use old a coil but outlet on rheem 23inch by 20 inc and a coil is only16 by 19 inch can i make it work or do i need a biger a coil box ididnt know if the smaller box would cut down on air flow ??? thanks for the help
Depends on a couple of things.
How much more CFM does the new furnace move than the old one?
If you are increasing the amount of air the furnace is pushing through the coil it is possible(probable) that you will have problems in cooling mode since you won't be cooling the air off as much as it passes through the coil more quickly, and loose your ability to dehumidify the air as it passes. You may also overload the condensing unit outside by absorbing more heat from the coil as more air passes through it.
How high off the top of the furnace will the old coil be located?
If the coil is located directly on top of the furnace(like only a few inches above the heat exchanger) you may well restrict the airflow through the heat exchanger, and overheat the furnace causing it to cycle on the limit control. If you have a bit of room between the top of the furnace and the coil this will be less of a concern.(14-18inches or more). If you have the space, you could install a bypass damper to circumvent this problem by taking air around the coil(as well as through it) during the heating season and closing the damper in the summer, forcing all the air through the A/C coil.
I think it can be made to work, taking these considerations into mind, but you will want to be sure that you do the proper tempurature rise accross the furnace upon start up and get the fan speed set up for both heating and cooling modes to ensure proper efficiency and tempurature drop/rise.