Are all heat-exchangers of 90+ high efficient furnaces made of stainless steel?

Question

can any qualified hvac out there show me a computation on how the returning air is affected by moving the A-coil on the RA . My hvac guy is arguing that I dont need any extra return opening above the area where the A-coil was moved to. Doesn't the presence of the A-coil in the RA cause an obstruction to the air flow inside the RA.

Answer 1

Not all secondary heat exchangers are stainless.

Carrier for instance uses a polypropylene-laminated steel secondary and an aluminized steel primary heat exchanger.

Most other manufacturers use a stainless secondary and an aluminized steel primary.

Adding an A coil to the return side could cause problems with condensation in the heat exchanger if it is not recommended by the manufacturer.

I have seen the results of an 80% furnace that had A/C added to the return side and caused large rust flakes to accumulate and fall on the ribbon burners and cause delayed ignition.

This is less likely in 90% furnaces as they usually are sealed combustion with in-shot type burners.

The restriction shouldn't be too much of a concern if the ducting was proper to begin with and doesn't have a filter access problem now.

The  temperature rise would need to be checked to ensure there is adequate air flow.

Why was the A coil moved? Or is this an A/C add on and they didn't want to move the furnace?

Answer 2

If the ductwork was set up properly to begin with, as long as the coil can be used for either "push through" or "draw through" operation then it may be ok (if the rated static pressure values of the coil are close).

Duct work is designed by an external static pressure (ESP) calculation. All the fittings, turns, sizes, etc are calculated in equivalent feet measurements. The ductwork is sized according to this calculation.

Have your technician show you his calculations to prove it will be ok.

One other thing to remember is that the condensate drain will need a p-trap installed to allow proper drainage if the coil is in the return air stream.

Answer 3

You do not put an evaporator coil in the return plenum of a gas furnace. This is a recipe for nightmares. It will cause the furnace heat exchangers and cabinet to rust due to condensation.

If your "HVAC guy" put it in the return, he's correct you don't need to add returns, you need to move the coil and install it properly.