Duct Furnace Downstream of Cooling Coil Rusting Out
Duct Furnace Downstream of Cooling Coil Rusting Out
(OP)
I've got a customer with an application in which every 5-10 years he has to replace a duct furnace. It rusts out, apparently due to moisture generated by a cooling coil.
I'm recommending he install a drain pan immediately after the coil.
Does anyone have other recommendations? Also, is there a rule of thumb for how far downstream to put the furnace?
I'm recommending he install a drain pan immediately after the coil.
Does anyone have other recommendations? Also, is there a rule of thumb for how far downstream to put the furnace?





RE: Duct Furnace Downstream of Cooling Coil Rusting Out
I'd suggest a duct heater with stainless steel heat transfer surfaces.
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
Horizontal run of duct with duct furnace and cooling coil in-line. I recommended the following:
1. Keep the unit as dry as possible. Locate upstream of the cooling coil, if possible.
2. When downstream, use a 321 exchanger instead of 409 (standard).
3. Install a catch basin or drain pan, with P-trap, immediately after the coil.
4. Install a stainless steel drain pan inside the duct furnace unit for any internal condensation.
5. Operate the flue power venter on the furnace during summer.
Other than a summer bypass, this is about all I could think of...
Thanks for your help.
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
Assuming that you do have a pan under the coil, look at the drain trap. It should be designed to maintain a water seal AND allow condensate to freely drain out without building a level in the pan.
A properly designed system will not cause the furnace to rust out.
---KenRad
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
HVAC68
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
To clarify my involvement here - we build duct furnaces. The customer initially wanted to know what was wrong with our furnaces & I had to explain that there wasn't anything fundamentally wrong with our equipment, just that you shouldn't wet it down and then expect it to last for thirty years. (Politely, of course.)
He's taking a look at his design(s) with this in mind. I'll forward the list on to him (the five above plus mist eliminator) and we'll see what happens.
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
If the dew point of the combustion air is greater than the leaving dry bulb of the coil there will be condensation in the heat exchanger. Unless the vent is sealed tight there will be some movement of air through the combustion side of the heat exchanger.
Stainless steel heat exchanger AND burners will help, but not solve the problem.
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
RE: Duct Furnace Downstream of Cooling Coil Rusting Out
Fran McConville
author of the Pilot Plant Real Book
www.pprbook.com