Duct Furnace Downstream of Cooling Coil Rusting Out

[Geof]

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?

 

[MintJulep]

If the cooling coil is providing dehumidification then the air leaving it will be essentially at saturation. It would take very little to cause condensation on the downstream heater.

I'd suggest a duct heater with stainless steel heat transfer surfaces.

 

[ChasBean1]

Any cooling coil should have a drain pan. Here, I'd be more concerned with air velocity through the coil. I would look at the fan airflow and the coil size. If you find over 500 fpm velocity through the cooling coil, you'll likely experience carryover of water droplets downstream. Is the cooling coil before the furnace in the airstream? How is this configured? Is the cooling coil on the vertical at the furnace outlet?

 

[Geof]

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.

 

[KenRad]

I'm not sure what you're getting at when you talk about installing a drain pan AFTER the coil. If your coil velocity is within design limits as ChasBean1 discussed, then condensation will form on the coil surface, and drop downward. There should be a drain pan below the coil, to catch and drain this condensation away. If you're carrying moisture downstream of the coil, you have a bad design that no pan after the coil will correct.

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

 

[HVAC68]

Agree with KenRad & ChasBean1. However, just in case the velocity across the cooling coil is higher (say, 500 fpm or 2.5 m/s), it may result in moisture carryover. Try installing an eliminator after the cooling coil to trap any moisture that may get carried over due to the higher velocity.

HVAC68

 

[Geof]

Thanks for all the input.

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.

 

[Rodale]

Where is the combustion air for the duct heater coming from?

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.

 

[lilliput1]

The furnace heat exchanger should be upstream of the cooling coil just like in my vertical forced air gas fired heater with a cooling coil in the discharge air plenum. The return air is filtered then the fan blows the air up through the furnace heat exchanger then to the cooling coil at the discharge plenum. During winter, the heater is on & cooling is off. During summer, the heating is off and the cooling on.

 

[Icanfix]

If moisture carry over is the problem then it seems to me that the duct would have the same exposure with the same results.

 

[ME69]

The general recommendation when a duct furnace is used downstream of a cooling coil, is to use SS HX, SS burners and a drip pan. Usually, the problem is condensation on the cobustion side of the HX. Note, while placing the DF upstream of the cooling coil would reduce this, it is usually impractical since the duct furnace must remain on the positive or discharge side of the fan.

 

[carlosgw]

Look at putting a moisture eliminator between the cooling coil and the furnace.

 

[lilliput1]

See Mistop Inc. Filtguard Eliminator - Stainless still brillo pad type mist eliminator.

 

[vannic]

The duct furnace is rusting because it is vented to the outside. Moist summer air is migrating into the heat exchanger from the outside. It has nothing to do with carryover. Replacing the furnace with a SS exchanger will prevent corrosion but you'll still have condensation problems. What I've done in the past is cool the space where the furnace is installed since typically it's a mechanical room of some sort. It seems to lessen the condensation problem somewhat.

 

[handee]

I have a very similar situation - we've got an air cooler that consists of a cold water coil and a blower. The only place we can locate the coil is upstream (return, or suction side) of the fan, so I'm also concerned about corrosion. But my first concern is draining the condensate from the ductwork at the coil - what kind a trap would allow the condensate to drain out against the negative pressure differential? Thanks for any ideas.

Fran McConville
author of the Pilot Plant Real Book

http://www.pprbook.com