A-Coil Above or Below Coil Blower Unit?

[hausdok]

Hi All,

So, here's a question for those of you from places where you cool your homes with mechanical devices instead of just opening the window. A very few times a year I run into split air conditioning or heat pump systems but not enough to gain real familiarity with them. I'd be asking the manufacturer, except it's Saturday and there's nobody there to take my call.

Yesterday I had a Lennox heat pump unit on a home where the A-coil is above the downflow coil blower unit instead of below it, so return air passes through the A-coil before it passes through the air handler and auxiliary heating coils.

This seems counter intuitive to me. As soon as I saw the configuration I thought, "Geez, won't the coil blower unit get all rusted being beneath the A-coil with all of that air passing through that coil dripping with condensate? When I opened up the coil blower unit my worries seemed to be confirmed because there was a capacitor on the side of the blower drum and the metal casing was all rusted - something I don't usually see inside of HVAC systems. Then I opened up the A-coil cabinet - which had been serviced in May - and found a bunch of nasty looking soggy gray gunk on the lower half of both sides of the coil that the serviceman probably missed.

I checked the HVAC referenced that I have but none seems to address this. Then I went to the Lennox site and perused all of the available literature there for heat pumps and coil blower units. Most of the configurations I saw diagrammed there have the coil blower unit above the A-coil but there were a couple where they are shown turned the other way around with the coil blower unit downstream in the airflow from the A-coil.

Your thoughts?

ONE TEAM - ONE FIGHT!!!

Mike

[davidlord]

The return air should pass through the coil prior to the aux heating elements and blower assembly. Some units are installed in an upflow config and some in a down flow config.

[Jim Katen]

With a gas furnace and a heat pump, the air should pass by the heat exchanger first, then through the A-coil. This way, any condensate spray won't rot out the heat exchanger. (The gas never fires while the heat pump is running.)

With an electric furnace and a heat pump, it's the other way around. The air should pass through the A-coil first, then through the electric heating elements. This is because, at low temperatures, the heating elements fire at the same time as the heat pump is running. If the indoor air were already heated by the heating elements, it wouldn't remove as much heat from the A-coil and that would screw up the refrigerant pressure. It might even cause slugging.

The problem you saw wasn't a configuration problem, it was a maintenance problem. Someone should clean the coil, the drain pan and the drain pipe. He might also check the blower speed -- a fast blower can cause condensate to spray right off the coil before it can make it to the pan.

- Jim Katen, Oregon

[hausdok]

David, Jim,

Thanks, now I get it (hope I'll be able to retain it). Yeah, I knew maintenance was lacking in the A-coil area. The fan speed thing is interesting; I've never heard that before - makes sense.

Geez, you guys make me seem to be smart.

Thanks,

ONE TEAM - ONE FIGHT!!!

Mike