Now that the subject has came up and we have a resident AC systems specialist here if a guy was to convert old AC units into dedicated dehumidifiers which way would be best for the air flow?

In through the condenser and out the evaporator coil or the other way around or independent air flow for each?

Just curious being I have a collection of old window AC units of assorted capacities and a few have unrepairable fans and motors but their heat pump systems are still good so if a guy was to convert one into a dedicated dehumidifier and use multiple and quieter muffin type fans what's the best way to have the air movement go?

 

I cannot imagine that the normal A/C configuration – separate air circulation around both coils – is ideal in DH mode. The DH manufacturers are trying to raise efficiency in the face of DOE pressure, and would probably use that configuration if it helped.

Every DH I've seen pulls in through the evaporator coil with the condenser coil immediately behind it. The condenser only ever sees cold air from the evaporator, so that's probably why the condenser coil on a DH is so much smaller than on an A/C. The latter sees outdoor, gawdawful heat. If you want to cool the condenser with ambient (not pre-chilled) air, you need more airflow and/or more coil area to make up for the lower ∆T.

You wouldn't want to heat the incoming air with the condenser coil before it hits the evaporator coil, since all that heat would have to be pulled back out before any water would condense out.

So, my vote would be to configure it just the way commercial DH units do: One fan, in over the evaporator and out through the condenser. The condenser coil is oversized for this application, but that's good for efficiency.

 

That's what I have been thinking as well but in all my years of playing around with heat pumps and air conditioning systems I have yet to ever do any experimenting with how to set something up for dehumidification purposes only.

To me, a dehumidifier is the lowest form of heat pump applications and if my house gets too stuffy the central air does quick work of fixing that.

 

I wonder if anyone here can verify something for me. I suspect that the evaporator coil of a DH has a lower operating temperature than that of an A/C. I think a pressure specification would be the most accurate way to tell but a IR gun measurement could be good.

I should be able to find the answer on line but I haven't searched yet.

 

And I would think the optimum air speed would be where the evaporator coil is just above freezing to extract the maximum moisture from the air but not frost up.

 

You could just observe a Refrigerator with automatic defrost. It collects Humidity on the coil over time and then defrosts the water. The collected water is evaporated off the condenser coil near the compressor. But, in your case it does down the drain.

kv

 

I found this pretty good overview of the design considerations, A/C versus DH. (1st bulletin listed there)

Looks like the temperature of the coil in a DH is indeed cooler, but minimally. It would be tough to confirm by measurement, I think. Airflow over the coil is lower in a DH, as I suspected when I turned down the fan of my A/C.