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From: John De Armond
Subject: Re: central humidity control?
Date: Sun, 27 Jun 1999 23:43:26 EDT
Newsgroups: sci.engr.heat-vent-ac
Message-ID: <>

"Kirk A. Kerekes" wrote:
> I'm a DIY-intensive homeowner who monitors this group for ideas.

Gonna assume this to be true.

> The weather appears to finally be turning summerlike, but this spring has
> convinced me that I need to find a way to incorporate automatic humidity
> removal into a central air system. It would _seem_ that it ought to be
> possible to incorporate a dehumidifier's "closed loop" inside the air
> path, but I have never heard of such an installation or device.
> Does such a widget or scheme exist for residential systems?

Two steps.

Install another coil downstream of the evaporator and pipe the
liquid freon line to flow through it before reaching the expansion
device.  This does two things.  It reheats the air to lower its
humidity and it subcools the refrigerant, making your system more
efficient.  The coil can be significantly smaller than the
evaporator - an auto condenser coil would work well.  It need not
cover the whole duct and indeed can be suspended across the duct
with the material of your choice - wire, steel frame, etc.

Install yet another coil downstream of the evaporator and pipe a
portion of the hot gas from the compressor through it. You will want
a hand valve to set the proportional hot gas flow and a refrigerant
solenoid so that this reheat flow can be controlled.  This has the
same effect as your running your heat and indeed is the same way
humidifiers work.  The air is first chilled to condense water out
and bring the dew point to approximately the evaporator temperature
and then the air is reheated to lower the humidity.  This is vastly
more efficient than supplying reheat from some other source because
not only does it recover some of the heat absorbed by the
evaporator, it also lowers the load on the compressor (and thus the
energy use) by cooling the hot gas.  This also functions as a
capacity control which dovetails nicely since the need of humidity
control is usually the highest when the cooling load is relatively

You'll need a control system.  The most basic would be a manual
switch to open the hot gas valve.  When the house starts feeling
stuffy, just flip the switch.  The next most basic would be a
humidistat to control the hot gas valve.  The best system would
integrate the two functions.  As cooling is called for, the AC would
operate with the hot gas valve open.  it would close either when the
humidity reaches the desired setpoint OR when the heat load becomes
great enough that the ambient temperature rises to a second, higher
setpoint.  This should be easily implemented with a multi-stage
thermostat such as the White-Rogers Digital Comfort Set II (~$100
wholesale with remote temp sensor).

Step 1 is a no-brainer.  Aids humidity control and reduces operating
cost.  If you'd like, you can include a bypass valve so that when
the heat load requires all the system's cooling, you can bypass the
coil.  Probably not necessary unless your system is really


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