Subject: Re: improved dehumidification in existing unit

From: [email-masked] (John Mills)
Date: 28 Jul 1995 01:04:05 GMT

>Besides, I've talked to a few of the older old goats around here about
>this idea of oversizing the evaporator in home units and they say their
>opinion is that the normal coil is too small to begin with as evidenced 
by
>high pressure differential across a new coil at startup and that if you
>use a larger coil it will take longer to plug it up with lint or dirt
>caused by poor filter maintenance.

Those old goats ought to see the new super efficiency coils that are out 
these days. Really funny to see them beside the new tiny furnaces. Some 
of the bigger Trane coils are 24" wide and 30" tall in the big sizes. 
Look funny on a 40" furnace. Our WK distributor bought some 3rd party 
coils to bump 11 SEER to 12 SEER cheaper and even the 3 ton is 2 feet 
tall! Sure won't work in a counterflow application and doesn't fit in 
some highboy basement situations either. Then look at the condensers of 
some of these 14 SEER units...

---

From: [email-masked] (John Mills)
Date: 28 Jul 1995 01:06:31 GMT

[email-masked] (Steve Bukosky) wrote:

>Your a man of my own thinking John.  Sounds like my kind of system!

Steve, I have YOU to thank for that 2 stage variable speed 80 I have. 
After you mentioned one time a distributor offering you and others 1/2 
price furnaces, I hit the A-S distributor up for a deal. Told him of what 
your's offered. So I got the $1000 furnace for $500. Was a scratch and 
dent model, like a forktruck smashed it, but works super and still on 
same ignitor 2 years later. I love it.

---

From: [email-masked]
Date: 28 Jul 1995 01:34:04 GMT

In <[email-masked]>, [email-masked] (paul milligan) writes:
>In article <[email-masked]>,
>   [email-masked] (Little joh) wrote:
>~~>Boy this is getting around the track.
>~~>Putting in a bypass will help with dehum. but not as much as the return
>~~>air.  The dehumidifier is set up to do several things. 1st warmup the air
>~~>going into the unit by way of the condenser. Thus raising the temp.  2nd
>~~>going thrugh the evap. cooling the temp of the air causing it to remove
>~~>and condense the water in the air.  I was always led to believe that the
>~~>higher the delta temp. of the air and the coil will give the best
>~~>dehumidification. Has to do with the btu of the water at specific temp,
>~~>dew point, relitive humidity etc.. 
>
>	I may be mistaken, but I think they put the air on the evap first,
>then the condensor, don't they ?  After all, if you warm the air first,
>that's just that much more sensible to be removed before the air on the
>evap starts losing latent.
>
>	I think the important delta is coil temp vs dew point, not 
>incoming air temp, no ?

To improve dehumidification with an existing a/c unit, you want to
pass the air flow across the evap first, and then warm it up afterwards
if necessary.  Doing it the other way around causes the system to
chase its tail...

>~~>    We always recumend that people put in an oversized coil iue.. 2.5 ton
>~~>evap with a 2 ton condensor for better dehum. 
>
>	I've never heard of that being done on purpose.  Don't you kill your
>efficiency, not to mention the back pressure problems ?

Placing an oversized evap on a condensing unit will (1) increase suction
pressure; (2) decrease the ability of the unit to dehumidify the air; and
(3) increase the efficiency of the unit.  High SEER units typically keep
evap temps up around 45F or so.  This decreases compression ratio allowing
the compressor to operate more efficiently.

Great idea for the Phoenix area (where humidity is low).  Lousy idea for
Houston.   :-)

>Comments invited, as always.
>
>Paul
>	

Andy Schoen
[email-masked]

---

From: [email-masked]
Date: 28 Jul 1995 01:47:19 GMT

In <[email-masked]>, [email-masked] (Allenbau) writes:
>Actually, I know several installers who routinely use an evaporator coil
>that is rated for higher capacity than the condensor unit ( 2 1/2 ton
>condensor, 3 ton evaporator coil). They use it for lower back pressure
>against the circulating fan and for generally making a unit run "easier"
>or under less load.

Mike,  something's amiss here.  Oversizing an evaporator will raise
suction pressures, given the expansion device is doing its job properly.
Higher suction pressures mean higher flow rate through the compressor,
causing the compressor to work harder.  Granted, lower suctions cause
the compressor to run "easier", but that won't happen if you oversize
the evaporator.

>
>I have oversized evaporators in commercial installations in order to keep
>head pressure down but only when I had good control of refrigerant flow so
>as to maintain a reasonable superheat and get refrigerant back to the
>compressor at a temperature low enough for adequate cooling.
>

Wait a minute...we're backwards here.  Undersizing the evaporator will have
a tendency to lower head pressures provided the unit is properly charged.
Lower suction = lower compressor flow rate = lower TD at the condenser.
Oversizing the condenser, however, is an effective way to reduce head
pressure.

>Unless you have pretty good control of the system it's probably not too
>good an idea to vary far from specs but I do know several good installers
>of home units who regularly size the evaporator 1/2 ton to one ton larger
>than the condensor. They are happy enough with the results that they have
>been doing it for many years.

No problem with this practice.  In fact, the unitary a/c manufacturers
will allow some oversizing of evaporators.  However, it will (1) reduced
the unit's ability to dehumidify, and (2) increase its efficiency.

Andy Schoen
[email-masked]

---

From: [email-masked] (Steve Bukosky)
Date: Fri, 28 Jul 1995 02:01:46 GMT

>Steve, I have YOU to thank for that 2 stage variable speed 80 I have. 

Glad to hear that, John.  I think that if more dealers experienced the
top of the line equipment, they would try and sell more of it.  By
chance were you at the Honeywell Perfect Climate presentation in
Indianapolis today?

Exec-PC  World's Largest BBS