# Whole-house dehumidifier/fresh-air intake?



## mahohmei (Mar 26, 2009)

I currently have an American Standard 3-ton heat pump/4.8 kW electric backup system, installed in 1998. My house is one floor, 1647 ft^2, and the system is upflow with only one return through the HVAC closet door and the filter in the air handler; see picture:










We live in north Florida, where the spring and fall present us with extremely high humidity without the heat, so the only way to dehumidify the house (short of a standalone dehumidifier) is to overcool, which can get uncomfortable when the ambient temperature is already down to 70.

I'm interested in installing a dehumidifier, namely, the Honeywell TrueDRY DR65 or DR90--both are more than enough to dehumidify my entire house, with the DR90 carrying the added benefit of having a fresh-air intake.

Taking a cursory look at those dehumidifiers' instructions and seeing that all the install diagrams assume return _ductwork_ as opposed to a "return closet", I'm tentatively considering doing the following:

- Saw two rectangular holes in the closet floor of appropriate cross-sectional area to match the dehumidifier's round duct diameter.

- Install the dehumidifier in the crawlspace (6-7' high, more than enough room to comfortably install it). Duct the dehumidifier's input to an air register in the floor of the HVAC closet. Duct the dehumidifier's output up the closet between the air handler and wall, entering into the supply ductwork just above the air handler.

- If I get a DR90, duct the fresh-air intake to an appropriate location outside.

- Wire the whole thing up to a Honeywell Prestige IAQ to control temperature, humidity, and, if I get the DR90, fresh air ventilation.

Here are my questions regarding this potential setup:

- Will it work? It is a good idea?

- The Honeywell TrueDRY dehumidifiers have filters in them. As the dehumidifier would be in a crawlspace, I'd rather not have to go down there every 3 months to change a filter. Would it suffice to install a filtered return grille on the HVAC closet door and have no filter in the dehumidifier? (It seems that this is not an option on the DR90, whose filter also covers the fresh-air intake).

- We have neighbors who like to burn their yard waste, and we can smell it as soon as we step outside; this is an almost-weekly event. Would this be an issue with a fresh-air intake?

Thanks in advance for all the advice!


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## TarheelTerp (Jan 6, 2009)

mahohmei said:


> 3-ton heat pump,,, installed in 1998.
> one floor, 1647 ft^3 one return through the HVAC closet door
> I'm interested in installing a dehumidifier...


A $200 portable would be less of a waste.

If you're ready to spend some money though... you should explore improving the return duct system and the general efficacy of your heat pump (which should be doing all the dehumidifying needed).

hth


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## yuri (Nov 29, 2008)

mahohmei said:


> I currently have an American Standard 3-ton heat pump/4.8 kW electric backup system, installed in 1998. My house is one floor, 1647 ft^3, and the system is upflow with only one return through the HVAC closet door and the filter in the air handler; see picture:
> 
> 
> 
> ...


 
......


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## bobelectric (Mar 3, 2007)

Is the unit oversized?


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## mahohmei (Mar 26, 2009)

- Install the dehumidifier in the crawlspace (6-7' high, more than enough room to comfortably install it). Duct the dehumidifier's input to an air register in the floor of the HVAC closet. Duct the dehumidifier's output up the closet between the air handler and wall, entering into the supply ductwork just above the air handler. should work OK except when the air handler is running it will blow backwards into it so no it is not a good idea, find another place to exhaust the dehumidifier like going thru a closet and its door.
Would this still be an issue with a backdraft damper where the dehumidifier enters the HVAC supply ductwork? I'm looking at the plans in http://customer.honeywell.com/Honey...getliterature.axd?LiteratureID=69-2089EFS.pdf, page 5, option B.​ - Wire the whole thing up to a Honeywell Prestige IAQ to control temperature, humidity, and, if I get the DR90, fresh air ventilation. sounds like expensive overkill as it cannot control temp just humidity so a simple DE-humidistat will work or it may come with its own internal one. you could use the IAQ for the AC and heating but it is not cheap. not sure if they do heat pumps easily but some of our guys may know.
Yeah, a Prestige IAQ would be kind of pricey. Less expensive alternative: mount a mechanical dehumidistat on the wall next to the thermostat; accomplish the same thing. ​ - The Honeywell TrueDRY dehumidifiers have filters in them. As the dehumidifier would be in a crawlspace, I'd rather not have to go down there every 3 months to change a filter. Would it suffice to install a filtered return grille on the HVAC closet door and have no filter in the dehumidifier? (It seems that this is not an option on the DR90, whose filter also covers the fresh-air intake). going down there once every 3 month won't kill you and yes you need a tight fitting proper filter or dirt will get into it and damage it.
Good point; it won't kill me. Alternatively, I could install the dehumidifier in the basement, but that would involve knocking out a CMU to get the duct from the basement into the crawlspace. (Our house was built on a very steep hill, so the lowest 13' of crawlspace were floored, walled, and made into a basement.)​ - We have neighbors who like to burn their yard waste, and we can smell it as soon as we step outside; this is an almost-weekly event. Would this be an issue with a fresh-air intake? yes
Now that you've pointed out it would be an issue, the DR65 would be more than enough dehumidification capacity. You just saved me $300. ​


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## mahohmei (Mar 26, 2009)

bobelectric said:


> Is the unit oversized?


Nope; 3 tons for 1647 ft^2 is just about right. The issue is when it's too cool outside for air conditioning, the house starts to get muggy inside, and running the air conditioner would make it uncomfortably cold.

A standalone $200 Home Depot dehumidifer with a continuous drain could solve the problem too, but I'm looking at something more "elegant". I might do this whole-hose dehumidifier when our HVAC system ultimately fails and has to be replaced (which might not be too far off; we already have a Kickstart capacitor).


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## yuri (Nov 29, 2008)

it may or may not work and I don't like it. if the air handler pressure is greater than the dehum pressure then it may fully close that damper and poor airflow thru the dehum may freeze it up and damage the compressor. they are hoping enough air sneaks by the damper from the pressure of the dehum fan but who knows if that is enough. not a good idea or one that I would choose unless there was no other option. I would try find another area to exhaust the dehum. the extra strain on the dehum motor working against the air handler is going to shorten the life of its motor too. 

send the $300 to your favorite charity or the one I use:

http://www.worldvision.org/


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## beenthere (Oct 11, 2008)

Tap both the intake and supply of the humidifier to the supply duct of the A/C. The IAQ stat is what you want to control your system.

Yes, it ill pick up the burn odor from your neighbors burning their trash.


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## DryDog (Jul 25, 2011)

*Your plan will work*

Your plan will work. Make sure you properly install the back-draft damper on the dehumidifier supply line.

The air mover in these dehumidifiers is powerful enough to overcome the duct pressure created by typical air handlers and should not be a concern.

This installation should be performed by a qualified and experienced installer as the wiring, ducting, condensate drain, and Prestige control are not simple to install and commission. That said, I am always supportive of in informed homeowner/customer.


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## mahohmei (Mar 26, 2009)

DryDog said:


> This installation should be performed by a qualified and experienced installer as the wiring, ducting, condensate drain, and Prestige control are not simple to install and commission. That said, I am always supportive of in informed homeowner/customer.


We have a 1998 American Standard SEER 10 system, and had to get a Kickstart capacitor after it wouldn't turn over in heat pump mode. Failure is probably a single-digit number of years away, and I'm considering doing a dehumidifier when we finally get a new system, to get everything at once.

My only really big issue would be knocking out a single CMU in the CMU basement wall; this would allow me to have the dehumidifier in the basement, where I can change the filters without squeezing through a crawlspace hatch.


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## mahohmei (Mar 26, 2009)

*Reheat?*

I just realized the following: a lot of commercial HVAC systems dehumidify by reheat, which, while extremely inefficient, does the trick without needing a separate dehumidifier--simply run the cool and heat coils at the same when the stat calls for dehumidification.

Our heat pump is 3 tons, which is 36,000 BTU/h, or 10.6 kW. The backup heater is 4.8 kW, but only because only half the elements are connected--the air handler has a total of 9.6 kW of elements.

This 9.6 kW would be sufficient to reheat the 10.6 kW cooling coil back to roughly ambient temperature, and would dehumidify the house without overcooling.

The drawback: energy use. Our heat pump is SEER 10, so we'd be looking at 14.2 kW to run the dehumidifier. I'm more along the lines here of asking if it's possible, and if a residential air handler can support reheat. If we all care about is dehumidification without comfort in mind, it would be more economical to connect a dehumidistat straight to the cool wire and just let the air conditioner make the house frigid to keep it at 50%.

Thanks!


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## yuri (Nov 29, 2008)

Doubt a air handler can do reheat because of the fan relays and interlocking with the elements etc. you have different relays and fan speeds and interlocking of them for heating versus cooling so both cannot run at the same time. would be VERY expensive to run . if you cool the house down very cold you run the chance of the return temp getting too cool and not enough load on the AC coil which could freeze it up and damage the compressor with liquid refrig slugging. not a good idea to fool around with these systems and ask them to do something they were not designed for.


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## bobelectric (Mar 3, 2007)

If it's a problem,then start researching high eff. multi-stage units.


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## mahohmei (Mar 26, 2009)

bobelectric said:


> If it's a problem,then start researching high eff. multi-stage units.


My parents' house has a two-stage system, and they love it...their thermostat is set to dehumidify by overcooling at stage 1 down to 3 below setpoint. Their house also has downflow (built in 1964, probably heat-only when built), so they have no vicious downdrafts that make cold spots during the summer, which I dearly hate about my upflow house.

The only issue with any sort of dehumidify-by-overheat is the fact that north Florida is extremely humid year-round--even in the spring and fall. If the house is 72F inside, I'd rather not have to chill the house to 69F just to dehumidify.


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## yuri (Nov 29, 2008)

Lennox makes a reheat/dehumidifying AC system which is patented and industry unique and gets great reviews in areas like yours. pricey but may suit you. requires extra head room to fit the extra coil in and I am not sure it works with reverse/counterflow units but probably does. they also have airhandlers with ecm motors and if you are SERIOUS about climate control may want to do some serious research and find a quality Lennox dealer who sells them AND is familiar with them and has references to prove that. I saw one here and they are quite complicated to setup so you must get an experienced Lennox co. to do it.


http://www.lennox.com/products/indoor-air-quality-systems/HD/


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## mahohmei (Mar 26, 2009)

Talking with a coworker, I realized a much faster/less expensive solution to installing a whole-house dehumidifier: place the dehumidifier itself on the floor of the HVAC closet pictured in the OP, and simply duct the output up the side of the closet to the supply duct.

I checked the NEC, and it appears to require metal conduit in air handling spaces, which would completely invalidate placing the DR65 on the closet floor, since it has screw terminal connections and an IEC input, rather than conduit knockouts. That having been said, I've seen plenty of residential construction (my house included) with NM and thermostat wire in the HVAC closet.

Any thoughts here?

Thanks!


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## beenthere (Oct 11, 2008)

It would block off too much of the return for the air handler,.


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## ionized (Jun 8, 2012)

You should be aware of this and look for like studies:
Found at building science.com

Conditioning Air in the
Humid South—Creating
Comfort and Controlling
Cost
Research Report - 0214
November-2002
Building Science Corporation


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