Hello.

I am considering the purchase of a UA-90H, UA-100V or UA-XT150H whole house dehumidifier for new construction.

As background: 

We had a horrible mold problem in our previous home.  The problem was related to direct water infiltration (numerous leaks) and not necessarily just the normal transference of moisture from outside to inside that occurs in hot and humid climates.

The house will be located about 50 miles south of Atlanta GA.  Very hot and humid summers.  The house has a 2700 sq ft basement constructed of Superior Walls precast concrete – this should keep the basement drier and more moderate in temperature than a standard basement as the walls are insulated and more watertight from the beginning.  The main floor is 2000 sq feet and the second floor is 2500 sq ft. (The square footages are correct as the basement extends under the garage).  There is the very real possibility that we will not be finishing the basement immediately.

Air handlers will be two GeoThermal Water Furnace ENVISION series 6-ton dual speed (6 and 4 tons in high and low compressor modes respectively) with MERV 11 filters.  These will handle:  Second floor for one ENVISION unit, main floor and basement for the second ENVISION unit.

Both the basement and the main floor will have fully furnished kitchens.

Insulation will be Icynene spray-in foam insulation.

I have a knowledgeable HVAC guy, but I like to stay on top of the issues.

The issues:

First: I would like to keep my indoor relative humidity around 48% during these hot and humid months in Georgia.  This would help control mold growth as I am sure some of our personal belongings, even though they were professionally cleaned, will still carry some contamination.

Second: Obviously the house will need mechanical ventilation.  I have looked into ERV’s; however, the energy exchange isn’t that impressive as going from 95 degree air to 75 degree air just doesn’t gain you that much energy savings (as opposed to the great white north where HRV’s exchange 70 degree inside air with -10 degree outside air – quite a temperature difference) Also, none of the ERV’s have stellar moisture removal properties – most seem to give a portion of the outside air’s humidity to the outgoing dry stream from the house.  As far as I can tell, this just increases my moisture load the A/C units will have to deal with.

Third:  I want to avoid tying the whole house dehumidifier into the second floor heat-pump if at all possible because I don’t want do circulate air through ductwork in the attic (thus heating it substancially in the summer) unless it is being actively conditioned by the air handler.

Forth:  Mild winters may make ERV’s’s or HRV’s virtually meaningless anyway as the heat recovery is negligable with temperature differences of 25 to 30 degrees from indoor to outdoor temps.

All that being said, here are the questions:

1. How big of a unit do I realistically need and will any size unit give me meaningful help, even if it is only a 90 pint system?

Thank You.

1. How big of a unit do I realistically need and will any size unit give me meaningful help, even if it is only a 90 pint system?

A 90 pint will do a 2,500 sqft. isolated basement.  It will have an sight impact on the first floor. With an open stairwell the impact will be on the first floor greater and the basement control less.

2./3. Will the unit help dehumidify the whole house even if it is only directly tied into the basement and main floor HVAC system? 

An UA XT 150 H will dehumidify the basement and first floor.  The impact on the second floor depends on the isolation of the individual rooms.  A couple individual ducts to the larger, more isolated areas will help.

4. Can any of these systems interface with more than one humidistat? Specifically, if any one humidistat needs dehumidifying, can it alone trigger the dehumidifier?

Several dehumidistats can be connected to one dehu.  Each will operate the dehu.  Balancing dampers on the dehu also help.

5. What effect does the whole house dehumidifier have on the HVAC sizing and what modifications will need to be made to any “Model J” calculations? 

The dehus do not effect the sizing of the a/c.  Properly installed a/cs will control the %RH while operating at high cooling loads.  The dehu will only be needed while the a/c is at a partial load cycle.  Many use the dehu to control the %RH when the home is routinely not occupied. The a/c is t-stat is set-up.  This raises the house temperature will reducing the %RH.  This method of controlling %RH conserves large quantities of energy.  Best to have a small amount of extra a/c capacity to cool the home down prior to occupancy.   

My compliments on the quality of your design and questions.  Keep us posted on your results.  Ken

Thank you for your responses.  I have a couple follow-up issues if you do not mind.

Simply stated: I am trying to determine if it is worth my time and money to attempt to help the A/C's on the main and second floor of the house and secondly, if a UA XT 150 H is worth the upgrade considering I am already over budget.

Lastly, how well do the MERV 11 and MERV 14 filters seal up inside the units themselves? Any airflow get by the filters?  I understand other manufacturers have some issues with the filters sealing.  Are yours tight?

Hope these are not too many questions...

Thanks

You need 75-100 cfm of fresh air ventilation when occupied.  The moisture in the fresh air  is a 90 pint per day dehumidifier load.  Combined with the occupants moisture load and other misc. moisture infiltration, you need a minimum of  one UA XT150 H.  Going with less equipment decreases the time you be ability to main 48%RH.  A 90 pint whole house dehu provides better contol than two residential dehus in the basement and an ERV.  When your a/c is short cycling during low/no cooling load, your supplemental dehu load is the greatest. The duration of the wet cool weather is difficult to forcast.  Therefore I am unable to assure you of 48% RH during the wet cool weather one UA 90H. You may consider starting with a 90 pint and monitoring results.  Although not as neat, you could add addition dehu capacity if needed.   Replacing two residential dehus and an ERV ventilating system with one UA XT150H is worth the extra money.  The balancing with manual dampers was my intent.  The ratio you suggest is reasonable. 

Filter fit is ok on the UA 90H.  Better fit on the UA XT150H. In both cases, fit is adequate to maintain a clean unit.  If on a tight budget, standard merv 11 filter is good.  Keep us posted. Thanks again. Ken

What are the filter demensions for the UA-90, UA-100 and UA-XT150H systems?

I ask because on page 4 of the XT150H manual it says 2x16x16 and on page 25 it says 20x16x2.

I assume these are standard sizes and any standard sized 20x16x2 will fit in the XT150H housing.  I can get "MERV 14 plus" (just shy of MERV 15) filters all day long for about 20 bucks that will fit right in that housing and give me better than factory filtration (I know you guys don't want to here that, but with the mold problems I have had I have to go aftermarket for some solutions to my problems)

MERV 11 filters capture all mold spores/pollen.  I assume the pressure drop of a MERV 13 is OK.  Our option for premium filters is a 20" X 24" X 4" MERV 14.  It requires an additional filter box that has a 2" MERV 11 prefilter with same size.  The MERV 14 last for 3 years when used with the prefilter.  The prefilter needs replacing yearly. 

Suggest you change your MERV 13 yearly or when the pressure drop exceeds .15"WG.  Thanks Ken