Whole house dehumidifier Ultra-Aire XT105H counterflow question

[mbalaev]

Hi All,

I am planning to install a whole house dehumidifier Ultra-Aire XT105H. The manufacturer recommends adding a separate return to the dehu unit and connecting the supply to the main supply line of the HVAC system. The back draft damper will prevent any back flow through the dehu unit when HVAC is on and the dehu fan is off:
http://ultra-aire.com/pdf/Ultra-Aire...Spec_Sheet.pdf
http://www.ultra-aire.com/pdf/Ultra-...05H_Manual.pdf
My question is: When the dehu fan is on and the HVAC is off and the air is blowing from the dehu supply line into the supply line of the HVAC duct work, will the air travel both ways - to the supply registers and the the return registers, creating a counter flow in the main HVAC unit? The installation instructions do not mention the need for any additional back draft dampers in the main HVAC duct work and I am not sure if there is enough space for such a damper in my duct work between the HVAC unit and the place where the dehu supply will connect to the HVAC supply.

Thank you.

[teddy bear]

Most of the dried air will move throughout the home via the supply ducts of the a/c. A small amount will flow through through the returns. The supply side is a direct open pathway to the home.
Keep us posted on your results. Where are you located? Thank you for the support.
Regards TB

[mbalaev]

Most of the dried air will move throughout the home via the supply ducts of the a/c. A small amount will flow through through the returns. The supply side is a direct open pathway to the home.
Keep us posted on your results. Where are you located? Thank you for the support.
Regards TB

Thank you for the info. I was thinking the same way. I am in Greensboro, NC, so it does get pretty humid here in the summer. We made a lot of improvements in the house in the last year (spray foam in the attic, blown in cellulose in the walls and new energy efficient windows and doors. We have not done an energy audit yet (still need to install two fireplace inserts to close two major gaps around the flues), but the house is pretty tight and even on the hottest days with a/c working long cycles the relative humidity stays at about 60% at 76°F. When we use kitchen or bath fans or clothes dryer, new hot and moist air seeps inside from all those tiny holes around the house. So, we thought putting in a decent dehumidifier with the intake of the outside air will increase the pressure inside so when the air is exhausted, it will not suck new moist air. (Btw, I am not trying to educate anyone, just offering my thoughts in case I should consider other options).

Another question I have is whether I should buy Ultra-Aire XT105H or Ultra-Aire 90H. The price difference is $670. 90H is 60% more expensive to operate to remove the same amount of water from the air. So price-wise it seems to boil down to how long it will take to break even if I buy the more expensive and more efficient XT105H. I am not an engineer, but I was pretty good in sciences in HS, so I did some calculations, which I attach in a PDF file. It seems that the main issue is to remove the moisture from the outside air when it comes inside. In fact 60cfm of fresh air for 12 hours will add 165% of the total air volume in the house (e.g. replace all air and 65% more). Does this sound excessive? Our house is occupied for at least 12 hours every day. At any rate, it seems that I will need to remove at least 80 pints per day when dehumidifiaction is needed. Some of it will be removed by the a/c, so I think I am okay with either unit. If my calcs have nothing majorly wrong, the break even period is 5.5-6 years between the more expensive XT105H and less efficient 90H. Does this sound about right?

Thank you!

[newenglandhvac]

I have never heard of a dehumidifier being used on a whole house duct system. have you given any thought to using an air handler that is capable of dehumidifing. Your capacity for dehumidification would be much greater and more controlled. Most ECM veriable speed air handlers are capable of doing the same thing with the right thermostat.
Http://www.newenglandheatingandcooling.com

Sent from my MB855 using Tapatalk 2

[teddy bear]

Another question I have is whether I should buy Ultra-Aire XT105H or Ultra-Aire 90H. The price difference is $670. 90H is 60% more expensive to operate to remove the same amount of water from the air. So price-wise it seems to boil down to how long it will take to break even if I buy the more expensive and more efficient XT105H. I am not an engineer, but I was pretty good in sciences in HS, so I did some calculations, which I attach in a PDF file. It seems that the main issue is to remove the moisture from the outside air when it comes inside. In fact 60cfm of fresh air for 12 hours will add 165% of the total air volume in the house (e.g. replace all air and 65% more). Does this sound excessive? Our house is occupied for at least 12 hours every day. At any rate, it seems that I will need to remove at least 80 pints per day when dehumidifiaction is needed. Some of it will be removed by the a/c, so I think I am okay with either unit. If my calcs have nothing majorly wrong, the break even period is 5.5-6 years between the more expensive XT105H and less efficient 90H. Does this sound about right?

Thank you!

I would expect your a/c to be able to provide 50%RH during high cooling load conditons. Make sure your a/c is setup with a cooling coil temperature that is 25^F colder than the space temperature. Dehumidification is needed when the a/c is short cycling. If you want the best dehu in the world, get the UA 105H. Paying for it self in 5 years is OK. An air change in 5 hours would be 2.4 air changes. Fresh air when occupied is a good strategy.
Keep us posted on your results.
I see we have another a/c contractor posting that a/c will dehumidify without a dehumidifier. He should read the previous posts. Thanks again for the support.
Regards TB

[newenglandhvac]

My reply was not that the a/c should dehumidify the house, only that im unfamiliar with the whole house dehumidifier being used in this situation and being installed into the duct. The blower section of this unit is capble of moving the necessary air for dehumidification and is piped in a by pass configuration as to allow the central hvac system to function correctly. Ive seen these units independantly ducted for spot treating areas such as bsmnt areas and sensitive structures, however im not so sure that increasing the overall tonage of the a/c in combination w a multi stage condenser tied to a variable speed blower (with the right controls) wouldent do as good if not a better job. Am i missing the point.... If so please direct my attention to the area of intrest i fail to see. Im not only from N.E. where humidity is no where near comprable to the carolinas, but im also a Honeywell True Dry fan if such a piece of equipment were to be necessary. That said, im just guessing at overall cfm and sqft living area, but i've over come some damp climate challanges by adding larger evap surface area and air flow modification. A dehumidifier also stands to heat the air a bit dosen't it?

[teddy bear]

My reply was not that the a/c should dehumidify the house, only that im unfamiliar with the whole house dehumidifier being used in this situation and being installed into the duct. The blower section of this unit is capble of moving the necessary air for dehumidification and is piped in a by pass configuration as to allow the central hvac system to function correctly. Ive seen these units independantly ducted for spot treating areas such as bsmnt areas and sensitive structures, however im not so sure that increasing the overall tonage of the a/c in combination w a multi stage condenser tied to a variable speed blower (with the right controls) wouldent do as good if not a better job. Am i missing the point.... If so please direct my attention to the area of intrest i fail to see. Im not only from N.E. where humidity is no where near comprable to the carolinas, but im also a Honeywell True Dry fan if such a piece of equipment were to be necessary. That said, im just guessing at overall cfm and sqft living area, but i've over come some damp climate challanges by adding larger evap surface area and air flow modification. A dehumidifier also stands to heat the air a bit dosen't it?

I forget that some of us, including me shared you convictions in the past. If you want <50% RH in home during all of variations of weather that we get in green grass climates, you need moisture removal when there is low/no cooling loads. This is a perfect application for dehumidifiers. Get yourself a %RH meter for your home and watch it throughout the seasons. With occupants generating moisture indoors and when there is moisture in the necessary infiltrating fresh air, you will exceed the 50%RH levels. Basements smell like basements because of mold growth. Dust mites grow in bedding. Homes are not as comfortable as if <50%RH. If you have none of these problems, you do not need a dehumidifier.
You should be good for 6 months, in fact you may need humidification.
Thanks for raising the issue.
Regards TB

[mbalaev]

From what I understand whole house dehus either need their own duct work (not feasible in existing houses) or their own return and connection to the current supply lines:
http://www.ultra-aire.com/pdf/Ultra-...Spec_Sheet.pdf
I think this is fairly standard. I have a WaterFurnace NDV038 geothermal unit that we put last year. I experimented and it cooled the house to 69°F when it was 105°F outside. It could have gotten lower, but I just got too cold. At that time RH was about 50%, but the next day it went back to 60-62% with the temperature inside the house 76°F. I do not see how I can use the a/c unit effectively to remove moisture.

[philjafo]

To prevent the dehumidifier from pushing air out of the return registers a relay is used to turn on the air handlers fan when there's a call for dehumidification.

[teddy bear]

r

To prevent the dehumidifier from pushing air out of the return registers a relay is used to turn on the air handlers fan when there's a call for dehumidification.

Whats wrong with a small amount of dry air flowing throughout the a/c return? This will slowly dry out the entire duct system. Runing the air handler uses upto 500 watts which is as much as the dehumidifier. No is fan operation is required when an seperate dehumidifier is used.
Regards TB

[teddy bear]

From what I understand whole house dehus either need their own duct work (not feasible in existing houses) or their own return and connection to the current supply lines:
http://www.ultra-aire.com/pdf/Ultra-...Spec_Sheet.pdf
I think this is fairly standard. I have a WaterFurnace NDV038 geothermal unit that we put last year. I experimented and it cooled the house to 69°F when it was 105°F outside. It could have gotten lower, but I just got too cold. At that time RH was about 50%, but the next day it went back to 60-62% with the temperature inside the house 76°F. I do not see how I can use the a/c unit effectively to remove moisture.

Ideally, I would suggest that the air flow through your a/c be slowed slightly. From your numbers, I would varify that your a/c has a 47^F coil temperature with a 76^F a/c return temp. This would optimized the a/c--dehumidifier interaction. Your a/c appears to have excess air flow during normal operation. During the hottest days of the year, your a/c should be able to maintain <50%RH. The dehumidifier is needed during evenings and during weather when the a/c is short cycling.
Keep us posted on your results.
Regards TB

[philjafo]

Fan operation isn't required but if using filter grills blowing air through them backwards will blow dust out of the filter, in fan on mode the fan speed is reduced so depending on the size of the system wattage for the fan can be less then 100 watts,

[teddy bear]

Fan operation isn't required but if using filter grills blowing air through them backwards will blow dust out of the filter, in fan on mode the fan speed is reduced so depending on the size of the system wattage for the fan can be less then 100 watts,

Good come back. The small amount of air that flows backwards through the coil/filter will not blow dirt off of the filter.
Yes, VS fans on low do not use much energy. Ok, VS fan on low is the best way to provide IAQ and uniform comfort to the home. An additional benefit is that the ducts/coil/pan are dried out throughly between cooling cycles. This reduces the possibility of mold/bacterial growth in these areas over the long term. The slight down side is that moisture on the a/c coil re-evaporates faster into the home. With the dehumidifier inplace, there is no problem. Without a dehumidifier and a short cooling cycle, expect slightly higher %RHs. I also operate my VS fan on "low" 24/7. Of course I maintain <50%RH with a whole house ventilating dehumidifier.
Regards TB

[srivenkat]

Folks,

I am looking at the XT105H as well. I am in a 2150 SFT ranch with a full basement. What I am thinking of doing is to have the dehu draw air from the return duct of the HVAC system (in the basement) so that it hopefully draws air from all the return (and possibly some supply registers as well) registers in the home and have the dehu supply to a large general area of the basement using a single short duct connection to a single register. The reason I am thinking of supplying to just the basement is that, I heard there's normally negative pressure in the basement due to the stack effect so I am thinking supplying to just the basement will serve to balance that out to whatever extent it can.

Will this work?

Thanks in advance.

[teddy bear]

Yes, this will work. It will also cause circulation between the rest of the home, which is a good thing. During the moderate temps of summer, the stack effect is minimal. When the stack is strong, outdoor dew points are low and dehumidification will not be needed.
Keep us posted on how this works.
Regards TB

[srivenkat]

Yes, this will work. It will also cause circulation between the rest of the home, which is a good thing. During the moderate temps of summer, the stack effect is minimal. When the stack is strong, outdoor dew points are low and dehumidification will not be needed.
Keep us posted on how this works.
Regards TB

Thanks for the info. One other question I have about the XT105H is that the spec sheet specifies the input air temp as 56F to 95F, but no specs on the operating temp. If I place it in a garage which can dip down to single digit temps in the winter, will it work (not the dehu part, but the vent part)?

[teddy bear]

Thanks for the info. One other question I have about the XT105H is that the spec sheet specifies the input air temp as 56F to 95F, but no specs on the operating temp. If I place it in a garage which can dip down to single digit temps in the winter, will it work (not the dehu part, but the vent part)?

I have concerns about circulating house air through any device located in an extreme low temperature. The concern is about condensation forming on the cold surfaces of the dehu. I suspect that supplemental ventilation will not be needed during extreme cold weather. If the unit operates 24/7 and is kept warm, it will not have condensation. Basement is a better location for the unit.
Keep us posted.
Regards TB

[srivenkat]

I have concerns about circulating house air through any device located in an extreme low temperature. The concern is about condensation forming on the cold surfaces of the dehu. I suspect that supplemental ventilation will not be needed during extreme cold weather. If the unit operates 24/7 and is kept warm, it will not have condensation. Basement is a better location for the unit.
Keep us posted.
Regards TB

The problem with the small utility room in my basement is that there's no easy/practical way to get another 6-inch insulated duct to be routed in for fresh air intake. Also, I was thinking placing it in the garage as opposed somewhere else in the basement would help isolate some of the noise. I understand the unit would only work to pump in fresh air during the winter, so there won't be any circulation of the warm home air thru the unit right? Would it be possible to disable recirc/dehu function while leaving the fresh-air intake function on?

[srivenkat]

One thing I failed to mention is that the exterior walls of my home are ICF (Insulated Concrete Form) and the wall of windows to the north condensate on the inside in the winter. So, I am hoping the cold dry air pumped in by the unit would serve to bring the humidity down some in winter.

[teddy bear]

Yes, if your windows sweat during cold weather, more fresh air is needed in the home. With some special treatment, it may be possible to operate in the garage during extreme cold weather. This would require additional insulation and insulated ducts. You need to use some 2" blue board to surround the dehumidifier.
The 2" foam box needs to be removable for service.
Regards TB