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ERV, Whole house dehumidifier & proper ducting.

user-7716234 | Posted in Mechanicals on

Questions on ERV & whole house dehumidifier – ducting.

Background: San Antonio TX (2B), new residential construction completed June 2021. Single story, slab on grade, 3Br 3Ba, about 3700 sq ft. 10’/12’ ceilings. Zip R3 sheathing over conventional 2×4 stud walls, open cell spray foam, walls/ attic roof. 4ton Carrier Infinity series heat pump feeds great room, kitchen, office, laundry. 2 ton infinity series heat pump feeds all 3 bedrooms and guest & master baths. Air handlers & insulated ducting located in the sealed non vented attic. On original install. our HVAC contractor installed 2 carrier 198 cfm variable speed ERV’s one to each air handler using “simplified system” (indoor exhaust air & fresh air intake each tied to return air duct plenum upstream of air handler for each ERV). The tie ins for each of the ERV ducts into return plenum are approximately 40”-46” apart ctr-ctr. However, the exhaust air duct tie in is only 50” in each case in front of the air handler filter. The Carrier thermostat set up appears to allow for an ERV “auto setting” whereas air handler fan kicks on when the ERV auto mode is set to cycle on (ERV on auto operate at minimum speed for 20 min each hour). The internal residence temp is set to 75F and a target 50% humidity setting (I was advised by my HVAC installer not to set below 50%) and to me the rooms feels muggy and at times stale, indoor humidity levels range from 57% – 47% and seem to vary throughout the summer days. The only time we are dehumidifying is when condensers are running.  We have outdoor terminated exhaust fans in each of the 3 baths & laundry and use them regularly for when rooms are occupied & then some.

I have some concerns about the effective performance of the ERVs adding proper ventilation as currently ducted. 1) Would I benefit by modifying to a Hybrid ducting of the ERVs (modify with dedicated ductwork to pull stale air back to ERV) and 2) adding a whole house dehumidifier to the 4 ton unit with separate ducted intake and output to supply plenum? And… would there be any benefit to replacing one of the ERVs (currently tied to the 4 ton) with a ducted ventilating dehumidifier like Ultra-Aire 98H?- As I understand, this would bring in fresh air much like the existing ERV? I am getting a bit overwhelmed on how best to rectify my indoor air quality   Many Thanks for your consideration.

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  1. user-2310254 | | #1

    I’ll give you a bump.

  2. Expert Member
    Akos | | #2

    The first problem is you have 6 tons of cooling in your house. If you look at the thermostat run times, I would be surprised that the system runs even half the time. The turndown the Infinity units is not the greatest, with that amount of oversizing, they will be cycling which is the opposite you want for good humidity control.

    Before doing any major rework, I would try to get more runtime out of the system.

    Options are to run the bedroom unit at lower temperature during the day, essentially use it is a dehumidifier for the rest of the house.

    The other thing you can look at is to have the installer reduce the fan speeds on low for both units. If possible, getting the airflow down to around 250CFM/ton will increase the amount of humidity removal.

    I think once you have the humidity under control and more runtime on your units, the air won't feel as stale.

  3. walta100 | | #3

    Do you have the Carrier Infinity thermostat?
    If so, did you set up the humidity controls?


  4. user-7716234 | | #4

    Thank you for the comments- I agree that the total of Units may very well be oversized, interesting idea of setting the bedrooms to a lower temp than the great room.

    Yes, the Carrier Infinity thermostat(s) (installed) allows for setting a target humidity- and has a setting that "tells the unit to overcool up to 3 dg to dehumidify". The target humidity settings ranges from 58% to 46% in 2% increments. Full disclosure; during this current summer - I set the 4 ton thermostat to target 75 dg @ 48% for several days...(bedrooms set to 76/48) and the 4 ton compressor failed sometime within a week or so (1 year since install). Per my installer, Carrier warranty dept said - "compressors are on back order", you can either wait or we offer a 1 time full equipment replacement (for outdoor unit). I opted for the onetime replacement. My installer suggested I don't target anything below 52% on the thermostat going forward.......
    I didn't think running the equipment would make it fail so prematurely.

    This got me thinking about the ERVs, ducting configuration and separate dehumidification. I think I am still in the camp of adding a whole house dehumidifier, but will see what I can achieve with the current configuration.

    1. Expert Member
      Akos | | #5

      I call BS on the compressor failing from you running the unit. Most early failures on HVAC are due to contamination during install. The unit would have died no matter what setpoint you used.

      For example, I have a undersized mini split that is used for both heat and cool on a studio. This thing runs 24/7/365. Because it is undersized, it is running at above 50% load from late fall to early spring and 100% between Jan and Feb. It has been doing this for at least 5 years now.

      I would still see what you can get out of your existing equipment before changing things.

  5. walta100 | | #6

    I say what ever setting you select will not make the compressor fail.

    Yours sounds like one of the Carrier’s higher end unit and would have enough sensors that would protect the compressor and I don’t think you can kill a scroll compressor if you did somehow manage to freeze the evap coil and feed liquid to the compressor.

    This line of BS makes me think you need a second opinion from a different contractor.


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