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  1. #1
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    Apr 2011
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    Question HVAC-ERV-Dehumidifier Layout for Humid Climate

    Looking to optimize the HVAC design of a house in NC near beach on pilings. House is 2580' tight insulation design, inverted floor plan with all closed-cell foam (attic, walls, below 1st floor, etc.) and had planned for 2 zones (one for each floor) with an additional zone on the 2nd floor for the master suite. After a couple of Manual J iterations with an independent engineering firm to get the correct information the results show that 1 ton is needed for 1st floor and 1.2 tons for the 2nd floor. Current plan with HVAC contractor is to now have two Trane 15 SEER XL 1.5 ton units with the new variable speed 2-ton air handlers with an ERV for fresh air for each floor, plus the humidity controlling Tstats, but because of the high outdoor humidity it was suggested to also include a whole-house dehumidifier just on upper floor. After looking at all this I am wondering if this is the optimal design from a cost, energy use and duct-integration considerations.
    I am now considering dumping the multi-zone idea and considering other options:
    1. Go to single zone for the whole house with a single right-sized 15 SEER XL or even the 20 SEER XL system (2.5 tons?) with single ERV and dehumidifier integrated somehow (I have lived in other houses with a single system for multiple floors, but they never seemed to be comfortable on all floors at the same time);
    2. Upgrading to the 2-stage 20 SEER XL and dual zone it for the two floors since it has a two-stage compressor that may allow this (not sure if it's possible to get a good duct layout with this house design...).
    3. Use a mini-split system just for the 1st floor (not a big fan of these though after initially looking at costs and reliability, plus they all seem to be non-U.S. made...).
    I need to decide on the final concept before following up with the final Manual D, S and T. There seem to be no shortage of ideas out there on these topics, but for this application does any one have better suggestions, and the best way to integrate the ERV and dehumidifier? Thanks!

  2. #2
    Join Date
    Jan 2004
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    PA
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    68,793
    2 stage units don't come in half ton sizes. So you would either need to go to a 2 ton or 3 ton if you go with a 2 stage. Best to have 2 systems. Or a single unit zoned is ok.

    The whole house dehumidifier would probably be better on the first floor system. Since the second floor would have a load on it sooner since heat rises, and probably run enough to keep the humidity down.
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  3. #3
    Join Date
    Aug 2003
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    Fort Worth, TX
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    We can't engineer your house HVAC from here. Can't see it, for one thing.

    Additionally, you mentioned that an independent engineering firm has been involved in the design, but for some reason you now do not trust their design? Other than what you touched on in general in your OP, do you have specific qualms as to why their design does not sit well with you? What specifically led you to question their recommendations?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #4
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    Apr 2011
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    Quote Originally Posted by shophound View Post
    We can't engineer your house HVAC from here. Can't see it, for one thing.

    Additionally, you mentioned that an independent engineering firm has been involved in the design, but for some reason you now do not trust their design? Other than what you touched on in general in your OP, do you have specific qualms as to why their design does not sit well with you? What specifically led you to question their recommendations?
    Thanks, I can certainly send all the drawings and manual J reports if you would like me too. The firm doing the Manual J, S and D is doing exactly as we requested and they noted it may take a 2 or 3 iterations of J, S and D before everyone would agree on the best solution; the initial manual J they did not have the right inputs for local conditions, windows and upper floor volume; so that is corrected on the latest iteration of manual J, but the results show that both floors would be undersized (more so for the 1st floor) for the equipment that we assumed. Based on the results of the latest iteration of the Manual J we owe the engineering firm an update on the desired equipment, so they can match up the coil, etc. with Manual S and edits on their initial Manual D. My increasing frustration is that I have a GC, HERS rater, local code official and HVAC contractor that all seem to have different ideas about solutions. As background I have been planning for almost a year for the design of this house, working with a local designer and so forth for a house that meets both fortification and energy efficiency. The designer did not do any HVAC design at the beginning, noting that the "HVAC contractor" would do all this. That approach is obviously a mistake as I would have probably have done things differently knowing I had margin available in the equipment.

  5. #5
    Join Date
    Apr 2011
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    Question

    Quote Originally Posted by beenthere View Post
    2 stage units don't come in half ton sizes. So you would either need to go to a 2 ton or 3 ton if you go with a 2 stage. Best to have 2 systems. Or a single unit zoned is ok.

    The whole house dehumidifier would probably be better on the first floor system. Since the second floor would have a load on it sooner since heat rises, and probably run enough to keep the humidity down.
    Yes, you are correct, I should have looked more closely at the Trane web site. However, for the 2 ton unit I know the 1st stage kicks in at 1.5 ton, but for the 3-ton unit does the 1st stage come in at 1.5 ton also?

  6. #6
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    PA
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    On the 20 its 1.5 ton for first stage on the 3 ton.
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  7. #7
    Join Date
    Jun 2003
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    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by trmac51 View Post
    Looking to optimize the HVAC design of a house in NC near beach on pilings. House is 2580' tight insulation design, inverted floor plan with all closed-cell foam (attic, walls, below 1st fl
    I need to decide on the final concept before following up with the final Manual D, S and T. There seem to be no shortage of ideas out there on these topics, but for this application does any one have better suggestions, and the best way to integrate the ERV and dehumidifier? Thanks!
    The most critical component is the ventilating whole house dehumidifier. By using a return from the open part of the top floor and suppling the dehu output to the supply on the lower level of the a/c supply, you can get whole house humidity control. I assume the stairwell is open between floors. If a single a/c is used, The dehu return canbe from any open part of the home. With all of the exhaust devices in the home and because on the very mild local conditions, an ERV is not as critical as a cold northern climate. Your clothes drier, kitchen exhaust, and bath fans all need make-up, which the whole house dehu with make-up air ventilation would be best. Positive pressure in your climate has the advantage of preventing unfiltered, humid air from infiltrating the structure.
    Also I would encourage you to use the Ultra-Aire 90H high efficiency dehumidifier as the most efficient, durable unit. If you desire to continue with the combo of the ERV/dehumidifier connection of the are possible.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  8. #8
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    Apr 2011
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    8

    Hmm

    Quote Originally Posted by beenthere View Post
    On the 20 its 1.5 ton for first stage on the 3 ton.
    Thanks, I guess that leaves an interesting trade-off, balance the increased duct complexities, cost and potential control issues using either a single zone or perhaps two zones across two floors using the more efficient single system; vs. conventional type arrangement of two less efficient systems with one on each floor. Has anyone tried zoning across two different floors with a 2-stage system and what kind of problems am I going to run into? It seems logical that from a long-term support perspective it would be better to have to deal with the maintenance, repair and eventual replacement of a single HVAC unit vs. two units; but then I don't know how much additional trouble it will be balancing and maintaining a zoned system; and whether from an IAQ perspective it would be as good as a more conventional approach.

  9. #9
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    Apr 2011
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    Quote Originally Posted by teddy bear View Post
    The most critical component is the ventilating whole house dehumidifier. By using a return from the open part of the top floor and suppling the dehu output to the supply on the lower level of the a/c supply, you can get whole house humidity control. I assume the stairwell is open between floors. If a single a/c is used, The dehu return canbe from any open part of the home. With all of the exhaust devices in the home and because on the very mild local conditions, an ERV is not as critical as a cold northern climate. Your clothes drier, kitchen exhaust, and bath fans all need make-up, which the whole house dehu with make-up air ventilation would be best. Positive pressure in your climate has the advantage of preventing unfiltered, humid air from infiltrating the structure.
    Also I would encourage you to use the Ultra-Aire 90H high efficiency dehumidifier as the most efficient, durable unit. If you desire to continue with the combo of the ERV/dehumidifier connection of the are possible.
    Regards TB
    Thanks - will discuss this duct arrangement with the installer. Also, many new and existing houses in NC beach area do install only the dehumidifier and not the ERV, per the code official I talked with. But he also stated he has noted a trend of both the ERV and dehumidifier being added. The HVAC installer has also done it both ways (and has used the ultra-air brand). My initial plan was to use only an ERV, due to the increased energy efficiency; but I am learning it may not be sufficient for the local area. I am beginning to think it may be overkill to have both an ERV and de-humidifier; even though it might help a little on energy costs.

  10. #10
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    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,644
    Quote Originally Posted by trmac51 View Post
    Thanks - will discuss this duct arrangement with the installer. Also, many new and existing houses in NC beach area do install only the dehumidifier and not the ERV, per the code official I talked with. But he also stated he has noted a trend of both the ERV and dehumidifier being added. The HVAC installer has also done it both ways (and has used the ultra-air brand). My initial plan was to use only an ERV, due to the increased energy efficiency; but I am learning it may not be sufficient for the local area. I am beginning to think it may be overkill to have both an ERV and de-humidifier; even though it might help a little on energy costs.
    ERVs transfer a small amount (<30%) of moisture from the fresh air to stale exhaust when the exhaust is dry. Without a dehumidifier to maintain <50%RH, the home will become the same dew point as outside after a day of fresh air ventilation.
    Exhausting damp air humidifies dry fresh air, not desirable. So exhausting a damp bathroom humidifies the fresh air.
    So the whole house dehumidifier is a critical component to maintain <50%RH during +60^F outside dew point ventilating/infiltration. Plus you have the mositure from the occupants that is added to the inside of the home. The energy saved by homes in your region, is not enough to have any payback. Also there is no make-up air for the cloths drier, kitchen hood, and bath exhaust. Also a slight positive pressure is maintained on the home which prevents infiltration of unfiltered damp air into the structure during wind and stack effect.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  11. #11
    Join Date
    Apr 2011
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    8
    Quote Originally Posted by teddy bear View Post
    ERVs transfer a small amount (<30%) of moisture from the fresh air to stale exhaust when the exhaust is dry. Without a dehumidifier to maintain <50%RH, the home will become the same dew point as outside after a day of fresh air ventilation.
    Exhausting damp air humidifies dry fresh air, not desirable. So exhausting a damp bathroom humidifies the fresh air.
    So the whole house dehumidifier is a critical component to maintain <50%RH during +60^F outside dew point ventilating/infiltration. Plus you have the mositure from the occupants that is added to the inside of the home. The energy saved by homes in your region, is not enough to have any payback. Also there is no make-up air for the cloths drier, kitchen hood, and bath exhaust. Also a slight positive pressure is maintained on the home which prevents infiltration of unfiltered damp air into the structure during wind and stack effect.
    Regards TB
    Thanks, upon further discussion with the HERS rater, HVAC installer and above commentary I am eliminating the ERVs out of the design. I am going to re-run the Manual D for for a single HVAC (Trane 1.5/3 ton SEER 20 XL) with variable speed air handler, with three zones (1st floor, 2nd-floor master suite, 2nd-floor living area), with a single UA 90 dehumidifier / fresh air hookup. If the single HVAC system won't work due to duct restrictions I will fall back to the 2 HVAC system.

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