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  1. #1

    Confused Geo + Aux Heat: Setback and Economy

    I'm the new owner of a residential geothermal HVAC system and have encountered some troubling surprises concerning the way the system uses the aux (gas) heat. First, I learned that using the programmable setback feature of the t-stat is not advised, because: "If the geo is called upon to raise the temperature more than 2 degrees, it will trigger the aux heat, which is much more expensive to run." What's more, "when the aux heat is triggered, the geo shuts down," thus deriving zero benefit from it.

    Is this correct, and is it necessary? I can understand that an A-coil mounted above the furnace (that is, in the path of the gas-heated air) won't be able to transfer much, if any, heat to air that has been preheated, perhaps even to a temperature hotter than the geo fluid. If that is the reason behind shutting off the geo, then couldn't the A-coil be placed ahead of the furnace??

    And if there is no getting around it, is there a way to make the system run only on geo? Could it lock out the aux heat as long as the indoor temp remains above a programmed threshold? There are many times (overnight, for one, or when the house is empty during the day) when I would choose economy over comfort.

    (The system consists of a Bryant Envision 120,000 BTU gas furnace and Waterfurnace NDZ064 with de-superheater and 5-ton loops; NRAC 060 coil; Honeywell VisionPro IAQ thermostat.)

    Thanks for any insight, options, suggestions, or other help you can provide.

    - MM / montgomd

  2. #2
    Join Date
    May 2000
    Indianapolis, IN, USA
    With the IAQ control and an outdoor stat you have quite a bit of flexibility on when backup runs and doesn't run. Set 345 and 346 to 0 and backup will never come on no matter what. It will change to gas only at the changeover outdoor temp.

    You can't have the coil before the furnace, the heat exchanger will get cold in the summertime and rust out prematurely.

  3. #3
    Thanks for quick reply. I tried 345 and 346 at 0 last night, along with setback, but the aux heat kicked on anyway. Outside temp was around 30F. Am I missing something?

  4. #4
    Join Date
    Feb 2009
    Portland OR
    Whoever designed your system should be able to give you the setpoint of where the geothermal system can no longer keep up(assuming a low EWT) and should be able to lock out the gas heat any temperature above that. I would also recommend changing to a 4 degree change over so you can set your stat back about 3 degrees at night(what I prefer with my heat pump).

    Is your loop vertical or horizontal. What kind of EWT's have you been seeing from the system and what was the heat loss on your home?
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  5. #5
    Join Date
    Aug 2004
    S.E. Pa
    frumper15, This is the Ask Our Pro's or AOP forum. In order to post responses here, you must have verified qualifications and have been approved by the AOP Committee. Please see my signature line for links to register and review the AOP Forum Rules. Thank you. Your post will now be deleted. Further infractions may result in loss of posting privileges.

  6. #6
    The loops are vertical closed (3x250'), using a chemical heat transfer fluid. I don't know the EWT, but understand it to be no higher than 50F. The same company also did an energy audit of my home (blower test included). They did not give me the estimated heat loss, but I have a call into them for this.

    Your questions suggest to me that I would do well to ask my installer for system documentation beyond the user manual, for example: EWT, installer settings for thermostat, and, if there are any options, how those settings affect the system performance, cost of operation, and longevity. What else would be useful to document baseline system parameters, especially for future use for maintenance as well as asking questions of third parties?

    [NOTE TO MODERATOR: Should I repost the last question either in this forum or elsewhere?]

  7. #7
    Join Date
    Sep 2008
    Western PA
    Quote Originally Posted by montgomd View Post
    [NOTE TO MODERATOR: Should I repost the last question either in this forum or elsewhere?]
    Your post is fine where it is.

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