Running supply duct above heat pump water heater

Discussion in 'General Discussions' started by gbig2, May 10, 2012.

  1. gbig2

    gbig2 New Member

    Getting estimates on geo at my home. Current forced air propane system with electric hot water tank. One guy said he'd go desuperheater, then another storage tank (pre-heat tank?) and then a new hybrid heat pump water heater. Of course the concern is that a hybrid water heater will pull all the heat out of the basement in the winter (current basement is not heated, but is insulated).
    So he suggested running a supply duct above the hybrid water heater. I guess figuring it's better to use the heat from the geo in the winter vs using the electric hot water heater. Does that make sense? Is this something that you guys do in your installs? The hybrid tank is in a tiny closet so a very tiny supply duct would certainly warm the area up.
    A hybrid tank would help cool and dehumidify in the summer.
     
  2. Looby

    Looby Member Forum Leader


    Highly dis-recommended. Check the manufacturer's requirements regarding
    the minimum volume of the room containing a heat pump water heater (HPWH).

     
    Last edited: May 10, 2012
  3. gbig2

    gbig2 New Member

    I should say medium sized closet and the door would stay open. But I guess my point was, a very small supply duct would send a very small amount of heat and would keep that area warm so the hybrid water heater would run very efficiently. I'm just wondering if anyone does installs like this? My current water heater is 8 years old so I'll need to replace in the next couple years, so I'm thinking a hybrid tank is a good idea with a very small supply off the main truck going to the tank?
     
  4. zacmobile

    zacmobile Guest

    I've seen some hybrid water heaters (Chinese imports mind you) where they are actually meant to have ducting attached to the in & out, not necessarily connected to your central air system though, seems to be intended to be ducted outside (for milder climates only I assume!) Not that this would work in you case, just thought it was an interesting thing to mention.

    I don't see why it would be an issue to run a small outlet into the space except it may make it less than an ideal temperature storage space for certain things and if you have cooling as well who's going to remember to close the duct in the summer?
     
  5. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Demands

    Your hot water demands and your home heating/cooling are two independent systems. You won't want your hot water demands dictating when you need to turn on the air conditioning for example. The temperature in your closet won't be dictating the home's temperature is another example.

    So, it may work. But accidentally so I'm thinking.
     
  6. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Fleshing out what Urthbuoy writes

    Tucking a HPWH into a small closet only works if that closet is well-ventilated, and that ventilation must be available whenever the HPWH runs. The HPWH will run for long periods after deep hot water draws, such as a long shower or 2-3 showers back to back.

    To recover a full tank from scratch may take 4-5 hours depending on incoming water and ambient air temperature. During recovery the HPWH will flow 100+ CFM of air and lower that airflow's temperature by 15 deg F every trip through the unit.

    If the HPWH is in a small closet, ventilation of that closet needs to be controlled by something other than supply air to the closet flowing only when the parent zone or system calls.

    Possibilities include continuous ventilation, driving a vent fan whenever the HPWH runs, or a temperature controlled ventilation scheme such as operating mechanical ventilation of the space whenever space temperature is below 55-60.

    Beware supercooling a closet housing an HPWH to the point where that closet's wall temperature is anywhere near ambient dewpoint - surefire recipe for moisture, mold and rot.

    Don't get me wrong, I'm a huge fan of HPWH, most of my energy retrofit clients wind up with one, but know the product's features and limitations.
     
  7. zacmobile

    zacmobile Guest

    But the water heater would dehumidify the space as it cooled would it not?
     
  8. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    YES - while the HPWH operates (essentially like a small window airconditioner that doesn't need to be in a window) it will cool, and, if dew point of its room air exceeds evaporator surface temperature, dry the air in the room.

    BUT:

    If while placed in a closet, per the extensive Advanced Energy Corp test, it cools the closet's interior to below the dewpoint of the air in surrounding rooms, that surrounding humid air will encounter elements of the closet's wall assemblies and leave water on them. A closet's wall assemblies likely include yummy mold food, such as wood and particle board, as well as mold Cavier, otherwise known as unpainted drywall backing paper.

    To wrap your mind around this takes a little walk around a psychrometric garden; principally one must understand the interaction between relative humidity and sensible (dry bulb) temperature changes. Try this question:

    Given: An air conditioner of any description (central, window, geo, air source, heat pump water heater, minisplit, automotive...whatever) is operating and has been operating for awhile (10+ minutes) in addition to producing nice cool air it is also dripping water via the condensate drain.

    Question: What is the approximate relative humidity (percent) of the air leaving the evaporator of the above-described air conditioner?
     
    Last edited: May 19, 2012
  9. CharBroiled

    CharBroiled New Member

    hi gbig

    just had a geo system much like what you are describing installed. 3 ton horizontal loop with desuperheater that preheats a dummy 50 gallon tank (actually an electric tank not hooked to electric) and then a GE hybrid air source hot water heater in an unheated unfinished basement.

    asked to owner of the company when he did the final checkout and he said basically the ground heat from the basement floor/walls ( floor is 5.5 feet below grade) and heat that gets through the duct insulation and unit itself will keep it warm enough (low 50s ?).

    the hybrid htw is great for summer though, its cooling and dehumidifying the basement well. we used to have to run a dehumidifier close to 24/7 during the summer months, so far it hasnt kicked in that I know of (its there as a backup).

    guess we will see come winter though :)
     
  10. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    A louvered door on the closet would be the easiest way to provide constant air exchange to the HPWH.
    The next question is "is this closet in an area where the noise will offend?"
     
    Last edited: May 27, 2012
  11. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I'm not certain a louvered door alone will provide for enough air movement to prevent an HPWH operating in a confined space from lowering air temp below ambient dewpoint.

    If such air temp dips are fairly short-lived, it shouldn't be a problem. OTOH if the interior of the HPWH closet walls sweat for any length of time, look out!
     
  12. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    In our biz sometimes a known air circulation requirement is easily quantified and allowed for as warm moves to cold. I do not see a problem in a closet with enough "free" area of gravithy ventilation.
    Sometimes the old fashioned ways are just fine.
    That said, I'm still not sure I want a compressor in a closet with a louvered door in my living area. I'd hate to have to turn up the TV cause someone is showering.
    Frankly the reward on these heaters has yet to off site the price in my AO.
     
  13. gbig2

    gbig2 New Member

    I can get a GE Geospring hybrid for $1000 from lowes and get $300 rebate from electric company or I can just get a standard 50 gallon tank for $500 plus $50 rebate. Figure same installation costs so hybrid tank costs $700 upfront and standard tank costs $450. Difference of $250. I hate the idea of basement being even colder in the winter. It's not a humid basement, dehumidifier barely runs. I'm thinking at this point to just go with desuperheater and keep my 8 year old standard tank. Geo company will install second pre-tank.

    I just don't think I'm going to save that much electric running hybrid vs standard since desuperheater will take care of most of my hot water in the Summer and it's the Summer when the hybrid tank would save $, and again, a colder basement in Winter. So I think I'll just go desuper and keep standard. So my original question is moot, no need to run small duct to closet containing hybrid tank.

    I'm in PA, any idea what % I can count on saving on hot water costs with the desuperheater? %30?

    I was even looking into solar thermal since I do have south facing sloped roof, but even with %30 federal tax credit and %35 state grant, it's still $2500 and I don't see the point if I'm going geo with desuperheater.

    Thanks for the feedback, I appreciate it.
     
  14. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Depends on design. One consequence with heavy loading is less hot water contribution due to shorter run times.
    30-50% isn't out of the question.
    One other drw back to geo spring you mentioned is much lengthier recovery time.
     
  15. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    A very important consideration while evaluating hot water options is the number of people in the home. Hot water use increases more or less linearly with number of people.

    It would be hard to justify investment beyond a basic electric resistance tank fed by a desuperheater buffer in all but the largest households.
     
  16. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    My painful GeoSpring story can be seen in the attached graph of energy demand. Moved in on May 1st, so the billling lags a month. I got the GeoSpring as a Sears floor model for $700. (I understand they discontinued this model line) Coupled with a $350 utility rebate, I was happy camper.

    Completed install of Geothermal and GeoSpring HPWH in mid-June. All was swell. I ran in pure e-heat mode (heat pump only). Only once in our household with 6/7 people did we have a cold shower thanks to 2 DSH and an 80 gallon buffer helping meet the needs.

    On Thanksgiving, I was in the basement showing some guests my set-up. Noted the flashing lights on the GeoSpring. Heat Pump Failure. Sealed System Requires Service. Now in Electric Resistance Mode.

    In hindsight, I did hear the heat pump runnning for an awfully long time prior to the absolute quietude of no fan noise in November. I had it in pure electric mode from Thanksgiving through Dec 22, and my January 2 bill is back down to earth, even with resistive heat. Failed heat pump ate up a good chunk of my expected savings.....
     

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    Last edited: Jan 9, 2013

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