What is more efficient: Water at 80 deg F or Air at 60 deg F

Discussion in 'General Discussions' started by jake72, Aug 16, 2012.

  1. jake72

    jake72 New Member

    Our ground loops are warm as we end the cooling season. At the same time, temperatures outside are beginning to be consistently cooler than the ground loop temperatures.

    At what point does an air-sourced heat pump become more efficient than a ground-sourced heat pump?

    Thanks for any insight you can provide,
  2. ChrisJ

    ChrisJ Active Member Forum Leader

    I think of it like this, early in the cooling season we (with GSHP) are cooling most efficient because loop water is cooler from winter heating, while air-source uses hot air all season. Same thing happens at the beginning of heating season.

    60*F outside open the windows. ;)

  3. jake72

    jake72 New Member

    Let's restate the question


    Good point!

    For discussion purposes, let's say loop temperature and outside air temperature are equal. Which is more efficient?

  4. ChrisJ

    ChrisJ Active Member Forum Leader

    I am not a pro, but I would think they are close, air may have an advantage since no water pump is needed.

  5. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    non-technician, engineer geek would say

    HVAC Rule of Thumb 800 CFM/ton condenser flow (I’ve seen 600-1000 Range)
    Air density 0.075 lb/ft^3
    Gives you about 60 lb/min of air for each ton going with air cooled
    Heat Content of Air 0.25 Btu/lb deg F
    15 Btu per minute per degree F delta T with ASHP

    GSHP Rule of Thumb 3 GPM per ton
    Water at 8 lb/gallon
    Gives us 24 lb/min of water
    Heat content of Water 1.0 Btu/lb deg F
    24 Btu per minute per degree F delta T with GSHP

    So 60 Deg Air vs 80 Deg water
    (x-60)* 15 = (x-80) * 24
    9*x= 24*80-15*60
    x = 113

    So large disclaimer, using various rules of thumb, it’d appear that these systems would perform similarly at 113 degree refrigerant discharge temperature. Higher and water wins, lower and air wins. Gotta figure the pump energy vs. condenser fan energy.
    As mentioned earlier, if it is 60 degrees, open the window. Don’t expect too much cooling need with 60 deg OAT.
    From what I'd expect in cooling mode, refrigerant temp will usually be above this 113 number unless you are in a heating dominated climate where 110+ refrigerant discharge temperatures are not so easy to come by.
    Last edited: Aug 16, 2012
  6. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Interesting analysis of an issue I wrestle with now and again. In Fla. we have months afternoon highs around 90 but also elevated deep ground temps, 70-75.

    In theory, it is much cheaper to move a given mass flow rate of water than air since water is incompressible - advantage - water. Couple that with water's 4x greater specific heat - advantage - water.

    There is one huge difference - outdoor air moves through an ASHP at a pressure drop of less than 0.1" (one tenth of an inch water column). That explains why the outdoor blower on a typical 3 ton ASHP is 1/4 HP or less, moving 800 CFM per ton, whereas the indoor blower motor is 1/2 hp, while moving only 400 CFM per ton. The indoor blower operates against 0.5" pressure drop, owing to ductwork, among other things. Twice the power moves half the air.

    On the water side of a geo unit, even though we only have to move 1/4 as much mass owing to 4x specific heat, the pressure drop is in feet of head, rather than tenths of inches as with air. Water is denser and more viscous so needs much higher delta-p for velocity and heat transfer.

    I swapped a condenser fan motor in the course of a service call over the weekend - I measured blower amps at 1.5, which probably works out to 300 Watts with an allowance for power factor. That's fairly near what some geo water loop pumpd draw.

    60 OAT makes for excellent comfort at 50% RH, not so much at 100% RH

    FWIW older, SEER 10 era ASHPs heat outdoor air by 15*F or so. Newer higher SEER systems heat outdoor air by 5-7*F in low stage, 8-10*F in high stage. Slower turning condenser fans with optimised propellors move more air more slowly through larger coils. Higher SEER is achieved by making everything bigger.

    Don't confuse refrigerant discharge temperature (generally includes a bunch of superheat) with saturated temperature / pressure, the conditions where 90% of the heat transfer takes place - phase change of the refrigerant inside the coil, flowing air (or water) on the outside of the coil.

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