Where's the primary electric consumption in a geothermal system?

Discussion in 'General Discussions' started by BlueHawk, Jan 25, 2020.

  1. BlueHawk

    BlueHawk New Member

    Hi all – The electricity savings for a ground source geothermal HVAC system are substantial compared to a conventional central AC and furnace setup. But they're less than I expected, so I'm curious where the electricity usage is coming from.

    I had the impression that ground source geothermal was virtually "free" because it was leveraging the constant temperature underground, and capturing it in the heat transfer fluid. So I figured the fluid pumps would be one source of electricity consumption. So what else is there? The heat pump itself? I guess I don't know how a heat pump works. Is it using the bulk of the electricity?

    Thanks for your feedback.
  2. mrpac

    mrpac New Member

    Hi BlueHawk,

    Your energy consumption is broken down into 3 major devices in the Geo.

    Heat Pump (compressor)
    Recirculation pump (s)
    HVAC delivery Fan (usually variable speed DC)

    The main culprits are the Heat pump and recirculation pump.

    There could be a backup system (electric heat) that if energized could double the loads very easily.

    I agree, the costs of running the compressor are quite high. Definately NOT free....nothing is free

    Hope this helps
    GeoPuzzled likes this.
  3. SShaw

    SShaw Member

    I have a 4T 7 series. Averaging snapshots of the energy use over the past week the breakdown of energy use is:

    compressor: 86%
    fan: 11%
    pump: 3%

    Average COP: 4.98

    The efficiency is 4.98 times that of electric heat, so it's about 1/5th the cost, but still not free.
  4. mrpac

    mrpac New Member

    There is a big difference between the 7 and 5 series, in that the 7 is purely VFD controlled?

    My 5 series has a 2 speed scroll compressor with a 2 speed Recirculation pump. I am assuming by documentation that the 7 is capable of complete PID control, which when coupled with variable speed devices should save lots of $$$ in energy savings.

    The world is going VFD, due to the overwhelming cost savings in electricity on the startup of these devices. In my industry (Water&Wastewater) all line voltage starters are being replaced by either soft starters or VFDs.

    I guess it's the upfront costs that matter as well...
  5. BlueHawk

    BlueHawk New Member

    Thanks, that's neat that it lets you see the breakdown. What is the 4T 7 series? Who makes it?
  6. SShaw

    SShaw Member

    It's a Waterfurnace four ton 7 series. The 7 series has variable-speed everything: compressor, fan, and water pump.
  7. SShaw

    SShaw Member

    Waterfurnace's analysis software projected a 20% savings for the 7 series versus the two-stage 5 series. The 5 series can be had with the option to control a variable speed pump, but I don't know whether the pumping costs are included in the analysis. The 7 series cost about $4,000 more upfront though, before tax credit.
  8. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Check if you can slow down your fan, 11% are quite high. We are trying to have the fan consuming around 5%. you can save 6% on the fan, although you're compressor might go up by 1-2%. Higher comfort too, lesser draft and lesser noise.
  9. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    We replaced a 6 ton dual stage TT27 climate master with (2) circulation pumps (26-99) with a 5 ton 7 series variable speed heat pump with a variable speed circulation pump in 2016.

    41.3% electric energy savings. Weather normalized. Interestingly, supplement heat went down as well, indicating that the (5) ton 7 series has a higher heat capacity than the (6) ton climatemaster, which is indeed the case looking at their specs.

    Attached Files:

  10. SShaw

    SShaw Member

    The fan is currently set on 2/3/9/10 for ON/LOW/HIGH/AUX. I suppose I could lower the fan speeds and see what happens.

    The Symphony energy readout looks like it cannot report a fan wattage less than ~47W, as that's what it reports for fan speeds 2, 3, and 4.

    I'd be interested to see how my fan wattage compares on an absolute basis.

    Do you have any data showing the absolute fan wattage vs fan speed for a 4T 7 series?
  11. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    1/2/6/10 is what I would recommend. Do you have an aid tool?

    If yes, what is the delta T over the air coil? And the delta T for the source coil? How is the pumping power set. Minimum versus maximum percentage?
  12. SShaw

    SShaw Member

    I can use the virtual aid tool to change fan speeds.

    When running C4/F5: EWT=42.9, LWT=38.0, Flow=7.5 gpm, HE=17824, EAT=70.0, LAT=86.4.

    Pump power is set to 25%/85%. Symphony reports 9W on compressor speed 1 and 159W on compressor speed 12. My loop pressure drop estimate is 20.4 ft-hd. Ge0-flo's pump sizing calculator estimates $38/yr pumping cost, so there's not much savings to be had there.

    I will run it on 1/2/7/10 for awhile and see how that does.
  13. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I would shoot for a 20F delta on the air side in C4,

    Pumping power is good.
    Loop looks very good.
    Your ductwork sizing looks good too.
    Take the fan down, and let us know the EAT and LAT
  14. SShaw

    SShaw Member

    Thanks, @docjenser

    I lowered it to 1/2/7/10 last night and the system went to C4/F4. The delta-T went up to 19 degrees. EAT=70.2, LAT=89.2.

    I didn't go as low as 6 for High. I see the manual says High "must" be 7 or greater.

    I'm curious. How do you tell the duct sizing looks good?

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