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     My related HVAC pages      
Geothermal hot-water heating efficiency analysis.  (See here for Radiant Heat Efficiency)

On Jan 5, 2004, I had the opportunity to repeat a GSHP efficiency test that I'd performed two month earlier. The specific nature of the test was to see if the hot water heating efficiency of my geothermal (ground source) heat pump was within manufacturer specified limits.  In my home, all the hot water for the radiant floor, and for domestic uses (shower, washing etc.) is generated by a single Ground Source Heat Pump.  Since so much of the house's heat comes from this source, I wanted to make sure I was getting my full money's worth :)  Since a GSHP efficiency depends on the temperature of the water it's delivering, this page just documents the Domestic Hot Water side, part way through the heating cycle (when the tank water is at about 100F).

I'm using the Water Furnace Premier Water-to-Water Heat Pump, 
model number P034W10NVAASSA.  
See the table at the right to decode this number.

The Storage Tank has an 80 Gallon capacity.

Note the identifiers for the 4 ports on the Heat Pump. 
G1 and G2 are the two Ground-Source pipes.  D2 and D3 are the two Domestic-Load pipes.  These labels match the Live/Historic graphs shown on other pages. 

P Premier Family
034 034 MBTUH
W Water to Water
1 208-230 Volts
0 No Desuperheater
N Cupronickel Source coils
V Vented Copper Load coil
AASSA No options, current vintage

Historic Note:

When I first performed this test, my analysis of the results indicated that I had several deficiencies in overall system performance.  However, over the last 2 months I have established several reasons why this was not the case, and so I'm retesting, and presenting the updated data and analysis here. 

Reasons for revised Test/Analysis.

 I have identified the following reasons why I need to re-do the analysis

  1. Since the initial test I had performed was dynamic (the tank temperature was increasing) I needed to account for the additional system masses, rather than just the mass of the 80 gallons of water in the DHW tank.  Without an accurate thermal mass number it's hard to determine the heat transfer rates based solely on rate of temperature change.  For this reason, I'm not going to use this approach.  Instead, I'm performing a static test.  Water Furnace (the GSHP manufacturer) provided me with an accurate pressure gauge so I could determine the source water flow rates.  By combining flow rates and source temperature differentials I can determine amount of  heat being extracted from the source water loop (HE). 
  2. My electrical power measurement method was too crude to account for the Power Factor of the Heat Pump compressor motor.  This may have caused a 20% over-read in electrical load.  To rectify this problem, I've installed a Watt-Node power meter on the HVAC sub-panel.  This meter measures and reports true RMS power with a 0.5% accuracy.  This is a very reasonably priced device which provides a pulsed power-usage output.
  3. The operational specification which I had been using for my heat-pump was the "Generic" specifications for the basic family.  My unit has a vented double-walled heat-exchanger for potable water heating, and consequently it is less efficient, and has other variations in operating parameters. Water Furnace supplied me with the actual specifications for my unit type.  I've extracted the key part of the revised spec, and included it below. (It's marked as VXW036 Heating Data):

 
    SOURCE 7.0 GPM Flow rate SOURCE 9.0 GPM Flow rate
ELT EST LLT HC KW HE COP LST PSI LLT HC KW HE COP LST PSI
 100 50 110 24.3 2.33 16.3 3.1 44.5 4.6 110 22.8 2.35 14.8 2.9 46.5 6.9
108 24.7 2.30 16.9 3.1 44.6 4.6 108 25.2 2.33 17.3 3.3 46.0 6.9
106 25.2 2.28 17.4 3.2 44.6 4.6 106 27.6 2.31 19.7 3.4 45.5 6.9

New Test Procedure.

  1. I first dropped the storage tank water temperature down to below 100 Degrees F,  by running several radiant zones.
  2. Next, I disabled all equipment except those components required to generate Hot Water.  This included disabling the Radiant Floor zone circulators, ERV, Air handler and well pump. 
  3. I then ran the W-W heat pump with all the other HVAC devices disabled.
  4. I monitored the heat pump's operating variables.  This included the electrical load, source water pressure readings & all water temperatures.  I recorded these variables at the point when the Entering Source Temperature (EST) was 100 degrees.

Here are the results:

Electrical Load:

Heat Pump Off  48 W
Heat Pump On     3502 W

Water Temperatures:

EST  49.6 F
LST  45.0 F
ELT 99.9 F
LLT 106.9 F

Water Pressure:

Entering Source Pressure:     16.0 psi
Leaving Source Pressure:     10.8 psi

Analysis:

Electrical Load.    This is now easy to determine with the new watt meter.  However, some numbers do need to be subtracted from the indicated power load.  There are two source-side circulator pumps which each draw  420W (1.75A @ 240V), and a single load side circulator which draws 84W (0.7A @ 120V).  This is also a minimal housekeeping load of  48W from other devices.  
For my system, KW = 3502 - 972 = 2530 W.  This is 200W higher than expected, but it's within reasonable limits.

Heat Extracted:  This is an all-important number, it indicates how much heat is being extracted from the ground loop. This can be calculated based on the water flow rate and temperature drop across the Source Heat Exchanger.  Flow rate can be determined by measuring the pressure drop across the heat exchanger and then using lookup tables.  My measured pressure drop was 5.2 psi, which extrapolates to about 7.5 GPM based on the chart above.  Heat extraction can be calculated as GMP * Temp Diff. * 500.  
For my system, HE = 7.5 * 4.6 * 500  = 17,250 BTUH. This is well within the expected range.

Heat Capacity: Water Furnace assumes that in addition to the heat extracted from the source water loop, the electrical energy consumed by the heat pump is also converted into heat and fed into the load fluid, so the total Heat Generated is HE + (KW * 3.413).
For my system, HC = 17,250 + (2530 * 3.413) = 25,884 BTUH. This is well within the expected range.

Coefficient Of Performance:  COP is defined as:  Heat Energy Generated divided by Electrical Energy Consumed.  Expressed in BTUH this would be HC / (KW * 3.413).  
For my system, COP =  25,884 / (2530 * 3.413) = 3.0.  A respectable number. 
To put this in perspective, it's 3 times better than a baseboard heat, electric DHW system would be.

So in the final analysis my system seems to be performing at about par with what should be expected.  
My earlier disappointment was due to invalid expectations and poor power measurements.

 




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This site is all about building a cool, energy efficient house, that makes maximum use of earth sheltered design, passive solar heating and cooling, geothermal exchange energy management, and right sizing of the house for it's designated use. The home's placement is on a south-facing hillside in Deep Creek Lake, Maryland. This site describes the design process, the technologies used and the expected results. We also have a comprehensive Links Page for anyone who is also interested in designing a similar project.