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
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
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.
||Water to Water
||Cupronickel Source coils
||Vented Copper Load coil
||No options, current vintage
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
Reasons for revised Test/Analysis.
I have identified the following reasons why I need to re-do the
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).
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.
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
||SOURCE 7.0 GPM
||SOURCE 9.0 GPM
New Test Procedure.
- I first dropped the storage tank water temperature down to below 100
Degrees F, by running several
- 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.
- I then ran the W-W heat pump with all the other HVAC devices disabled.
- 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:
|Heat Pump Off
|Heat Pump On
|Entering Source Pressure:
|Leaving Source Pressure:
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
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. *
For my system, HE = 7.5 * 4.6 * 500 = 17,250
BTUH. This is well within the expected
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
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
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