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A year after doing my DHW heating efficiency test, I've had the opportunity
to re-plumb my radiant heat system to get the most efficiency possible from my
Ground Source Heat Pump (GSHP). Since
a GSHP efficiency depends on the temperature of the water it's delivering, this
page just documents the Radiant Heat side, and attempts to calculate the actual
efficiency of the system.
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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 Radiant loops are plumbed directly to the heat pump, which was a somewhat controversial configuration. Note the identifiers for the 4 ports on the Heat
Pump. |
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| 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 |
| 60 | 50 | 73.5 | 31.4 | 1.92 | 24.7 | 4.7 | 42.1 | 4.6 | 74.1 | 32.5 | 2.03 | 25.6 | 4.7 | 44.0 | 6.9 |
| 70.2 | 31.5 | 1.89 | 25.0 | 4.9 | 42.1 | 4.6 | 70.9 | 32.7 | 1.94 | 26.1 | 4.9 | 44.0 | 6.9 | ||
| 67.0 | 31.6 | 1.85 | 25.3 | 5.0 | 42.1 | 4.6 | 67.8 | 32.9 | 1.85 | 26.6 | 5.2 | 44.0 | 6.9 | ||
| 80 | 50 | 91.9 | 27.8 | 2.14 | 20.5 | 3.8 | 43.3 | 4.6 | 92.0 | 27.7 | 2.19 | 20.2 | 3.7 | 45.3 | 6.9 |
| 89.1 | 28.1 | 2.10 | 20.9 | 3.9 | 43.4 | 4.6 | 89.6 | 29.0 | 2.13 | 21.7 | 4.0 | 45.0 | 6.9 | ||
| 86.3 | 26.4 | 2.06 | 21.4 | 4.0 | 43.4 | 4.6 | 87.1 | 30.3 | 2.08 | 23.2 | 4.3 | 44.7 | 6.9 | ||
Test Procedure.
- This test was performed during a cold period (10-20F) and the slab had been warmed to a typical operating temperature.
- I set the thermostats on two zones to engage the GSHP and provide maximum load flow rates.
- I monitored the heat pump's operating variables. This included the electrical load, source water pressure readings & all water temperatures. I recorded these variables over a 10 minute period and took the average of the 10 sets of readings.
Here are the results:
Electrical Load:
| Heat Pump Off | 48 W |
| Heat Pump On | 2934 W |
Water Temperatures:
| EST | 48.03 °F |
| LST | 43.48 °F |
| ELT | 72.52 °F |
| LLT | 88.13 °F |
Water Pressure:
| Entering Source Pressure: | 11.7 psi |
| Leaving Source Pressure: | 6.5 psi |
Analysis: First I'll determine the actual GSHP efficiency, then I'll calculate the overall Radiant Heat system efficiency.
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 * 6.82 * 500 = 18,075 BTUH.
This seems very low.
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 two load side circulator which each 84W (0.7A @ 120V). This is also a minimal housekeeping load of
48W from other devices.
For my system, KW = 2934 - 1056 = 1878 W.
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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An exercise in Energy Smart, Not So Big living. www.OurCoolHouse.com - Ideas @OurCoolHouse.com
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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.