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     My related HVAC pages      
Geothermal Radiant heating efficiency analysis.  (See here for DHW Heating Efficiency)

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.

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. 
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

    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.

  1. This test was performed during a cold period (10-20F) and the slab had been warmed to a typical operating temperature.
  2. I set the thermostats on two zones to engage the GSHP and provide maximum load flow rates.
  3. 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.
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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.