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Geothermal hot-water heating efficiency analysis (first cut).

This page reflects my initial analysis.  
Since then, with some help from Water Furnace, I've revised my results based on new (more accurate) information.  
My revised analysis can be found here.

On October 9th 2003, I had the opportunity to repeat an efficiency test that I'd performed a month earlier, so I was interested to see if I obtained the same results. 
(If you want to cheat and jump to the results, here they are)

The specific nature of the test was to see if the hot water heating efficiency of my geothermal 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.  See the diagram below.

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 Charts below, and 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

To perform the heating analysis, I did the following:

  1. First I depleted all the hot water in the Storage Tank by running the slab circulators overnight with the Heat Pump circuit breaker shut off.  The temperature of the water in the Storage Tank was drawn down to about 68 °F.  The tank temperature was given ample time (several hours) to stabilize at the slab temperature.
  2. Then, I disabled the slab circulators and any other heat source/drain which might effect the Tank water temperature.  
  3. I performed a fine calibration of the four temperature sensors used to measure the Heat Pump Source and Demand water temperatures.
  4. I connected an Amprobe recording current meter to the A and B phases of the Heat Pump supply.
  5. Finally, I turned the Heat Pump circuit breakers back on and recorded all currents and temperatures.
  6. When the Water Tank temperature got up to 120 °F. the Aquastat turned off the heat pump & I stopped recording.

Here's a picture of what happened.  You can also download a spreadsheet of this data. 

Chart 1: Temperature/Elect. Load vs. Time graph for the Hot Water Recovery process. 


The test started at 10:25 and ended at 12:02 (duration 97 minutes).  The temperature values were recorded at 1 minute intervals, and are plotted against the left hand Y Axis.  The Current (Amps) readings were transcribed off the chart recording at 5 minute intervals, intermediate values were interpolated, and converted to KW.  These values are plotted (in yellow) against the right hand Y axis.

Certain trends can be observed from this chart.

  1. The Tank Temperature (shown in blue) increased at a nearly constant rate. This indicates a relatively constant heat output.
  2. The difference between incoming and outgoing load fluid temps (D2 vs D3) decreased slightly as the temperatures increase (from 7.0 to 5.5) .
  3. The electrical load drawn by the heat pump (shown in yellow) increased proportionally as the incoming load fluid temperature increased. This indicates a reduction in efficiency with load temp.
  4. The difference between incoming and outgoing source fluid temps (G1 vs G2) stayed essentially constant throughout the test.(3.74)
  5. The Ground Source water temperature (shown in green) actually began to rise within 30 min of starting the test. (Nothing wrong with my loop length)


Without being able to measure water flow rates, it's hard to determine the actual heat transfer rates just from the static temperatures.  However, based on the rates of change of the Tank water temp, it is possible to calculate the amount and rate of heat being transferred.  Follow my chain of logic:  (Apologies to my metric brethren, all these calculations are done in USA units, which never seem to sound right)  

  1. The starting Tank temperature was 68 F
  2. Let's assume that the final Tank temperature of the tank was 122.5 F (halfway between the FINAL water in and water out temps). This equates to an overall temperature increase of 54.5 F
  3. The duration of the test was 97 minutes (10:25 - 12:02) or 1.62 Hours.
  4. The capacity of the storage tank is 80 gallons (US), plus add another 5 gallons just to account for the water in pipes etc.  This equates to 708 Lbs of water.
  5. Therefore the total heat added to the tank during the test is Delta T * Mass of water  or  54.5 * 708  =  38606 BTU.   
  6. Therefore average rate of heat transfer is  Total / Time or 33276 / 1.62 = 23,880 BTUH

BTUH can be converted to KW by dividing by 3413.  So 23,880 BTUH equates to an electrical equivalent of 7.0 KW.  What does this mean?  Well, if we had a 100% efficient electrical water heater which generated the same amount of heat as our heat pump, it would consume 7.0 KW of electrical energy. This number can be compared to the actual energy consumption of the heat pump to determine efficiency. 

Since the efficiency of a heat pump depends on the relative temperatures of the source and load, I've graphed my calculated COP (Coefficient of Performance) against ELT (Entering Load Temperature).  I feel OK doing this as my Entering Source fluid Temperature (EST) was a pretty constant 53 F during the test.

Chart 2: COP vs. ELT graph for the Hot Water Recovery process. 

From this chart you can see that the COP, or efficiency, of the system (shown in blue) starts out at 306% and eventually drops to about 174%.  To someone in the traditional HVAC world this may seem fantastic.  But I was somewhat disappointed by the much lower efficiency at the high temperature end.  Since I use my Hot Water for domestic purposes, I keep the tank temperature up at the 120 F level all the time, so a constant 174% efficiency wasn't floating my boat.

So I pulled out the manufacturer specs to see if if I was justified.  The relevant except is shown below.

I've only included data for EST values of 50 F since this is where my unit was operating, but I've included a wide range of source flow rates as I'm not sure what my actual flow rates are. The worst case numbers seem to be for slow flow rates, so I'll assume the worst and use the Source 5.0 GPM section.  For each ELT/EST combination, there are three rows if numbers.  These correspond to 3 different load flow rates, so I've shown them for completeness.  Once again, the worst case numbers are on the third row, so I'll use these in my discussions.

There are three very interesting numbers in these tables.  KW, HE and HC.  KW is the actual electrical energy being used by the heat pump.  This energy is used to extract heat from the source fluid, but it also has a side effect of indirectly heating the load fluid.  That's what the other two numbers represent.  HE is the heat extracted from the load fluid, and HC is the total Heating capacity which adds the thermal equivalent of the KW number to the Heat Extracted.  We can see this in the green section.  KW is 1.84. We know that if we multiply KW by 3413 we get BTUH (1.84 * 3413 = 6.3 KBTUH).  Then if we add this to the listed HE number (24.3) we get the HC value (6.3 + 24.3 = 30.6).  


Unfortunately my numbers aren't anywhere near what the manufacturer says I should be getting:

  • My HC is way too low. Their chart calls for about 30K BTUH at all ELT's, I only get 23.8K BTUH.  This is 20% low.
  • My KW is way too high.  Their chart calls for 1.84, 2.4 and 2.99 KW.  At the same ELT's, I get 2.4, 3.4 and 4.0.  This is at least 30% high.
  • My resultant COP's are super low.  Their chart calls for 4.9, 3.7 and 2.9.  At the same ELT's I only get 2.7, 2.2 and 1.7.  This is 45% low.

So not only am I not generating enough heat, my power consumption seems too high as well..   What's could be up?   
I though about errors that could be in my test results.  

  • My temperature (T) monitoring is pretty accurate.  After the fine calibration I did, I would expect less than 1% measurement error.  So this doesn't explain the 20% low HC values which is only dependant on temperature. 
  • My current (I) readings are no where near as accurate.  In addition to the fact that my measuring device requires me to interpret a line on a piece of paper, apparently I also need to take Power Factor into account.  This is caused by the Current and Voltage being "Out Of  Phase", something that happens on inductive loads like compressor motors.  A Power Factor of less than 1 will cause my simple P = I x V calculation to give a value that's too high.  I need to determine my Power Factor!

I guess I need to call Water Furnace to get some suggestions regarding the low BTUH number, and my expected Power Factor.  
Off I go.


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An exercise in Energy Smart, Not So Big living. -


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.