Published Monday July 15, 2013:  Updated 9/29/2013

Geothermal 5-Months Later

July 18, 2013 marked 5-months since the Geothermal heat pump was installed.  
I have since finished a basement apartment in our home for my mom. This adds a second set of kitchen appliances, another bedroom and another bathroom onto the energy demand of the house.  The added HVAC demand isn’t very significant since we were already conditioning that space, but another occupant in the home further adds to the total home energy usage. 

On a particularly hot 100 °F day, the heat pump ran continuously all day.  The house stayed cool while the HWG heated up the passive water tank 2 degrees hotter than what was coming out of the active tank.  After 2 baths, 3 showers, washing dishes and several loads of laundry the water heater only used 1.8 kWh of electricity all day.  I estimate over 10kWh of water-heating energy was contributed that day by the HWG.  

On these hot summer days, the HWG is reducing the energy demand of the hot water heater by about 84%.  

On June 29, 2013, we hosted a large family party.  There were 20 people in all in attendance in my home.  It was also 101 °F outside that day.  I was anxious to see how the heat pump would handle the high outdoor temperatures along with all the warm bodies adding additional heat load inside the house.  We also did a bit of cooking in the morning in preparation for the party and had a crock pot running throughout the day. The home theater TV and equipment was also running all day as various guests watched movies and played video games.  

The house stayed cool and remained at its temperature set point (75 °F) most of the day.  The heat pump kicked into stage 2 (full blast) by 12PM and stayed at max until later that evening.  By the hottest part of the day (around 2PM), I noticed the sliding door in the bedroom was wide open, with only the screen door closed.  One of the younger cousins must have unknowingly left it open.  Who knows how long it was left open that way.  I checked the thermostat temperature in the hall just outside the bedroom and it was at 77 °F (2 degrees above the set-point).  But because of the superior ventilation of the heat pump equipment, the house still felt comfortable.  Doubtful the heat pump would have the capacity to bring the house temperature back down quickly enough, I turned on the ceiling fan for good measure.
The party was a success and so was the Geothermal heat pump.  In all, the heat pump used 31.2 kWh of electricity that day (my old A/C would have drawn over 50 kWh on a day like that).  But it was well worth it.  In all, the house (including driving/charging an EV) used 59.6 kWh that day, exceeding the solar panel production by 23.5 kWh for the day. 


For the past few months, I have been monitoring temperature along several points of the geo-thermal heat pump. 

  • Ground loop input temperature
  • Ground loop output temperature
  • HWG input
  • HWG output
  • Air output temperature
  • Return air temperature
  • Passive tank output temperature
  • Active tank output temperature
  • Cold water input temperature

The ground loop acts like a large sponge that absorbs or releases heat.  Typically, at my location, it remains at 55 °F.  But when the heat pump is running hard in heating mode, the ground loop temperature drops into the low 40’s until such a time when heating of the home is no longer necessary.  At that point the ground loop temperature slowly goes back to equilibrium around 55 °F. 

In cooling mode, the ground loop temperature rises.  On the hottest days this summer (101 °F outside), I have seen the input temperature of the ground loop as high get as 86 °F  and the output as high as 76 °F.  By morning, the ground loop temp has dropped back down to about 62 °F before the next day’s demand for cooling causes it to raise back up again. 

With every hot summer day, more summer heat is being dumped deep into the Earth via the ground loop.  This is causing the Earth surrounding the vertical ground loop to rise throughout the summer.  I anticipate this heat will be beneficial come winter time.  

Remote Monitoring:  

I have since purchased an 8-port temperature data logger from a site called PCSensor.  It is lots of fun collecting data and studying the trends that take place throughout the day and cooling season.  

8/3/2013:  Free Hot Water

On days that the heat pump runs continually, the HWG (hot water generator) is able to heat the domestic water hotter than the temperature set-point of the active water heater. In July alone, there were 8 days when the electric water heater used less than 1 kWh of electricity.  Almost no energy was consumed in heating domestic hot water on those days. 

With no hot water usage at all (like when we are all on vacation), the energy required to just maintain the temperature in the (hyper-insulated) water heater is about 0.5 kWh (equivalent to a 20.8 watt load running 24-7).  

On Monday July 22, it was 98 °F outside.  We did, lots of laundry, washed dishes and took showers, but amazingly, the electric water heater didn't use a single watt-hour of electricity.  I attribute this amazing result to the HWG heating water hotter at the same rate that we were consuming it. Really hot water was flowing from the passive tank into the active hot water heater fast enough to overcome the effect of any passive heat losses in the water heater, preventing its heating element from needing to turn on.  As a result, the water heater didn't even kick on once.  

Next Article:  1 Year Later