Eliminating 1000 gallons/year
Geothermal Heat Pumps

Bob Bruninga, PE
IEEE National Committee on Transportation and Aerospace
Electric Vehicle Association of DC
wb4apr at amsat dot org

Enthusiasm and experience in converting from 1000 gal/year of oil burning to clean, emission-free renewable solar powered Geothermal energy:

The image below is our new basement arrangement with Ground Source heatpump in the center, ground loop pumps on the left, and new heatpump water heater on the right with old tank preserved in line as a backup for Christmas visitors. Note, our house is heated by hotwater cast-iron radiators and so the heatpump is a water-to-water unit and not as common as most modern homes with HVAC duct work. Since we have no ductwork, we can only take advantage of the GS heatpump for heating only (no cooling yet)... In the far back is a homemade 10 kW electric backup heating system for emergency backup also described.

Having switched to Hybrid and Electric cars in 2007/8 to reduce our use of oil by 50%, and Solar in 2011 to eliminate our coal emissions, our #1 remaining fossil fuel source was the 1000 gallons of heating oil we were burning at ever increasing price. In 2012 oil was up to $3.50 a gallon only slightly less than gasoline! The knock-outpunch was when we realized that at $3.50 a gallon for heating oil, it was as expensive as stright electric resistance heat, the most expensive form of heat! Time to switch to the 4-to-1 savings of a ground-source heat pump.

Efficiency is the Goal: The main purpose of this page is to share my lessons learned which are counter to all advice received from all sources. In a nutshell, The HVAC industry is based entirely on "performance" just like Detroit and the Auto industry that for decades hangs all their production and advertising on the singular focus of "performance". When in fact, some people want "efficiency" foremost. What we discovered was that the typical ground-source heat pump system is designed for maximum performance and fastest response times even though such design significantly sacrifices efficiency sometimes over 30% on milder winter days!.

No Storage Tank: What is notably absent in the above system photo is the very large hot-water storage tank that is normally "required" in a water-to-water hydronic heat-pump system. In such typical systems, the heat pump is designed to always maintain this storage tank at a set high temperature and then as the house demands heat, the radiator system water is circulated from this tank, giving a quick response. The heatpump operates independently to simply maintain the storage tank temperature. What made no sense to me was this design to always pump heat to the tank at 120F even if the house would do fine with 100F water in the radiators during most winter days.

Heatpump performance is inversely related to temperature as shown here in blue. The black line shows the increasing electrical requirement for higher temperatures and the red line shows the resulting efficiency in BTU per kWh. As you can see, the lower the radiator water temeprature, the higher the performance and lower cost. The difference between operating at 120F and 100F is a coeficient-of-performance (COP) difference of 2.6 vs 3.1 or a 20% loss. If the storage tank temperature is set to 125F for worst winter day and most winter days can do with 95F water, then the diffrerence is almost 33%!

It is also worth noting, that without the storage tank, every cycle of the heatpump system begins with the radiator water having cooled down below 70F. So the first majority of each cycle is actually operating in the more efficient upper left portion of the red curve.

Fighting for a Tankless System: None of the HVAC contractors would consider a tankless system because "thats just now how we do it. A tank is required." Yet the installation manual from the manufacturer was less stringent... that is, "recommending" a storage tank in some paragraphs and saying it was "required" in others. My frustration was the inabilty to talk directly to a real thermal engineer. Getting blocked at every turn by salesmen, installers, and middlemen of all stripes. Finally I got to someone at the manufacturer (still not an engineer) who did agree, that operating the heatpump at a lower temperature was more efficient and less demanding on the hardware, but he still could not "recommend it" for the following reasons (and my counterarguments):

  • Danger of radiator scale getting into the HP heat exchanger - N/A since there would be a filter on the input
  • Danger of loss of circulation on HP output - N/A since all our radiator valves are rusted open
  • Danger of short-cycling - N/A, our old radiators hold 180 gallons of water compared to their 80gal storage tank
  • Poor response time - Only when doing big thermostat set-backs, and no worse that with oil.

    Tankless Install: Other thermal and mechanical engineers where I work (though, not specifically HVAC engineers), all agreed that operating at lower temperatures as a matter of routine would be more efficient so I finally found a contractor that would install the system without the storage tank and would install the added temperature, pressure, and flow gages that I requested. Though, he put a clause in the contract that without a "storage tank, performance was not guarnateed". Besides, his price was also the best price. Oh, and the other contending contractor lost the job when he said something like "my father has been doing these for 30 years and that is just the way we do it"... implying to me that no one was really engineering these systems, just turning the crank.

    Trenches: The photo at right is the 4' deep trench with the header piping to three of the four 300' wells. The 4th well is 20' around the far corner to the right. The 4th is near the buldozer in the photo farther down the page.

    Primary Mode - Gotcha! Once the system was installed, it was immediately obvious why a storage tank was "required". And that is because the control system only does one thing. Drive the HP compressor until the return circulating water reaches the 120F setpoint (tank set point) completely independent of the house thermostat. So the house just kept rising in temperature until every radiator in the hosuse was at the "tank's 120F set point" even though there was no tank. This is called "Primary mode".

    Secondary Mode: Next we tried secondary mode, where the HP would respond to an external thermostat (in the house). This worked great except now, it ignored the maximum water "set point". This is not really a problem except in the case of deep temperature set-backs. If we set back the heat by 5 degrees, then when the timer later bumped it up by 5 degrees, the HP would run max out until the house was satisfied, and this could let the heat pump go all the way to its maximum output even if it was not needed.

    Engineered-Fix#1: What we came up with was to use spare contacts on the house thermostat relay to not drive the heapump directly, but to simply parallel a fake-themsister across the aquastat max-set-point themrister so that when the house was "satisfied" then the heatpump thought the water was already at max temperature and would shut down. When the house needed heat, the paralleled virtual thermister would be disconnected by the relay contacts and the heatpump would notice the water was below the set point and would crank up. In this manner, we would have the house thermstat control AND the auotomatic closed loop aquastat keeping the heatpump operating below its peak temepratre. (but we have not done this, see below).

    Engineered-Fix#2: Blow air across the radiators! Actually, this fix works so well, that I have not even gotten around to Fix#1. I just placed some of our summer fans directed at some of the radiators in the house. Now not only does the house warm up faster, but the circulating water never gets close to the maximum of the heatpump and so all of our heat is now coming at a much lower temperature. Note, I only placed fans where aesthetically acceptible to the wife, as shown here in the dining room (with outdoor plants in for the winter), but it turns out that with fans on 6 out of about 15 radiators, the house temeprature is wonderful. We now set at 68F and are enjoying winter life, where with Oil, we only set to 65F for guests and shivered around 62F the rest of the time.

    Fans for convection: The fans aren't all new to us. We had already grown accustomed to placing a box fan in front of a radiator in a room where we were going to be for a while to improve convection there while keeping the rest of the house more economical. It also has the advantage of not having to go open and close radiator valves throughout the house to move heat around. See our other fans in the living room, playroom, game room, and kitchen. The kitchen has our only baseboard heater and so I had to build a small box with some box fans in it as shown here flipped over. The two box fans are wired in series to make them very quiet and the box makes a nice sleeping platform for the fat cat that then overlaps onto the narrow baseboard. Though this arrangement only blows air under about 1/4th of the baseboard and is still inadequate to fully heat the kitchen without an aux electric baseboard heater. I have added a "tower" fan as well, but it is too noisy even on low and we both dont like the looks, so it goes away before guests.

    Shopping around for $5 fans at the flea-market found enough fans that we could select the most quiet ones and chuck the noisy ones. I also ran a circuit around the house with small outlets by each of these radiators driven from the heat-pumps load circulating pump circuit. This turns the fans on and off with the heat and also makes it easy to connect the fans once a year between seasons.

    Emergency Backup Heating: To provide emergency heat during power outages or in case the HP needed work, I installed a pair of 4800 W electric water heater elements inside a 3' piece of 2" galvenized pipe as shown at right This 9600 W worth of heat can provide about 33,000 Btu but at a cost of about $1.50 an hour or about $30 a day for 20 hours run time on very cold days. Surprising, straight resistive electric heat (at 10c/kWh) is no more expensive than heating oil when oil is at $3.50 a gallon or higher. Caution: There must be circulation! If the circulator pump would fail, the water will boil and over presurize the system. Presumably the relief value would blow and the heating elements would then self distruct (though contained safely within the pipe). So I will use a surface contact over-temp thermal cutout to sense any abnormal heat rise. When it steams, the 3" pipe heats up to boiling almost instantly from its normal 95 F to 100F normal operation.

    PRIUS Emergency Power: The plan is to use my Prius for Emergency power to power these heater elements. The beauty is that these two 4800 W elements can be connected directly to run at 240 VDC from the Prius High Voltage system. My car has outlets on the back for access to both 10 kW of 240 VDC power and about 1 kW of 120 VAC. The car should be able to provide the 10 kW from its 18 kW MG1, but the challenge is to make sure the engine stays running. Normally the Prius will start and stop the engine as needed to keep the HV battery charged, but at this 10kW load, I don't want the engine to be starting and stopping every fraction of a minute. I need to find the right load so that it runs all the time (this has not yet been tested). Bob Wilson suggest spoofing the engine thermostat to make it appear cooler than 40C and the engine will automatically run to maintain engine temp (to keep emissions low).

    Solar Emergency Power: Although my grid-tied solar also ceases operation when the grid goes down, I still have about 8 kW of solar DC power available. The system consits of three arrays of panels providing about 2.8 kW each at 480 volts to three separate grid-tie inverters. During power outages, I can simply parallel the 480v outputs of all three arrays and connect this to the two 4800W heating elements in series to provide a nice matched load. Caution: All high voltage DC wiring inside a dwelling must be in metalic conduit since a slight bad connection becomes a 3000 degree welding arc and is not self-quenching like AC.

    Conclusion and Phase-2: So with the solar during the day, and the Prius at night, we can at least get 33,000 Btu of heating during power outages. Phase 2 of the system would find a way to take all the waste heat of the Prius gas engine and get that inside the house. That would double the heating capacity! Call it co-generation.

    Geothermal Installation Impact: For us, the installation impact was enormous! The reason being, that the only place we could get the Well drilling truck was in our front yard, and our front yard was 100% driveway, plantings, or gardens. The drilling truck was 30 feet long and needed at least 13 feet of overhead clearence just to get in, and then needed 33' of clear sky vertical clearance over the wells. This involved supstantial tree trimming as well. The photo here shows the impact on our driveway after the well drilling rig has left and then the connecting trenches were dug. The entire 3400 sqft concrete circular driveway had to be dug up and hauled away to clear room for all th is work.


    Other Related Pages:

  • EV Misinformation Page. Most that we hear about EV's is wrong (see Powerpoint).
  • Payin-to-Plugin for easy use of 120v outlets at work. See NPR interview by Jessica Gould or hear it (2 mins).
  • See some successes with 120v Charging in Maryland. Download EV speech at Driving Maryland Green 11 Oct 2012
  • Download IEEE Paper on L1 Charging. Provides supporting justification for this concept.
  • Download a Charging OUTLET sign or a Charging STATION sign.
  • Charging at Park-N-Rides the common sense approach!
  • Building a DIY Charge cord for 120v for under $100
  • EV Charging, Payin-to-Plugin at US Naval Academy
  • Federal, State, City and Corporate authorzation for EV charging at work
  • Parity between EV Road Tax and Gas Car Environmental Tax
  • Get the Charging-at-work Presentation from the DC EV Forum 12/12/11.
  • Download the EV Position Paper on Charging Infrastructure
  • EV/Gas Road and Environmental Use Taxes - maintaining Parity. download a 1 page paper on the road-tax topic.

    See also my Solar PHEV, a work in progress...

    Bob Bruninga, PE
    IEEE National Committee on Transportation and Aerospace
    EV Association of DC