My Geothermal Heat Pump Project

 

 

 This is the old farmhouse I installed a geothermal heat pump in. Actually, my heat pump is technically a water source heat pump, since it uses the heat from a water source to heat the home. I live close to a river, and my source of drinking water is not the usual drilled well, but a small spring, which flows year-round. It has less than 5 gal/min in the summer months, but in the mid-winter months the flow increases to over half-filling a 4 - inch pipe. The water temperature is always 55 degrees, summer or winter. Since there is around a 200 - 300 foot rise in elevation on the hill the spring flows from, (to the south of the house) the water table that is emerging is really originating from about the same water table depth as having a deep well, it comes from deep underground and emerges where the river has cut a channel eons ago.

 I had been interested in installing (or having a geothermal heat pump installed) for many years, but I never could find an HVAC person that was interested in my project. Last spring things sort of fell into place for me when I saw a six ton Climatemaster GS Series horizontal Packaged water source heat pump on Ebay for the incredibly low first bid of $250. The only thing was that it was (a) very slightly freight damaged, and (b) it was a 460 volt, three phase unit. I went ahead and put a bid of $250 on it, and surprisingly, no one else bid, so I got it for the $250 opening bid, plus another $180 in shipping charges. I calculated that I could replace the compressor and blower and still save money over purchasing a new unit of the proper input voltage requirements. The freight damage was limited to some bent fins on the air condenser unit, no damage to the copper. Amazingly enough, about a week after I took delivery of the heat pump, some brand-new Copeland Scroll compressors came up on a Dutch auction on Ebay! I bid on two compressors, model ZR61K3-PFV at $80 each. I don't understand the Dutch auction bidding process, but when it was all said and done, I found that I won the two compressors for only $60 each, which was the lowest Dutch auction winning bid - apparently they all sell for the lowest bid! So, for only $120, plus another $150 in shipping, I got my compressor plus a spare! I also found a new 240 volt fan motor to replace the 460 volt motor in the heat pump on Ebay for around $70.

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 Luckily, I had access to the equipment and the knowledge to replace the compressor - I used a Robinaire refrigerant reclaim/recycle unit to pump out the freon from the heat pump, then cut the pipes to the compressor, and brazed one of my new units in place, using nitrogen to flood the piping so as to prevent oxidation in the lines. Then I pulled a vacuum for an hour or so, and then recharged the system. (I have my EPA 609 certification, so I think I was even legal!) I replaced the fan motor with the new one, a straightforward process, and also replaced the 460-volt control transformer with a 240-volt unit. Then I tested the unit, using tap water from a hose, and it all seemed to work very nicely.

 My spring really doesn't have enough flow in the late summer and early fall to support a six ton heat pump, so I scratched my head for some time, then happened by the local Climatemaster dealer, who was extremely helpful, and suggested that I install a large water storage tank - he suggested a septic tank - to hold a large quantity of water that I could draw heat from or pump heat into when the water flow is low. The tank would in essence act like an enclosed swimming pool - a big heat sink, if you will, and the four sides of the tank would provide a very useable heat transfer from the earth around the tank. I called around, and found a local manufacturer of cement septic tanks that also makes a water tank version, which is the same outer shell that a septic tank has, but without the internal divider.

I had him make me a 1500 gallon tank, and had him leave the top off so I could install a heat exchanger, then had him put the lid on and seal it, then he delivered it and put it in the hole I had dug for it. The cost of the tank was $775 delivered, and the one foot riser and lid was another $80. Ruts in the lawn were free! (The lid alone weighs over 1200 lbs.!) Let's go through that again, slower:

 First, my brother has a Ford 641D industrial tractor, with a backhoe and front bucket. I used it to dig a hole 8 feet deep, 8 feet wide, and 14 feet long (The tank outfit wanted a foot or so clearance on each side to make lowering the tank into the hole easier). I then lined the hole with a yard and a half of pea gravel in the bottom, well smoothed and as level as I could get it. When we lowered the tank into the hole, there was about a foot of space above the top of the tank, so I used a 12" riser pipe, 18" in diameter, and a green plastic manhole cover, both provided by the tank manufacturer. The riser is sealed to the tank, and the tank doesn't leak anywhere, so the water level comes up about an inch into the riser pipe above the top of the tank in my installation. I have the spring overflow running into the tank near the top, (4" pipe), and the tank's outlet pipe picks up the water from the bottom of the tank, where it is coldest, and the water then continues on it's way to the river, just as it has for the past 100 years or so that I know of. Everything connected with the spring and overflow is underground, so there is no visible change except for the 18" manhole cover in the yard. (And the ruts, which will go away in the springtime, when I redo the lawn.)

 While I had the backhoe I dug a 5-foot deep trench from the house to the tank, and also decided to improve my heat pickup by digging a loop in the yard.

I had been saving 250 feet of 1 1/2" black plastic pipe for 20 years or so, and used it to run between the house and the tank, then used the rest to start my ground loop. I also bought 400 feet of 1/2" black plastic pipe, and made two manifolds, which accept the 1 1/2" pipe and distribute the coolant to four 100 foot long pieces of pipe.

These are buried thusly: one on each side of my 2 foot wide trench at the five foot level, then one foot of dirt over them, and the other two at each side at the 4 foot level. The levels are approximate, and vary somewhat due to the ground being so hard when I dug the trench. The ground was mostly a sand/clay mix, which held its shape very well while digging. So I actually have 650 feet of pipe in series with the heat exchanger in the water storage tank.

  The heat exchanger is built up of 10 1/2" copper pipes, each 9 1/2 feet long, and separated by about 8" each. They are connected in "parallel", i.e.; the coolant flow is divided amongst all 10 pipes. My thought was that the heat transfer should be very good at the low flow rate through all the copper, and the copper itself is an excellent conductor of heat.

The grid of pipes is located at about the 4-foot high level in the tank. I also installed two 3/4" lengths of copper tubing about 60 feet long each, arranged as coils about 1 1/2 feet in diameter, running across the bottom of the tank. These two coils of tubing are connected in parallel with each other, but in series with the grid at the top of the tank. The coolant comes from the house, goes through the plastic pipe ground loop, then enters the two coils at the bottom of the tank, then goes through the grid at the top of the tank, where the 55 degree water should be, then goes back to the house. I have a Grundfos UP-26-64 1/8 hp pump that circulates the coolant through the system. I calculated that the total volume of the system is just over 10 gallons, so I put in 7 gallons of RV antifreeze, which I got on sale at Lowe's. I also read in the pump's literature that it needs a minimum of 4 lbs. pressure at the pump, so I installed a 3" pipe, 5 feet high on top of a 3/4" pipe that is 2 feet high. This gives the pump around 4 lbs. of static pressure, since it is located in my basement. I filled the reservoir with straight RV antifreeze, or propylene glycol. Propylene glycol is one of the recommended solutions to use, as it is safe for human consumption, and is actually used in food processing. The reservoir gives the air bubbles in the system a place to exit, and also allows for expansion and contraction. I have found that the level stays about the same, once the air bubbles all worked out of the system. I brazed all copper connections in the system, and used Ace This heat exchanger grid is located inside the tank at the 4-foot level. There are two coils of 3/4" pipe 60 feet long located underneath it at the bottom of the tank. They are in series with the grid, and in parallel with each other.

brand pipe thread compound with TFE on all threaded connections, as I found the Teflon tape leaked, whereas the pipe-threading compound doesn't leak at all. It is best to avoid threaded plastic to copper connections if possible, although I have 8 of them in my installation. With the extremely low water pressure in my system, less than 6 psi, there's not much likelihood of leakage anyway.

 I relocated the heat pump to a more central location in my basement, and tried to "clean up" the ductwork - the house originally had an oil furnace installed, and they put it close to the chimney, near one end of the basement. Then an electric furnace was installed, but left in the original location, then a Coleman heat pump, which was still working fine when I disconnected it, but some of the ducts were as short as 4 feet, and others were almost 40 feet long! This made for uneven heating, to say the least. I moved the new heat pump to a more central location, and made a 16 foot long plenum 13 x 18", which I surrounded with fiberglass insulation, to keep the heat as high as possible at the registers. The result is even heating throughout the house. Additionally, the new heat pump's output temperature gets up as high as 110 degrees at the registers, which actually feels warm when it's running!

 So, here's how it's doing, after a few months of running - The inlet temperature to the heat pump's coaxial heat exchanger coil is around 55 degrees, but drops to as low as 43 degrees after a long cold spell. The outlet temperature from the heat pump is around 43 degrees, but drops to as low as 30 degrees. If the weather were to be real cold for a long time, it would probably get colder, but that's the coldest I've seen so far. We just came through the coldest weather in 10 years, where the outside temperature got down to 18, and didn't get above 23 for a few days, and the temps held steady, so it looks like I am getting a 12 degree drop in my water tank's heat exchanger and another 12 degree drop through the heat pump. This was with the heat pump running continuously for several days straight. (actually, the "DXM" microprocesser controller monitors the compressor's on time, and if it has been running for 4 hours straight, it shuts off the compressor, leaving the fan on, and monitors all the temperatures for 5 minutes. If everything is ok, it turns the compressor back on for another 4 hours, The reason for this is to protect the compressor against a possible low refrigerant pressure condition.) The compressor draws about 15 - 16.5 amps in heating mode. The heat pump, being a six-ton unit, is able to handle the entire heating load, there is no backup resistance heat installed. (Actually the compressor specs say it should put out around 73,000 BTU's at the temperatures I'm running) (The old system was a 3 1/2 ton unit, but had 30 kW of backup heat that would come on if the outside temperature got below 34 degrees) The old unit would also take reverse cycles whenever the outside temperature got below 42 degrees, to get rid of the frost on the outdoor coil. The water source heat pump just pumps out nice warm heat all the time, with no fuss at all!

 My Climatemaster heat pump has the "DXM control board" installed, which has dual microprocessors. I think that one microprocessor is dedicated to taking care of the core heat pump functions, such as monitoring all the temperatures, both water and refrigerant, high and low refrigerant pressures, and checking the water level in the tray underneath the condenser, etc., while the other microprocessor takes care of the ancillary functions like switching on the circulator pump, controlling any additional heat pumps, and other functions that I'm also not using. I think the Climatemaster folks did a great job of designing the board, and suspect it would be difficult for the compressor to be damaged, what with all the monitoring that goes on. The board even has a "Unit Performance Sentinel" built in (can be disabled with a switch setting if desired), that lets you know if the heat pump is operating inefficiently. The board has 3 LED's that provide various codes to tell you if any errors have occurred - so far I've not gotten any errors, although when I tried out the AC function in the winter it gave me a UPS code that said the incoming air was cooler than it should be, although not cool enough to cause an error shutdown.

 I'll let it run for a year or so (finished the install around Thanksgiving of 2003) and let you know how the electric bill is - I anticipate that it won't be quite the savings one would expect due to the fact that we are now heating the WHOLE house to a comfortable temperature, where before with the Coleman unit it was impossible to get some of the rooms warm in the winter - it's an uninsulated 3 story farmhouse with the basement and first two floors heated. As long as the bill doesn't go up, Ill be very satisfied, but I expect it to save at least 25 percent over the old pump, due to the lack of any backup resistance heat, and no longer needing reversing cycles, which wasted a lot of power. Time will tell. When I relocated the heat pump, I decided to let the return air come back to the basement through a pipe organ chamber, which is actually an old sun porch with the floor sawed out to give 11 feet of height for the organ pipes to reside in. This previously unheated space is totally un-insulated, as is the entire rest of the house, and has glass windows on two sides. The heat pump's compressor is very quiet - you cannot hear it over the fan, and the fan isn't that noisy either. Outside the house there is absolutely no noise from the heating system at all. Inside, there is a bit of fan noise, but the heat pump itself doesn't make as much noise as our basement refrigerator!

 I have around $2000 total invested in my system, but I suspect it would have cost well over $20K to have had it professionally installed. I consider the entire project to be a big experiment, and have learned a great deal from the experience. I found an interesting book that helped me decide how to make my grid heat exchanger, among other things - "Heat Pump Technology", by Billy C Langley, published by Prentice-Hall, ISBN 0-13-385766-2 (621.402) is a book worth reading. I got mine on Ebay. I also printed the old and new compressor specifications and operating charts from the Copeland web site, as well as all the information on the heat pump from the Climatemaster web site. Climatemaster has a very good site, with a great deal of information to be had. I was very impressed with their instruction/operation manual for the CXM/DXM circuit boards. I also researched other manufacturer's web sites and the geothermal consortium's web site, and found the Oregon Institute of Technology's excellent web site. I sent them an E-mail requesting some information, and they responded quickly, and were very supportive. They even assigned an engineer to help guide me as necessary! According to both the information in Langley's book and the Copeland compressor charts, my system is working as designed, and is putting out over 65,000 BTU from 16.5 amps, 240 volts input power (3960 Watts). Using the temperature differential measurements, this computes out to an EER of 16.4, and a COP of 4.8. These numbers are far better than my 25-year-old Coleman unit, which was highly touted when it was purchased. I seem to remember a COP of less than 3 for the Coleman unit. This would indicate that the Climatemaster unit is indeed running at an efficiency of around 40 percent greater than the old unit, which is what Climatemaster indicates in the information on their web site. I find it pretty remarkable that such an increase in efficiency is possible!

 I have also discovered some flaws in my installation! Firstly, don't use RV antifreeze to protect your system from freezing. I happened to make a business trip to Denver the week that we had a major cold snap and the heat pump stopped putting out heat. I had the wife shut it off and go to the ultimate backup, the old wood-burning kitchen stove, and looked at it when I returned. She had told me the temperature of the circulator pump was around 150 degrees, which I couldn't believe, but sure enough, when I returned, the circulator pump had some air in it, was at 150 degrees, and it caused the coaxial coil to mostly freeze up due to no water flow, and the system shut itself down. I had first installed the circulator pump with the inlet and outlet located horizontally, which allows air to become trapped in the pump, so I relocated it slightly to a position where the outlet was pointed straight up, which lets all the air bubbles leave the pump. I also tested the freezing point of my solution, (should have done that at installation!) and discovered to my dismay that it would start freezing at around 30 degrees! I guess there isn't enough propylene glycol in RV antifreeze to prevent freezing, just enough for pipes not to burst. At any rate, I looked around for some ethylene alcohol, and found some at the local hardware store in the form of shellac thinner. I put 4 gallons of it in the system, which my testing shows to now remain liquid at 24 degrees. After some deep thinking, I've decided that the antifreeze is very important, as the coaxial heat exchanger has a fluted copper heat exchanger pipe inside, and if ice forms in the depressed flute sections, there will still be flow through the heat exchanger, but the efficiency will drop way off, so it's essential that no ice can form at the lowest possible temperature inside the coaxial coil. After I added the alcohol to the system it works noticeably better. Second, make sure that your system has adequate circulating fluid flow. I have ordered out the next sized higher flow pump to try on my system, a Grundfos 26-99 1/6 hp unit. I think I have enough flow presently, but would like to make sure, and also want to have a spare pump in-house for the system. The overheated circulator pump brought to light another potential problem - I had assembled the indoor part of the system using schedule 40 1" plastic pipe, and all the non-glued threaded connections around the pump were loose due to the overheating. When I relocated the pump, I installed cast iron pipe on both sides of the pump to the shutoff valves. With the vertical repositioning and the extra metal involved, I'm hoping that the pump can't transfer a lot of heat up to the plastic connection anymore. By the way, I have an air drain at the top of the vertical pipe above the circulator pump. It is a handy place to release air, and also very convenient to siphon off liquid to do deep freeze temperature tests with.

  I'm also wondering what effect a longer ground loop would have on the system. I could easily dig up the front yard and add another 800 feet or so of pipe, but I'm not sure it would make a lot of difference. The input temperature of the water stays pretty close to 43 degrees, after dropping slowly to that temperature from 55, and I suspect that just means I have a 12 degree temperature drop in my water tank heat exchanger. Also, I don't know yet what the "balance point" of the system is - that is the temperature that the system can no longer keep up with the heating load. I do know that it can get below 29 degrees outside and the heat pump can handle it ok, although the upstairs rooms do feel a little "crisp" around the edges due to lack of insulation. I think that my next project needs to be adding some insulation somehow, as I think the heat pump is of ample capacity for the house.

rwooldridge2@juno.com

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