Geothermal Heat Pump Project

[techman]

I will give a blow-by-blow of the installation of a geothermal heating system in my home. First for the disclaimers and "don't attempt to do this at home": Although I am a trained engineer and a very mechanical hands-on guy, I also have a small side business installing and servicing HVAC equipment (A/C and heat pumps primarily). As such I am EPA licensed and have over 18 years of involvement with the HVAC trade. This is not a simple or easy project, and a wide range of skills are needed.

The project started when I decided early last year that I was tired of fixing the inevitable breakdowns that occur in my existing heat pumps every winter, usually when it is 15 degrees out and blowing snow. My existing heat was a 2 zone/2 air heat pump system, 1.5 and 2 tons, a for 2200 sq-ft house (2 years ago I added a new 1.5 ton unit for a new 500 sq-ft addition to the house). The existing York units have been very good, reliable units that provided good comfort at a reasonable cost (all electric average $175/mo the last few years). But given that they are 18 years old (7.8 SEER), every year seems to bring some little part failure. Usually not expensive, but a pain to fix at those odd and usually cold times. At least I have the benefit of not haveing to call a service guy, so I get it running more quickly.

Anyway, I had wanted to install ground source when the house was built, but the technology was relatively new 20 years ago and priced way beyond my budget. Now I decided it was time to do it to lower my utility costs and have a more reliable system. Part of this was pre-retirement thinking, although that is 10+ years away. Little did I know when I made the decision and planned the project that the energy costs would take off later in the year and make my decision all the more wise.

I did a lot of research through the internet, books, seminars and networking with other dealer/installers. By spring the project was going full speed.

Continued in Chapter 2

 

[techman]

Chapter 2

After concluding my research I made some conclusions that may be of benefit to others. There are several schemes available:

- Well based: These are known as open loop systems since the water is not in a closed loop, but pumped and discharged. They have the best efficiency and capacity since the water temperature is constant and relatively warm (40-50 deg) The negatives are: the cost of drilling at least 2 wells to support the water flow needed - can be costly and also subject to local restrictions (not in my area), and the pumping costs are somewhat higher. However the other negative was the show stopper for me - water quality. Water chemistry can cause corrosion, clogging, scaling and other headaches. Likewise dirt, silt and other water borne material can clog up heat exchangers. I found that the majority of all geo service issues are open loop water quality based problems.

- Pond based: using either pond water (water issues again) or coils of tubing at the bottom of the pond to provide a water/heat source. For me no pond, so this was not a consideration.

- Vertical loop: This uses vertical bore holes 200'-500' deep with a loop of tubing in the hole. Costly drilling is required and the hole with tubing needs to be grouted solid, from the bottom up, to thermally "couple" the tubing to the surrounding earth. The number of holes is determined by the unit sizing and other ground factors. A local firm would do the entire loop system, grouted and finished for $ 2K to $ 10K depending on the sizing, etc. This is an ideal choice for small lots, but since I have 11+ acres, it did not make sense for me.

- Horizontal loops: This like the vertical is a closed loop system (circulate water through a closed pipe loop) but uses pipes buried in a trench.

I chose the horizontal loop solution since I had sufficient land area. There are many ways to do a horizontal loop system, that is the number of tubes in parallel, how many in the trench, the trench length and depth, pipe size, etc. Most soil is a very poor thermal conductor (average 1 BTU/Ft/Deg F) so you need a lot of feet of ground to get a house's worth of heating. The rule of thumb is 500'-600' per ton. This meant I needed about 2000' of pipe. An ideal system would use a 2000' trench with a single pipe. Trenching cost would be high, and pumping losses would also be an issue. You can minimize pumping losses by using bigger diameter pipe, but than the pipe cost goes up. So the normal comprimise is to put 2 or more pipes in the trench. This lowers the heat capacity per foot of trench, but saves on trenching. Also a shorter trench allows smaller diameter pipes. The loss of capacity per trench foot is not too bad and the trenching and pipe costs are in your favor with multiple pipes.

I did hand calcs, talked to installers and finally had some runs done on a geo design program to finalize the loop design. The numbers were 420' long trench with 4 pipes in the trench and a trench depth of 5'. I decided to go to 7' depth since the seasonal ground temp variation at 7' was about 30% better than at 5' (+/-10 deg at 5', 6-7 deg at 7') This will help keep the efficiency and capacity up. I also rounded up the trench length up to 500'. If I went with 420', I could use 300' coils of tubing and have tubing spliced in the run (should be okay but why do it if you don't need to do it), or buy 500' coils and not have any running splices. If I had 500' coils, I hated to throw away the extra, so I rounded up to a 500' loop, using 4 coils of 500' for a total of 2000' of tubing. This extra length would further add to the system capacity at the cost of a bit more trenching. More on the tubing later.

My plan was to use a chain trencher for the loop. I told the wife that the yard would only have at most a 1' wide line of harm to the yard. She liked that. I found that there were 7' capacity trenchers made, but after 2 weeks of searching, I could not find a rental company or contractor who had one.

Having exhausted the chain trencher idea, I eventually called Bob the local little excavating company. He said he could do the work in the approximate time frame (planned for the week after Labor day based on my real job work schedule and it being a cooler time of year, and with minimal house cooling and heating demands for the changeover). Bob said "no problem. but what the **** are you burying 7 feet deep ?" I told the wife that I had to go with a backhoe for the digging. She said "okay" but I later learned that she did not grasp the magnitude of that change on the yard.

Continued on Chapter 3

paul

 

[rebel300r]

Techman, I am pretty stoked that you are writing this up. I have been trying to figure out all the ins and outs of doing this myself. Please keep it up.

Scott

 

[techman]

Chapter 3

The tubing used is a black polyethelene pipe similar to the standard well piping. The major difference is that 160 PSI rated is used, mostly for mechanical strength, and the tubing is supplied sealed and pressurized to keep it clean. It is nice to know that when you install it, as you cut the end off and hear the pressure escape, you have a no-leaking tube to start off with. The tubing is also "geo" rated and has a 50 year warranty.

The tubing also has a bunch of calculationd to do. I got a nice Excel sheet from a manufacturer to use. The pressure drops of the tubing and fittings needs to be calculated to determine the amount of pump you need for the circulator to provide the loop flow at the calculated pressure drop. This must account for the fluid temperature and the increased pumping pressure required due to the viscosity of the antifreeze used in the water at that temperature. You also need to determine the Reynolds number of the flow in the tubing. This is the amount of turbulance in the fluid as it flows through the tubing. If the flow is too slow, there is little turbulance in the fluid and the resulting "laminar" flow causes what is known as a boundry layer along the tubing. This is a "coating" of fluid along the wall of the tubing that is barely moving. It has the effect of actually insulating the fluid from the wall of the tubing and greatly decreasing the heat transfer. So turbulance is needed to prevent a boundry layer from forming and maximizing the heat transfer. By the way, there is a myth out there that the plastic tubing is such a bad heat transfer material that it hurts the geo system operation. In actuality the tubing is 10 times better at heat transfer than the soil, so the effect of the plastic is minimal.

The end of all of the calculations was that the loop tubing would be 1' ID. There would be 4 parallel tubes in the trench, connected at each end to a header manifold, and 1.25" ID line from the manifold into the house. The larger supply tubes would minimize pressure drops and there is no concern about the turbulance in the supply lines, since they are supply lines and not included as part of the heat transfer system.

To make all of the connections needed, without exception, the industry specifies that all in-ground joints must be thermally welded. Mechanical joints are not permitted since they can corrode over time and eventually leak. The couplings, etc look like a glue fitting, but the joining process involves simultaneous heating of the tube OD and fitting ID with a temperature controlled tool. After heating for a prescribed time, the pieces are joined together (as straight as possible) and held together for another prescribed time. Special thermal collars are applied to the tubing to control the melt, hold the tubing shape and concentrate the heat to the desired area of the tube. In a well done joint, when cut apart, it will be impossible to see where the two parts were joined.

I was able to get training on thermal fusion from a wholesale supplier. He gave me the instructions, checked my test joints and later rented the fusion tool for my job. Tools can be rented from a company on the internet, but you pay per day, including shipping time, and there is no training. You can also buy a tool for about $ 1K.

Once all of this was done I found a wholesale supplier for the tubing, and I ordered the tubing and fittings.

Continued in Chapter 4

paul

 

[techman]

Chapter 4

My other research was on the geo units themselves. The house had 2 splits (outside compressor with inside air handler), one for the 1st floor with the air handler in the basement, and one for the 2nd floor with the air handler in the attic. The best solution with geo units are unitized units that have the compressor, heat exchanger and air handler in one box. Just add water and power and you have a complete system. This would work fine for the basement unit, but I could not use it for the attic unit. This was due to the limited access to the attic (pull down stairway but tight clearances top and bottom) and no way to easily get the 1" water lines into the attic. I figured that I would use a split unit with the geo compressor in the basement, next to the 1st floor unit, and I could use the existing freon lines that were in the walls. This would require a new air handler in the attic (a later story) to match the geo compressor unit and meet the requirements for the SEER (efficiency).

I found a lot of units on the market. The big name guys were quickly scratched off the list, since they will not sell to a small operation like mine. I would be forced to buy from another dealer/installer, if one was willing to sell to me (not likely), and pricing would not be that attractive. Of the other geo manufacturers, who mostly make just geo units, there are several little, niche companies and a few bigger ones. After asking installers, reading manuals and specs, I narrowed it to 2 companies: ClimateMaster and WaterFurnace. Both had very similar units that feature a well built unit with high SEER, good controls and reasonably priced, and good reputations. I heard that WaterFurnace was started by guys that left ClimateMaster, which explains the similarity in the units.

In the end I chose ClimateMaster. There were a number of reasons in my case. Most of all was the support of a great wholesaler/distributor in the region. The price was a bit lower than WaterFurnace, and I heard more good things about ClimateMaster. Not that I heard anything bad about WaterFurnace, but there seemed to be a bigger installed base and network for ClimateMaster. Also there was the decision of using R22 or R410a (Puron to some) models. Even though R410a is the new standard, I do mostly R22 work as of now, my third air unit is R22 and there was no real disadvantage of using R22. ClimateMaster had a better selection of R22 units.

I chose a 2 ton for the 1st floor and a 1.5 ton for the 2nd, same as I had installed before. I went with a single speed blower as compared to the "ECM" type (variable speed). There is no measurable energy savings with a variable, only better "comfort" capability, mostly with A/C and the ability to run a dehumidify mode in the summer. I personally saw no significant advantage to offset the cost difference. I selected copper heat exchanger coils, since the costlier cupro-nickel coils are needed only for aggressive water conditions that might occur with an open loop well or pond system. I also added the desuperheater option to both systems. This is another small heat exchanger that is used to cool the heat of compression at the outlet of the compressor. The heat is used to help heat domestic hot water and lower overall energy costs (much more on that later).

More on Chapter 5

paul

 

[chili]

I would be interested in how much a system like you are going to put in is going to cost and how long would it take to justify the extra expense versus conventional systems. Thanks for letting us in on your project.

 

[bebster]

Thanks so much for sharing this with us! I'm right in the middle of trying to decide on a geothermal system for my new house, and your research and analysis is great! Can't wait for the next post!

 

[techman]

Chapter 5

To finish the equipment selection, I decided to put resistance backup heat in both units. Although the calculations showed thatI would not require it, I wanted it to have available heat if something failed during cold weather. It was intended as "emergency" heat.

The other items needed were a new coil/air handler for the second floor. There were models from most brands that would meet the spec. I chose a York unit since that was the brand I had mostly installed over the years, and I liked the product. Also I specifically chose the same model line as the old existing unit, with the hope that the similarity would mean easy fit up of the existing ducting, etc. (Ha Ha but more on that later).

The last item needed was the circulator panel. This is the assembly that contains the loop circulator pump and associated fittings and valves. I went with the ClimateMaster unit on that as well. Reasonable price, good quality and same supplier. I was able to get away with a single pump (a Grundfus). Depending on the flow/pressure requirements, they make single and dual pump panels, but I needed a single for both units. Both units would be fed by the same circulator panel, piped in parallel. The pressure drop characteristics of the 1.5 and 2 ton units was nearly the same so no flow control adjustment devices were needed. A relay is used to energize the circulator if either unit is running. If only one is running, the water flows through both, but the non running unit introduces no temperature change in the water and it results in the exiting water only being half as cold as compared to both running (due to water mixing on the return).

The final decision was the choice of antifreeze. The most common are methanol, ethanol, ethylene glycol and propolyene glycol.

Ethanol is considered non toxic but is expensive and must be denatured to be sold. The denaturant can create compatility with the pipe and pump, so the wrong product can be a disaster. It was not recommended by the manufacturer. Methanol is a very good choice for performance, but is considered toxic and is not allowed by code in some areas. The glycols can be used, but ethylene is considered toxic and has a lower performance compared to the alcohols. Propolyene glycol is non toxic but expensive, has poorer heat transfer characteristics and gets quite thick below 35 degrees and becomes difficult to pump easily. All have pluses and minuses. I chose methanol since there was no restrictions locally and it has very good performance and is low in cost.

With the decisions made, I ordered the equipment from the wholesalers and picked it all up early to mid summer.

Continued on Chapter 6

paul

 

[techman]

Chapter 6

September rolled around and it was time to start. Although I have a Kubota BX22, it was out of the question to consider doing the digging with it. But I did plan to do all of the backfill with the Kubota. So as mentioned, Bob showed up on schedule. We had discussed that he would use a 2' bucket on the BH, since 2' was the nominal spacing of the tubes, with 2 at the bottom, about 2' of backfill and the other 2 tubes.

I had carefully measured and marked the path of the loop in the yard. I wanted to be sure I was slightly under 500' so I would not come up short with the 500' coils of tubing. I planned strategy with Bob. If the whole trench was dug, it would create an island in the middle that would be very difficult to get to with the tractor. Likewise the spoil that was excavated needed to be piled up. If I piled it outside the loop, I could have a problem getting over it to get to the trench. If piled on the inside I would need to cross the trench with the Kubota to backfill, another problem. In the end we decided to do the "front" of the trench with the spoil on the inside and the "back" half with the spoil outside the loop.

As it worked out, I installed about 50-100' of tubing as Bob was digging. This allowed me to drive the Kubota around to get to the "inside" and backfill that stretch so I had a bridge to the inside of the loop.

Some pictures of the work now, and I will continue after them.

paul

 

[techman]

At the start - 2000' of 1" geo tubing in 500' rolls

paul