Home Geothermal System - PeachParts Mercedes-Benz Forum

Graplr · #1 02-11-2012, 12:18 AM

At the request of Kerry (in another thread) I am posting about the geothermal system that was installed in my home this past summer. Get ready for the novel! I'm sure way more info than Kerry wanted but I got carried away.

First, a bit about my home. I live just outside of Stillwater MN, which is a river town and a tourist-y area about 15 miles NE of St Paul. My home was built in 1940 as a square 2-story farm house. In 1968, the people we purchased it from bought it and added straight on to the back on both stories and an angled 1 story addition on the front side and made a walk-out basement. The home is rather large at roughly 3700 sqft of finished space.

The home sits in the rural area on over 5 acres. I have no gas hooked up to the house and it is a septic system.

There were two heating systems in place when we moved in. A fuel oil boiler that uses baseboard heat to heat all parts of the original house and the backside addition. The front side addition is mainly electric baseboards, although parts of it are open to the rest of the house (not isolated by a doorway but open either through a hallway or part of the room itself). Every room in the house has electric baseboard heat as well.

My house has two electric meters installed. One is "off-peak" which means the electric company can shut it off at periods of HIGH volume electric useage but the reward for having one is a drastically reduced cost per kWh. When I moved in, the only thing hooked up to the off-peak was the electric baseboard heat. On average throughout the year I pay (changes with season and such) ~$.06/kWh for the off-peak and ~$.13 for the regular meter.

I believe the previous owners installed electric baseboards in all rooms in the more recent part of their ownership due to the increasing cost of fuel oil (but this is NOT confirmed) to help offset the need to run the boiler as much. I have never added fuel oil to the tank. From past records left by the PO, it appears I have a 1000 gallon tank (underground) as I have two receipts for about 920 gallons filling the tank. From my best estimate of fills, it seems they needed to fill it once every two seasons give or take a few months depending on the winter.

There was NO central air system OR ducting installed into the home when we moved in. There was one in wall AC unit in the kitchen on the main level and ironically two in the two basement bedrooms. It is ironic because much of the basement is mostly underground and thus stays fairly cool in the summer on its own. There was an attic fan installed on the upper level to help "pull" the warm air up and out of the home which did help some. But the main advantage to me seemed to be to open up the windows in the morning on hot summer days to cool the house quickly and then shut the windows to keep the temperature inside down during the hot days. It worked okay but on long hot spells of multiple days in the 90sF or 100sF it didn't matter because it didn't cool off enough at night.

So we made the decision we wanted to install central air as much of the house became unlivable much of the summer. Of course to do this, we would need to retrofit ducting into the house, which of course is no easy task physically or financially.

In doing the math and looking at when the fuel oil tank was due to be filled for this season (no gauge on it, just put a tape measure down and looked at the trace of fuel oil on it so far up from the tip on withdrawl) and that it had not been filled in two years I assumed it would need at least 900 gallons to fill it (and last 1.5 - 2 seasons). Fuel oil prices were well over $4/gallon, closer to $4.25-$4.50 mark. So a rough estimate was $4000 to fill the tank for the coming season. Plus I would still have to run the baseboards to heat at least part of the home.

I first looked into A/C systems on their own. I did not talk to a contractor but my rough estimates on a system were $5-7000 for the system itself plus a cost to install ducting which I roughly estimated at $6-8000. So all in all about $12+k for Central Air with a small rebate from the government.

In looking into government rebates, I noticed geothermal has a 30% tax CREDIT (not deduction) through 2016. I decided to contact a geothermal contractor to have them give me some numbers. He estimated my home to need a 5-ton ground source heat pump as a part of the system to cool my home in the summer and heat my home in the winter. He took a lot of measurements and got back to me in a few days with an estimate of about $32k for the most efficient top of the line system for everything installed including all new ducting throughout the majority of the house.

The home had an outdoor inground pool when we moved in that was rather large and NON-functional. My wife grew up with a pool and really didn't want one. I had a pool contractor come out and he thought he could get it working for maybe $5-7 grand but that may not last too long as the pool was over 40 years old. We had another contractor come out to look at redoing the entire area (landscaping, pool, etc) and we realized that in order to have the pool functional and as low of maintenance as possible we would basically have to take the pool out and put a new one in costing in the neighborhood of $30k.

Realizing that the geo guys would need heavy equipment to do the drilling, the salesperson offered to take out the pool as part of the package and he only added on the cost of fill to the total. We also added on ducts to a couple more rooms which brought the grand total to $37,550.

Now I ran some numbers. First take the pool fill numbers out (as we would have done this anyways) and that brings the cost down to about $34k. Next factor in the tax credit and a local electric company rebate that brings it down to about $22k for everything on the heating and cooling side. So if it would take about $12k for central air that leaves about $10k for heat costs. I didn't know exactly how much less my cost for energy would be for heat using geo but I was fairly confident it would be less.

Another nice advantage of this system is that the ground source heat pump (think compressor on your car) has a liquid jacket on it. There is a pre-water heater installed in my water heating system and there is a loop that runs to the heat pump to cool and thus heats my home water heating needs. Thus it reduces my energy needs (electric in my case) to heat domestic hot water.

So we decided to install it.

The original estimate by the salesperson was 5-7 days. Yeah right. The ducting install took over three weeks. The drilling took about 2.5 weeks. Since it was a 5 ton pump they needed to drill 5 separate loops of 500 ft each (250 ft out and 250 ft back). Evidently there is some thick groundrock at our location and the horizontal boring took much longer than anticipated.

Since the unit was installed in the summer the first mode we were able to use was central air. It worked flawlessly. The entire house was cool and comfortable even on the hottest days.

The entire geo system is hooked up to the offpeak meter so we are charged at a cheaper electirc rate. I noticed almost no change in my summer electric bill even though the whole house was cool and temperature controlled.

This winter I have been able to test out the heating mode of the system. When the temperature is below about 32F, the system runs pretty much constantly. It is designed this way in an effort to be as efficient as possible.

Running the ducts to the front addition was difficult and at best without tearing up the house, so the geo provides only part of the heat to this part (even though it seemed to cool fine) of the home. So I still have electric baseboards on in about 3 of the rooms.

In comparing electric bills from last season to this, there is a decrease in cost to this year. But, this season has been noticeably milder. On the coldest spell of the season so far I did turn on the boiler to as the geo didn't seem able to keep up completely (not sure if the electric company kicked it off or not) and the temp had dropped a couple degrees in the house. I don't know exactly how much fuel oil was used but it was very little. So the biggest cost savings comes in not having to use fuel oil which is at a minimum of $1800/year plus what ever electricity I save. I assume I will have to keep at least 100 gallons in the fuel oil tank for emergency purposes.

So looking at a cost payback of $22k heating and cooling would give me perhaps 10 years. But if I factor installing a central air system into the equation my pay back is much quicker at less than 5 years. (Since it would have cost 12k to install central air I only have the 10k to pay back on heating alone and this does not factor in the geo cooling is EXTREMELY efficient and costs very little energy wise)

Now to talk a bit of how geothermal works. It is very similar to your refrigerator or AC system in your car. There is a sealed coolant that is run through a refrigeration cycle which compresses a gas into a liquid and then evaporates back into a liquid. This gives off heat at the condenser and cooling at the evaporator. In the geo system in the summer, the ground is the condenser. It basically takes the heat out of the home and deposits it into the ground via the loops that were drilled into the ground. There is about a 10F difference in the line going out of the home versus the one coming into the home. In the winter the system runs in backwards. It takes the heat out of the ground (about 50F constant Earth temperature at my location/latitude) and then compresses it and uses the Earth as the evaporator side of the cycle. Plus the pump is cooled by water that is dumped into my domestic hot water system.

All-in-all, the geosystem has an efficiency percentage of around 420%. That means for every one unit of electrical energy used from the power company, I get over 4 units of energy into my home. The extra energy is "stolen" from the Earth's residual heat energy. If I did not use the domestic hot water heating part the efficiency drops to into the mid 300%.

I am very happy with the system and am glad I did it. I haven't been watching the payback numbers that closely but I do know eventually it will pay for itself no matter how one looks at it.

Thanks for reading my novel.

kerry · #2 02-11-2012, 10:01 AM

Thank you. Very helpful info. The city installed geothermal into a public housing high rise near my house a couple of years ago. Took a whole lot of drilling. I was curious as to how it worked for a private residence.
Seems to me that there must be a temperature below which the system cannot function to heat the house given the constant heat of the ground. I know my sister in law had a head pump in Tennesee with a set of electrical coils which would/could be kicked in when temperatures went below X. Do you have any idea of what that temperature is on your system?

jplinville · #3 02-11-2012, 10:33 AM

does the heat feel more like the forced air systems with cold spots in the rooms or more like radiant heat, where it's a more comfortable feeling?

I grew up with wood burning stove for heat in the main part of the house, and gas forced air systems for the rooms that didn't benefit from the wood burning stove.

Currently, our house has a 35 year old gas forced air system without A/C, which is a bit unbearable during the summer. I was quoted between $5 and $10k for a new system, near the $10k mark with central air. When I get the kitchen and bathroom done, I'm thinking of radiant floor heat.

My dream is to get rid of the old 55 gallon water heater and go with a tankless system, but the payback for one of those systems isn't in the time range I'm hoping.

Due to living in coal country, I'd think there would be problems with drilling for a geothermal system here...

When the time comes for us to move to our property in Utah, we've decided to build most of the house underground, with a modest dwelling above ground. This decision was made on the premise that heating and cooling prices would be much cheaper.

Graplr · #4 02-11-2012, 10:51 AM

Quote:

Originally Posted by kerry

Seems to me that there must be a temperature below which the system cannot function to heat the house given the constant heat of the ground.

I'm assuming you are talking about ambient air temperature. If so it is all in sizing the ground source heat pump. I have a 5 ton pump. There are 5 loops associated with it. For a 6 ton pump there would be 6 loops and so on. I went and looked at the stickers on my unit at the temp ins and outs of the coolant entering and exiting the ground. These were taken on 7/28/11. In cooling mode it came out of the ground at 70F and entered the ground at 79F. In heating mode it exited the ground at 68F and entered the ground at 59F. I'm sure these temps would be different if they were measured today in the winter. I place my hand on the tubes (pulled back the insulation) and both felt cool. I have an infrared thermometer in the garage. Perhaps I will check sometime.

There may be a limit but I would think the limitation would be the size of the loop field available to extract heat out of the ground. I know in my location they were able to bore horizontally which costs less (smaller machine used). In areas with less of a lot size footprint they drill vertically. I know there has to be X amount of space between each loop. But it is not much. They drilled all loops in one direction spanning maybe 100yds separating the end of the outermost loops (they use GPS like technology and mark them with flags--see below). They could have easily had many more loops horizontally, not to consider vertical loops if my system had needed it.

Of course the further North (or S in the S hemisphere) one gets the cooler the average Earth temperature is (near the surface) so I'm sure that factors in as well. I believe my salesperson told me that average temp in FL is 65F and here in MN it is 52F.

Quote:

Originally Posted by kerry

I know my sister in law had a head pump in Tennesee with a set of electrical coils which would/could be kicked in when temperatures went below X.

Sounds like this was an air source heat pump. These work similarly but are not near the efficiencies of a ground source heat pump since the air source heat pump takes the energy out of the air, and knowing one tries to cool in the summer when the air temps are hot and heat in the winter when air temps are cool they do not work as well as the ground since the ground is cooler than the air in the summer and warmer than the air in the winter.

Quote:

Originally Posted by kerry

Do you have any idea of what that temperature is on your system?

Which temperature? The coolant in the loops? If so, see above. The air temperature change across the heat exchanger in cooling mode on the same date above was 70F air in and 51F air out. In heating mode on the same date it was 70F air in and 96F air out. Again I'm sure those values are different in the winter.

Another very interesting part was how the drilled the loops into the house. As I mentioned earlier they use GPS like technology to locate the head of the drill and guide it that way. Basically they dug a 20' x 10' hole about 20' deep in my yard about 50' from my house. They bored down a bit but mostly horizontally 250' out and back then pulled the hoses through the holes.

Now for the cool part. When all 5 bores were done they turned the drill around and bored towards the house. Using GPS like technology in which they use a surface "detector", one guy walks along (and IN the house) to signal to the drill operator which direction to go via walkie-talkies. Then when they are at the correct location the drill is turned upwards into the house. It is amazing how accurate they are.

Then in the hole in my yard they connects the two large tubes coming out of my house to the 5 loops in my yard. Then they filled the hole back up.

buffa98 · #5 02-11-2012, 10:54 AM

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