One Year of Experience with Geothermal

java_manMay 28, 2005

We have just completed all the arithmetic for the first year of operation with geothermal in our new home. Our home replaces a smaller 50-year old home on the same lot. The old home had 4 inch fibreglass batt insulation retrofitting in some but not all walls and R28 fibreglass blown into the attic. The new home has an excellent air seal, 2x4 walls insulated with fibreglass batts, and R40 blown-in fibreglass in the attic. The new home has a high efficiency pleated filter, humidifier, and a UV sanitizer in plenum. The old home had the standard $5 fibreglass pad filter in the air handler for the furnace, and no other enhancements.

The old home was 2050 SF on 2 floors; the new one is 4771 on 3. The new home has aircon and an ERV, the old one had neither. The new home has electric hot water augmented by the geothermal system, the old home had natural gas furnace and hot water. The old home had 2 direct vent gas fireplaces, the new has one. Our climate is very similar to Seattle's.

Total energy cost in the old home was $140 per month (average of last 12 months in the home) while energy in the new one cost us $130 per month (average of first 12 months in the home). If we adjust these costs for the price increases in electricity (up 9%) and natural gas (up 7%) that occurred after the old home was demolished but before we moved into the new home, the price comparison is even more favourable. Before building we estimated the payback period for our geothermal unit at 8 to 12 years, depending on the rate of price increases for electricity and natural gas. That calculation still looks about right with the first year of energy costs known.

The geothermal system has functioned flawlessly after an initial glitch caused by a refrigerant leak that was found in the unit when first started up after installation.

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gina_in_fl

Please explain your geothermal system? Is it inground?

    Bookmark   July 17, 2005 at 2:24AM
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java_man

Oops, sorry -- I should have included more information.

Our system is a ground source heat pump, with PEX tubes buried in six 135 foot deep drilled holes. The fluid in our system cirulates under moderate pressure. The house uses forced air to distribute heat from the heat pump.

All costs are in Canadian dollars.

    Bookmark   July 17, 2005 at 6:29PM
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lucky_p

Wow, and if you'd used cellulose insulation instead of fiberglass, your monthly heating/cooling bills would probably have been 30-40% lower than they are now - and you'd also probably have been able to downsize your geothermal unit by similar 20-40%.

    Bookmark   July 18, 2005 at 5:42PM
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java_man

We checked several alternative insulation treatments ranging from 2x6 walls (with commensurate increase in insulation), various icynene foam/fibreglass batt combinations, and 100% icynene foam. None of them showed energy use reductions anywhere near 30%.

Of all the changes we modelled, the biggest variable in the heat loss calculations was always air infiltration. We modelled our home with an "equivalent leakage area" of 144 sq. in., approximately 35 to 50% of what we were told is "typical" of new construction in our region. With air leakage that low, the energy savings from added insulation showed a very long payback on additional insulation.

We checked with all 3 HVAC bidders on whether we could get away with a smaller heat pump, and ended up going from the recommended 5 tons to 4 tons as a result of our sealing calculations and the methods we planned to use to achieve a very low air infiltration rate. Two of the HVAC bidders were not willing to reduce the size of the heat pump -- they said it's too "iffy" to get air leakage as low as we were modelling, and we wouldn't know whether we had achieved the required seal until AFTER the heat pump was installed, when the house was complete. The third HVAC bidder said they wouldn't recommend dropping the size of the heat pump to 4 tons unless we used 100% icynene foam insulation. Their view was that air sealing is too dependent on attention to small details that is seldom achieved in the real world of construction.

We went with the 4 ton system anyway. We personally applied more than 60 tubes of acrylic caulking, hundreds of feet of foam "rod", and many rolls of "Tuck Tape" ourselves, improving on the builder's work. When the house was complete, we had an air test done, and ended up bettering the modelled ELA figure. The technician who did the air leakage test said he's seen only one tighter house in five years of doing pressure tests.

The lesson for me is that air sealing is the best investment you can make in energy efficiency. With an excellent air seal, we achieved energy costs that are as low or lower than more expensive insulation treatments would have yielded with "normal" air sealing practices.

    Bookmark   July 18, 2005 at 6:51PM
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jrdwyer

Thanks for posting. Very good info.

Do you feel the quality of the air in your home is good considering you have such a tight structure?

The reason I ask, is that I am slowly but surely making our 1982 home tighter by replacing old windows, caulking, door seals, etc. Will I reach a point where it becomes unhealthy and requires an ERV or is that unlikely considering typical builders' sealing practices?

We use a standard electric heat pump with forced air for heating and cooling and Aprilaire filtration. It is very cost efficient, expecially considering the current high prices for the natural gas heating alternative.

    Bookmark   July 23, 2005 at 2:32PM
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java_man

We have an ERV that runs continuously to exchange the air ~ every 90 minutes. We never open the windows. The quality of the air is the best we've ever had. It's filtered (Aprilaire), humidified and conditioned, and it shows up in the low accumulation of dust, greatly reduced allergy symptoms, and . . . . I don't know how to put this delicately . . . in my kleenex.

I would not go without an ERV in a tight home, especially a newer one. There is so much off-gassing from furniture, finishes, etc. that a tight house without ERV will be unhealthy. People exhaust so much moisture into the air that a tight home without ERV is likely to turn into a mold factory.

    Bookmark   July 25, 2005 at 11:37PM
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bydesignprez

This is interesting although the significant changes in the size and insulation of the house mean that apples are not being compared with apples.

The modeling I have done is with my existing house and its known insulation, air leakage and various other quirks and defects. The existing heating bill including electricity to drive the various fans is $2,667. [All CDN $] Using the efficiency figures etc. of a GSHP I calculate that my energy bill would drop to $1,206, so the net savings of $1,461 are available to "finance" a GSHP and preferably an ERV. Frankly, I have found energy saving investments with a better financial or aesthetic payback, but if we are going to stay in the house another 10-20 years it may be worthwhile.

This model was calculated with gas and electricity costs of a year ago and gas costs have increased faster than electricity here so it may change it. As it's a spreadsheet I can easily plug in new values. Now all I have to is win a lottery!

    Bookmark   July 28, 2005 at 9:21AM
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java_man

Bydesignprez,

Do I understand correctly that you're considering a retrofit? If so, and if your existing heating system is forced air, you may not find converting to a geoexchange system to be a satisfactory solution.

Since the heated air coming from a gas furnace heat exchanger is considerably hotter than that coming from a geoexchange system, geoexchange systems (and other heat pumps) need to move much more air than a gas furnace to get the required BTU transfer. That requires either new, larger air ducts or higher air velocities in the existing ducts.

If you use your existing ducts, you're likely to end up with a noisier, draftier home. The locations of some of your air ducts may cause annoying drafts in some living areas. But replacing all the ducts would add considerably to the cost of the system.

There are many articles on the Internet that discuss forced air duct size for heat pumps vs. those for gas furnaces. As I recall, some of the early heat pump installations used "standard" duct sizes intended for gas furnaces, which is one of the reasons why heat pumps were initially criticized for creating cold, drafty homes.

    Bookmark   July 28, 2005 at 11:54AM
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bydesignprez

Thanks Solar Gary that is a very interesting web site. I am in the middle of the Canadian prairies which goes from 95F in the summer to -40 in the winter, so insulation is the first line of defence. Given the wild swings in air temperature, GSHP are very attractive as they can use the lagged heat/coolness of the ground. Solar works for swimming pools, but again my experience is that unless it's kept covered at night the dang thing cools off so much that the solar cannot produce enough BTU's to bring it back up to temperature.

Always interesting, isn't it?

    Bookmark   August 2, 2005 at 10:48AM
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fsq4cw

This is primarily for Bydesignprez,

We live in Montreal, where the weather can be as severe as yours. Without any other major changes to our home, the energy savings weÂre experiencing using geothermal over oil has been 72.33%. This is based on what we paid for our last oil fill-up (2 1/2 yrs. ago) and electricity to run the systems (TX incl.).

Your calculations of savings are rather conservative. I would strongly encourage you to not let another heating season go by without making the change. I have not yet met the person who went to geothermal and was the least bit sorry.

I would only be sorry if I was STILL paying higher, and yet higher energy costs rather than investing that money in a project that adds real value to our home as well as saving energy.

To all those still pouring over and agonizing over the figures; like calculating pi to the last decimalÂ

Bottom line  JUST DO IT!

SR

Here is a link that might be useful: how-efficient-is-it-magazine.com

    Bookmark   August 3, 2005 at 12:21PM
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