I am in the process of investigating the replacement of my oil furnace by a
geo thermal heat pump. This is intended to be a summary of the things I
have learned during this process.
For background, I currently have a 4 ton A/C unit and a 92000 BTU oil
furnace, and live in a 3000 square foot house (built in 1978) near the shore
in New Jersey.
1) Buried ground loop HP.
The loop is made out of plastic tube, and is to be buried in about a 5 foot
trench. Joins in the tubing HAVE to be done by gluing, and you need about
600 feet of tubing for each ton. So in my case I need 3000 feet of tubing,
which translates as 3000 feet of trench. Now the tube can be laid on one
side of the trench, looped at the far end, and returned on the other side of
the trench. Instantly dropping the trench length to 1500 feet. Then you
can fill the trench with about one foot of fill, and repeat the above
process. This cuts the trench length to 750 feet, which is still a lot of
The problems are that the tube is filled with alcohol to reduce the freezing
temperature, and they have to be leak tested before the trench is finally
filled. This also appears to have the lowest efficiency of any of the
2) Ground water Loop HP.
In this system two wells are drilled, and water is pumped out of one well,
through the HP, and back into the second well. This has higher efficiency,
but well drilling suffers from serious cost problems in NJ. Also the
injection well may silt up, giving serious maintenance problems (at the very
least). When it works, its very good. This system skips one heat exchange
from system #1.
3) Direct exchange HP.
In this system, the actual refrigerant is passed through a ground loop.
This skips one complete heat exchange from #1, thereby increasing the
efficiency quite a bit. The system I am looking at needs a 40 by 50 foot
pit, 8 feet deep, with refrigerant run through copper tube. In this case
the major problem is the size of the pit, and the potential for the copper
to corrode; so you would then have to re-lay the pit and replace the
refrigerant. Under reasonable conditions the copper will last 50 years.
Also if a leak develops in the copper tube, you will have a serious problem
You will also get a choice of refrigerant. I would prefer to have R22 (I
think that's right) for the next 10 years, and about that time switch to a
drop in replacement. R22 is to be phased out in 2020, and I don't have much
faith that the other alternatives have been fully tested yet.
In my case it is estimated that I will save about $2000 per year over my
current expenses (Oil 2500 per year, hot water $600 per year and A/C $300
per year). I am seeing cost estimates ranging from $10000 to $30000 for
everything. My guess is that I can get a reasonable system for $15000, with
a payback of 5 years at current prices. If the price of Nat Gas and Oil
doubles, my payback will be about 3 years.
I should mention that I will also get a credit of $500 per ton from the
state for an acceptable system.
Anything you can add would be appreciated.
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