[TriLUG] Fwd: Re: OT: thermodynamics of A/C question

[Scott Chilcote]

[re-send - rejected by listserv this morning due to the byte count, i.e. 
included charts.  Sorry about the reply warp.]


Hello LUGGers,

I apologize for any potential thread-jacking, but I'd like to throw 
geothermal HVAC into consideration.  We had ours installed in May of 
2011.  So far, the savings have been outstanding.

Saving energy on HVAC costs is a topic of great interest to me, for some 
very good reasons.  A few years ago we moved from a two level house to a 
ranch style house, with most of the rooms spread out on a single floor.  
Even though the HVAC systems were relatively high efficiency (12 SEER), 
the energy bills were much higher.  We were paying at least half again 
more in the months of peak cooling and heating.  At our previous house 
the bills rarely exceeded $200, now we were paying close to $300 for 
peak electricity (summer) and natural gas (winter) billing.  The winter 
bills were especially bad because the home had hydronic heat, driven by 
a large gas water heater in the crawl space.

We learned over our first summer that the 18 year old AC compressors in 
both outdoor units were on their last legs, as the circuit breakers 
started flipping on very hot days.  We looked at the costs for replacing 
the systems with similar systems, with air sourced heat pumps, and with 
geothermal open loop and closed loop systems.  We even considered 
multi-room ductless mini-splits.  Despite the larger initial investment, 
geothermal was the front runner early in the process.

The federal and state tax credits make geothermal very attractive, at 
least until the current programs change in 2014.  If you install a 
geothermal system from a certified installer, you recoup about 30% in 
federal tax credits (Energy Star) and an additional ~40% in North 
Carolina tax credits.  With last year's tax filing we have already 
recovered about 50% of the initial outlay.  These savings have gone a 
long way towards making our geothermal investment no more expensive than 
a conventional gas-pack system.

We opted for closed loop geothermal, because we did not want to be 
dependent on the availability of local ground water to have our HVAC 
system operate.  We would have saved about $5000 by going with open 
source, but it was one less thing to worry about down the road.  Due to 
the limited space an access in our front yard (residential cul-de-sac 
with driveways) we had to go with vertical closed loop, in the form of 
two 300 foot wells.  The loops consisted of pipes going in both 
directions, top to bottom (like a large W with narrow Vs).  So in total 
it was over 1200 feet of approx 1" pipe, grouted from top to bottom.

Drilling the wells was the largest expense, and it also threw a big rock 
into the pond of our quiet neighborhood.  It took two days and created 
noise that could be heard two blocks away.  One of our HOA officers had 
a tizzy, in spite of the fact that the work was in full compliance.  The 
drilling rig also put some cracks in our concrete driveway, but we were 
told that might happen in advance.  People with lots of yard space can 
save money on the excavation costs.  I spoke with one fellow in a rural 
district who rented a vibrating trencher, and installed a closed loop 
coil field by himself.  He saved thousands of dollars.

We contracted for a 3-ton equivalent geothermal system.  The rule of 
thumb the installer follows is 200' of piping per ton, hence the pair of 
300' wells.  The yield on our system turned out to achieve 3.8 tons, and 
it cools our house effortlessly.  There's no outside 
compressor/condenser unit, just a quiet water pump mounted next to the 
air handler in our crawl space.  We've been running the house cooler 
than we did last summer, and the bills are less than half.  The 
difference over last winter was stark because our gas bills rarely 
exceed $20 now.  We only use gas to heat water, and the geothermal 
system (de-superheater) uses recovered waste heat to pre-warm the hot water.

The two attached images show our electricity and natural gas use before 
and after.

Note that I haven't attempted to adjust for the temperature variation 
between the two years.  Winter 2011 was mild compared to 2010, but the 
summers seemed fairly equal.  You can see that the energy savings over 
the summer was substantial though.

Comparison of electricity use per year 
<https://lh6.googleusercontent.com/-grtGkR1el4c/T8-WsCKftqI/AAAAAAAAALk/78IYLH8QU_g/s940/Electricity+Comparison+-+S+Chilcote.png>

Comparison of natural gas use per year 
<https://lh6.googleusercontent.com/-Ic8_JGLLgU0/T8-gP3LzjbI/AAAAAAAAAMo/SRldgu9UPuk/s824/Natural+Gas+Comparison+-+S.+Chilcote.png>

Just as a disclaimer, I'm only a happy customer.  I have no financial 
interest in geothermal systems other than the one we bought.  And to 
bring the topic back to Linux, I'm working on entering into a pilot 
program for a device that records status information from the system and 
delivers it to my computer.  If the device doesn't support Linux, I plan 
to give the manufacturer a strong push in that direction.

Let me know if I can help with any information on this HVAC project.  I 
have photos of the installation process online.

     Scott C.