Thinking about Options for Replacing HVAC system

I currently have a relative high efficiency gas furnace and air conditioning system.
The gas furnace exhaust is via a plastic pipe. Though the motors for the furnace fan and the AC are not multispeed so they are not the highest efficiency available but in 1991 they were pretty good for that time.
The furnace and AC are into their 15th year and it's my understanding that the useful life of these items is usually 15-20 years.
I'm wondering at what point I should go ahead and replace my system? With the price of Gas continuing to go up should I be looking sooner rather than later?
When I replace it should I go with another high(er) efficiency gas furnace and AC [location is north central Illinois]?
or Should I be looking at replacing with a geothermal system?
Any thoughts, comments, things to look for, etc.
Thanks.
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if dependability is what you seek, you could put in a new geothermal system independent of the working system with crossovers. then when the electric service is out you still have gas service to heat the house with a gas stove or space heater. if the severe hot or cold weather somehow exceeds your system limits for geo, you could run your gas system during the first summers and winters.
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geothermal uses latent heat in ground, with a heat pump.
no power, no heat pump, no heat.
using heat from ground makes heat pump more efficent in both heating and cooling mode since ground temperature is always around 50 degrees.
with a regular heat pump on a zero day, heat is all high cost electrical resistance.
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At the start, your ground temp will always be around 50 degrees, but with a closed loop, the earth can easily go below freezing if the loops are undersized.

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Just because its not multispeed doesnt mean it isnt as efficient as you can get now. what is it rated, or look inside the unit or on the papers and find input and output btu and calculate it. 94.5 %- 95 is about as high as they go . Even Carriers 96.7% line is BS as only the smallest unit 38000 btu is 96.7% the rest are near 94.5%. If you have had it cleaned and maintained it should be fine. VS DC motors save electricity but you need the 10 yr warranty.
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Although your system is 15 years old, it's still very efficient. You aren't going to save much money in operating costs with a new gas system. Keep it properly maintained, and look into getting a service contract from a company you trust. If it starts giving you trouble, that's the time to replace it. Geothermal units are expensive to install, and (depending on the type) can require a lot of yard work after it's installed. A geothermal will save you money, but the payback could take a long time, considering what you have now.

conditioning
than
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Now is the time to do some research (O.K., I guess that's what you are doing now). Ask your friends and neighbors who they would use to replace your system. Call up at least 3 or 4 different companies and have them come out and give you an estimate. Ask them the same questions you asked here. From what I can gather from your post, your sytem is still in good working order. That's good. That way you are not pressured into making a desicion. Depending on the efficiency of the units you have now, you may not WANT to change them out at this time. You may not get your money back in a savings/cost scenerio. At least you can get an idea on the cost and options....
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conditioning
than
I installed a 5 ton American Standard 12 seer in 1990. That was about as high as they went back then for the 5 tonners. I bought mine just before the 2 speed units were released. Old 5 ton FLA was 45 amps new AS was 28 amps. Electric bill dropped by half from the previous owner. I offer the following two sites that you can play with to see the "savings" you will get. My last house had a "builders special" circa 1999 10 seer. I looked into changing it and the savings would have taken 10 years, I passed. Sold the house and moved after a year anyway. http://www.hvacopcost.com / http://198.147.238.24/ac_calc/default.asp
calculators are on the left of the bottom page. Energy solutions Pay attention to the duct insulation section. There are some interesting numbers for the cooling cycle when I played with the calculators.
You had better find a geo thermal contactor locally and investigate your site. Some of the piping systems get to be huge. Do you have an acreage? You might need one. Geothermal can be pricey to install.
VFD's can help with humidity removal. But a cold coil works better, >22 F difference. I installed a VFD it lasted 5 years, and lunched. Took me a day to R&R the damn thing. Price for the part was not bad ~$100.00. Sure glad I could do it myself.
Best to find out exactly what you have. Take some base line readings of the performance. Add a wish list then call some contractors.
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