The theory is awesome and yes it does work but works the best with older 13 and below seer units. Some systems work well with HRU's and some not so well or not at all. I really wanted to go this route and researched and talked with a few hvac pro's that had issues like the unit would operate fine if it was 90 degrees or higher outside but if it was 80 the pressures were too low and the a/c unit wouldn't operate correctly. Its like Fsq4cw said. The system needs to be designed for it otherwise its anyone's guess how things will work. If everything works together they are pretty awesome. If not they can cause a huge headache trying to iron out the kinks. One other idea I brainstormed was using a HRU and a small ground loop or pit with a pump and make a hybrid geothermal/air unit. I doubt it would work though.

Update: Before the install, I did wind up modifying the track in the air handler for the 1-inch air filter. I used 2 pieces of steel banding, 2 inches wide, cut to length, and placed them over the manufacturer's track using a few dabs of automobile body putty (Bondo) to keep it secure. It is now a smooth track for the filter to slide on. Goodman used the track as part of the frame of the air handler so it had some crimpings in it and the filter would not easily slide in. I also trimmed away some foil faced insulation that prevented the filter from seating properly. A 20"x24" filter now fits perfectly. I am glad that I did these minor mods as I would have been fighting this monthly when I change filters. The unit was installed Friday, 6/22, after I had it for about 5 weeks waiting on the installers. It took two guys 8 hours. I fed them both breakfast and lunch (I treat my installers well). The old supply air plenum and the plenum after the air handler had to be replaced to match the new unit. The old 5/16" liquid line had to be replaced with a new 3/8" line. 3 holes were cut into the finished ceiling and 1 in the wall so that the new line could be run. I'll repair the holes when I get a chance - no big deal. The TXV sticks out about 8 inches from the air handler. Looks funky and I have to be careful not to hit it while moving things around in that area. The old outside unit had sunk about 4 inches into the ground. I filled the hole with soil that I was saving, packed it down, put a plastic condensor mounting pad on the dirt, and used 3" thick brick to elevate the outside unit (which is huge in size compared to the old 7 SEER unit; 14 SEER = more coils = larger unit). The outside unit is much more quiet as expected but the new mult-speed air handler is also a little more quiet than the old one. The fan is set on low speed and I'm not sure of the setting of the old unit as the lables on the motor had worn away after 21 years. The sheet metal in the new air handler is flimsy and the track for the air filter can be improved by Mr Goodman. The outside unit looks decent. These comments were also noted by the installers. I plan on taking the outside and inside coils to a local scrap facility and sell the copper. Maybe I can get a few $$ for a couple cases cases of my favorite beer. I can now start saving 40% on the cooling and heating part of my energy bill and use this to combat the 65% increase in electric rates in the State of Maryland. All in all, I thought that I got a good value for the money spent (only time will tell if I made the right choice with Goodman). I got a 14 SEER system with Copeland scroll compressor, thermo-expansion valve on the air handler, hard start capacitor, 15 kw heat strip kit, bypass humidifier, and 10 year parts warranty for $2477 from a local HVAC supply shop. I paid $1200 for a pro install (needed the pro install for the warranty). Total cost was $2k less than...

I just did a wikipedia search on heating oil. Heating oil has 138,500 btu's per u.s. gallon. I'm assuming thats gross btu's. So you would have to have 100% efficiency to get that. I also did a wikipedia search on therm. It says 1 therm is equal to 100,000 btu's. Thats 100% efficiency ofcourse. 1 therm or 100,000 btu's is equal to about 100 cubic feet of natural gas. 100 cubic feet is 1/10th of a metric cubic foot (MCF). Right now my natural gas costs .90 cents with all fee's delivered to my house per ccf. I think it takes about 1.3 ccf to equal the btu's in 1 gallon of heating oil. Heating oil would have to get down to about $1.25 per gallon to provide the same amount of btu's as natural gas at the price of .90 cents per ccf. Keep mind the price of natural gas usually goes up in winter. Its also kind of cheap right now. Its been higher in the past. Lets look at heatpumps. At 47 degrees fahrenheit my 3.0 ton 15 seer unit produces 36,000 btu per hour. It uses 2.82 kilowatts per hour. My electricity is .11 cents per kilowatt hour delivered. Thats about $.87 cents per therm. or 100,000 btu's. Keep in mind that the btu numbers for oil and gas are less then the numbers mentioned. The number mentioned are raw heat. Your furnace will get 95% for gas and a different % for oil. You don't have that problem with heatpumps. The btu's mentioned are the real thing. The heatpump will go into defrost mode sometimes and waste energy. If electricity is a good price heat pumps can be cost effective down to low temperatures. Some utilities give discounts if you consume alot of have heatpumps. If electricity costs .20 cents per kw it might not be a good deal. Its nice to have the heatpump option atleast incase fossil fuels skyrocket again.

"Last question. Does the dual fuel furnace determine when the temp is low enough for the LP to kick on?" sdionnemoore There are several methods in setting up dual fuel systems as far as the changeover point between HP and gas furnace. I prefer the restrictive method wwhere the homeowner decides the outside temperature that HP locks out and gas furnace becomes the heating source. I have a Trane dual fuel with a mech thermostat on my HP connected to the furnace control cabinet. My changeover is set at 30 degrees(my selection) and this works seemlessly back and forth between HP and gas furnace setting. I ran some numbers using your electric rate and LP propane cost. If operating cost is paramount to you, then straight HP with strip heat auxilliary/supplemental heat is by far your best choice. However, if you plan on using propane for other reasons such as cooking, fireplace gas logs, clothes drying, etc then I see nothing wrong with LP gas backup heating. Just know this will be more expensive now and in the future. LP gas prices just scare the heck out of me. It is correct that today's high eff heat pumps' efficiency drops as the outside temp drops. Again, I suggest you refer to the fuel calculator that I posted above. IMO