"The biggest cost in operating a furnace is the electricity that a fan uses. " I think your math is waay off. Read Mike's description above for what a multistage system offers. He prefers them, I personally don't but i also don't live in a humid summer climate. If I did, I might share his view. Here's an example for the money calculation. - My furnace's blower seems to be between 500-600 watts. Let's be generous and round it up to 1000 watts. Operating for one hour would be 1 kilowatt hour, which at my top tiered rate is 30 cents. Remember we rounded up. And in most parts of the US, this number averages about 15 cents and can be much less. My furnace is 120K btu/hr, input. Let's round that DOWN to 100K btu/hr. At the top tier, I pay over $1.50 per therm (which is 100 btus) but lets round that down too to $1.20. Operating my furnace for one hour costs $1.20 for gas, 30 cents for electricity. The real electricity number is less because I rounded up, and the real gas number is more because i rounded down. Did I get something wrong?

I realize this is single stage vs. two stage thread. My goal in stating what I stated was to show there is a realm above 2 stage, more or less as well as to show there are differences from one manufacturer from another. It wasn't my purpose to hijack a thread. There are those out there that say 2 speed equipment is unnecessary... so why would manufacturers make 5 stage equipment and variable speed equipment on top of an already unnecessary system? Many times in the field I've heard of people saying 'I don't want to gold plate anything'. Part of my response was geared to show that 2 stage systems these days are middle tier systems. People change homes so much: yes, this is why my job can be really challenging. Many times it's not replacing a whole system (which would be required to go to a 2 stage machine... in most cases*). Many times my job involves retro fit decisions. Replacing one or two pieces of an older system. In situations like that it's more about repairing the existing system and give a homeowner a solution they want. If I find out a home owner is only planning to stay in the home a few short years I rarely if ever even offer the 2 stage equipment. 2 Stage equipment is for those of you looking to stay in the home for at least a decade or longer. HVAC is not a one size fits all market. I am more likely to repair an existing system in some fashion than to replace. But replacement in some cases is the better option regardless of how long you plan to live in the home. If you live in my climate (Katy, Texas), you will have a tough time to sell a home without a functioning AC system.

I'm also in the Bay Area and presume the OP has PG+E as I do. Gas and electricity charges are tiered by order of the PUC, a so-called "baseline" allowance for the first small tranche and then the "excess" rate. For me, the baseline is enough for water heating and cooking and was $1.19 per therm on my last few bills. The next tier, immediately reached in any month when heat is used, is $1.75. Yeah, it's high, compared to rates elsewhere in the country. I replaced two systems a few years ago. The HVAC contractor recommended sticking with 80% furnaces because he said there would be no payback for the incremental cost of more efficient ones, so that's what I did. I guess it depends on how warm one keeps the house during the winter. We tend to stick to the high 60s, with daytime and nighttime setbacks, and in a larger house than this one. I don't think I've ever spent as much as $750 per year on heating and it's usually less. To save 10-15% isn't going to get me very far any time soon. Also, because of the mild temperatures, single stage units seem to be the most common. HERS testing is required for all HVAC equipment replacement and installation jobs. It's basically testing by a 3rd party, not the HVAC contractor, for duct leakage and requires reasonably easily located holes be patched. For my job, I had them do a complete rehab and reinsulating of my old hard duct system anyway, and in advance, so the HERS testing was a non-event. But it does cost about $1K.

we have a trane two stage compressor. with the tam7 airhandler. we are in sw florida, if that helps. i believe american standard is a rebranded trane, but im not sure of that... we have no complaints. our home was a new build, four yesrs old extremely well insulated, great windows, obvioysly new and well sealed ductwork, in semi conditioned space. . we almost never have the second stage kick in... if you're not well insulated, at least you will probably only run in first stage in shoulder season, saving some power and short cycling. if you're in a high humiduty area as we are, you mught also consuder a whole house dehumidifier. we. can set the ac at 79 or 80, and feel very comfortable. reversible ceiling fans in all rooms but kitchen and dining.