Having been a American Standard (Trane) distributor/wholesaler, I can tell you that we sold very little in motors and modules replacement wise. Go with the variable speed motor (engine) in your furnace, you will be very happy with the comfort added, reduced noise and energy saved.

Thermostat problems can mimic or hide other problems with your HVAC system. If the thermostat turns the system on, more often than not... the problem is with the system and not the thermostat. With that said, a HVAC repairman must have a good understanding of controls as well as the problems HVAC equipment has that tends to mimic one to believe there is a problem with a thermostat. Intermittent type problems can be very difficult to diagnose properly. If the HVAC repairman is suggesting you to replace a furnace because you're throwing money away on a part to repair the old one what happens if you put in a new furnace and the same problem happens with the new one a short time later? Sure you can replace the furnace to fix it again, but how much money are you throwing away by not just fixing the old furnace? The key in this is that you need someone who knows they can fix it. If your repair man lacks confidence the writing may be on the wall. A few words of advice if you look to a new service company..... don't try to diagnose your system as to what you think is wrong with it. You are paying for a service, it is their job to diagnose it. They may ask you for bread crumbs as in how the system is operating, this is perfectly fine to give technician clues as to where to look. Intermittent HVAC problems can take more than one visit sometimes. You need a HVAC repair man with a tenacity or determination to get to the bottom of the problem, otherwise you're left to replacing the furnace and hope the problem doesn't reappear a short time later. I can tell you I've solved tough problems just this last year alone on the first visit, but I've also had several calls that took two visits and one that took as many as 4. I say this so you understand the challenge. A true HVAC repair man is a dying breed. The way things are going in this industry your choices will be few. The bad part is, you will always need a good HVAC repair man... as you've found out the hard way replacing 4 furnaces at this property in less than 80 years.

I'm a little confused by the underlying issue. The point of two zones is so that you can have different temperatures in different parts of the house. It is expensive to run the blower all the time. As others have pointed out, running it continuously can mask problems, but doesn't really solve anything. You really need to focus on fixing the problem... high humidity levels and temperature differences (if that is really a problem) rather than running the fan continuously.

Thanks for the comments tigerdunes. We've been over this quite a bit already. The pricing is average for the North East....labor rates tend to be quite high and the $600 rebate makes the price of the furnaces excellent. I obviously have two stage thermostats and the sizes of the furnaces were decided after heat loss calculations. I live in an older home in a cold climate and my calculations were within 10% of my contractor's. The calculation for my two floor unit came in at 75kBTUs. The contractor initially wanted to install a 100kBTU unit, but I talked him down to 80kBTU. The furnace for the renters below me is a bit large, but it was the smallest two stage available from American Standard. I thought about installing a 92% efficient 40kBTU single stage furnace with selectable motor speed, but as the 60kBTU two stage with variable speed motor was the same price after the rebate I went with that one.so that both furnaces essentially the same and to get the variable speed blower. I imagine having the second stage will also be nice on very cold mornings.