My first suggestion is to have a load calculation done. The 80K BTU is likely to be the correct size but want to verify this.

I looked at the product data sheets of the 59TN6 and the 59MN7. The former is a 2-stage variable speed furnace. The latter is actually a 3-speed variable speed furnace. Carrier advertises it a a modulating furance, but it does not have the variation that other manufacturers offer.

I don't see why the contractors would discourage you getting a 3-stage furnace or a 2-stage model. I own the 58MVC which predecessor of the 59MN7. I think the addition of the intermediate stage is a big plus. In reality you really don't need more than 3 stages. I would recommend getting the 59MN7 with the Infinity controller.

You should have the duct work inspected for leaks and seal them as much as possible. If the return is too small have it made bigger. The variable speed motor will work as hard as needed to push the required air. I would also recommend getting a 4 inch filter.

The more stages the more comfort. Not convinced about the savings in operating cost.

IMO

>1. Putting aside price, given the age and bad insulation in my house, would the modulating be the better option to get heat to cold places places like my 3rd floor?

That's not really a problem that can be fixed by just changing equipment.

>2. If I am better instead with a two stage variable then is the Infinity 96 or Trane VC95 a better option (as similar price)?

It's a matter of whether it's worth it to you. Modulating furnaces are much better than 2 stage in terms of comfort. The only way modulating would be worse is price. What is the contractor's justification for avoiding modulating? Usually when contractors steer people away from modulating it means they haven't been keeping up with the latest technology and aren't trained on how to install and service them.

>3. If go with a two stage furnace do I need an infinity/trane thermostat or can i simply buy a decent programmable thermostat?

No need for the communicating stat if you go 2 stage. In a 2 stage application the only benefits are less wires needed, and diagnostics/more data available at the thermostat. Those benefits are mainly for the contractor, less so for the customer. Make sure though that if you go 2 stage that you get a 2 stage thermostat, not any old programmable.

>I looked at the product data sheets of the 59TN6 and the 59MN7. The former is a 2-stage variable speed furnace. The latter is actually a 3-speed variable speed furnace. Carrier advertises it a a modulating furance, but it does not have the variation that other manufacturers offer.

Actually the 59MN7 is fully modulating from 40-100% of capacity in 1% increments, when used with the Infinity control (which it always should be). I forget what it does when used without the Infinity control, it may very well become a 3 stage.

I found the modulating algorithm in the 59MN7 trouble shooting guide. You are correct in that the furnace can change the intermediate speed in 1% increments. The way it works is nn every heat cycle the intermediate speed is calculated based on the previous heat cycle. This is done by counting the number of minutes the furnace operated in the low and high stages. The calculation will determine an intermediate speed from 40-100%. However for a heating cycle there will be only one intermediate speed. Therefore on any given heat cycle the furnace can only operate in three stages. In some cases the furnace will only operate at the maximum stage if the calculation determine that is the way it should operate.

Thanks everyone for advice. I ended up going with modulating XC95 Trane furnance and XL950 thermostat with control link. Was tough decision between that and Infinity 98 with infinity thermostat. Prices were very comparable so ultimately went with dealer who seemed to be better. Getting installed today so fingers crossed.

>Posted by mike_home (My Page) on Sun, Oct 19, 14 at 23:52
I found the modulating algorithm in the 59MN7 trouble shooting guide. You are correct in that the furnace can change the intermediate speed in 1% increments. The way it works is nn every heat cycle the intermediate speed is calculated based on the previous heat cycle. This is done by counting the number of minutes the furnace operated in the low and high stages. The calculation will determine an intermediate speed from 40-100%. However for a heating cycle there will be only one intermediate speed. Therefore on any given heat cycle the furnace can only operate in three stages. In some cases the furnace will only operate at the maximum stage if the calculation determine that is the way it should operate.

This did not sound right to me, and I finally got a chance to read through the installation manual and check this. It confirmed that I was correct earlier. It will ONLY operate the way you say if it is used with a conventional, single stage thermostat. When it is used with the Infinity Control, it will match the load. This illustrates my point that it is CRUCIAL to use this furnace with the Infinity control. This is a true modulating furnace, unlike its predecessor the 58MVC/Infinity ICS.

I reread the description and agree with you. What I described is the algorithm the furnace uses when connected to a single stage thermostat which is the worst way to install it. Good catch on pointing this out.

I am still curious as to the maximum number of stages the controller will use in any given heating cycle.

1. Putting aside price, given the age and bad insulation in my house, would the modulating be the better option to get heat to cold places places like my 3rd floor? - Did the modulating furnace address this issue?