You may be misunderstanding the terminology. The term "variable speed" refers to the blower in the furnace. So, it is possible to have a single STAGE furnace, with variable speed blower. Your typical (older) furnace may have a multi-speed blower. where the installer 'selects' the required (single) speed through electrical taps. Further, in the above case, the speed of the blower is not variable (i.e., you don't have a choice of fan speed, while heating. As with any gas furnace, the fan speed must be selected, such that the air flow through the furnace will produce a temperature difference (known as temperature rise) of the return air to the outgoing air, within specified limits. What a VS blower WILL do is ramp up in speed if restrictions (clogging filter, poor duct system)prevent the selected airflow from flowing. It will (with proper thermostat) reduce airflow in cooling to increase dehumidification. A 2-stage furnace (also available with VS blower)and proper thermostatic control, provides greater comfort from having longer running cycles. NOTE: A 2-stage unit provides greater comfort, not greater efficiency. Carrier/Bryant produce a 3-stage furnace, which they refer to as 'modulating', but there are larger gaps in output between stages. A modulating furnace provides many stages of heat and utilize variable speed blowers. Again, the ultimate in comfort, but not more efficient than a single stage unit of the same AFUE. You should note that newer furnaces blow a lot more air than older units of the same size. Combine that with a VS blower, if the duct system is not sized for the airflow, the system will be noisy and efficiency may suffer. So, proper sizing and duct system analysis is essential. On a personal note, I replaced my oversized SS furnace with a Rheem Modulating unit (13 stages). Before changeout, the upstairs was always cold; the downstairs would warm, but would chill before the unit would restart. I had to use a space heater in an upstairs bedroom (computer). Now, I have relegated the heater to the basement, and frequently sit at my computer in short sleeve shirts. Insist on a manual J (heat calc) and duct analysis. V

You need to have a load calc done to determine sizing. There's no guarantee that a new 3 ton will be any better if sizing is a problem. Also, you should have your ductwork inspected for possible issues. Many issues are ductwork related. Have a load calc done and ductwork inspection by a COMPETENT salesman/contractor (the key here is competent--many times they will make up excuses or avoid load calcs, but they are necessary). You're looking at a great system (however a heat pump for the top floor and a 90%+ furnace for the first floor may be more advisable). I like the Honeywell touchscreen thermostats; I have one myself. I would not drop the variable-speed blower--that's the way the market is moving and it will provide comfort and efficiency. You can run the fan constantly to even out temperatures and constantly circulate the air with a low air speed of the variable-speed blower. They vary their speed to ensure you get the proper airflow, so they can compensate for SLIGHTLY deficient ductwork systems (again, keyword slightly--a poor ductwork system will make the blower work harder and use more electricity, hence the important of ductwork inspection, which is easily done in an attic). Hope this helps--kinda typing on a whim here.

Sorry if I confused- it would depend on whether the VFD control module is integral with the motor or not, depending on the manufacturer. And in fact one diagram I checked it was 12 wires. PJB, follow the line voltage wires feeding the unit. You will see the nuetrals (white) all on one block on the board and the power going through the door safety switch going to the board that powers the TX and blower via a relay on the board. I bet your blower leads are connected to HT, CL and one marked park? Park is the unused blower tap speed. A volt meter to nuetral or ground and one of these terminals will read 120VAC while operating. I'm not sure how Canada works but, in the US when we had Energy Star tax credits available, the furnace model must be listed on the official listing and install date within a certain time frame was required to get the VS blower credit. Merely converting, especially individually wouldnt count. FYI, there is one manufacturer, Nordyne that labels Maytag, Frigidaire, Westinghouse, Tappan, Kelvinator and a few others that had available and sold variable speed, complete slide in blower assemblies for field conversion. Nice concept for energy savings but did nothing for the heat inputs. My experience has been using both Trane and Carrier equipment energy comparison programs has shown that a VS blower operates at less than 25% of a standard comparably sized PSC blower motor. There was a time where every model A/H or furnace required a factory programmed and matched motor and module. What a headache for stocking parts distributors, which resulted in parts acquisition problems for techs in the past. That has gone away with simpler field programmable units. Below is a sequence of operation so, a proper ECM setup senses what the blower compartment is doing along with AC/Ht input/output. Merely driving the motor is not just the only thing that happens. VS units are capable of adjusting to airflow changes to maintain proper AC airflow and temp rise during heat. Here is a simple explanation on how the work from GE: The G.E. Electronically Commutated Motor (ECM) shown in figure 3 is different than previous generations of variable speed blower motors. This motor has all of the capabilities of previous ECM blower motors but it does not have to be pre-programmed at the factory. Instead the variable speed furnace control programs the ECM blower motor upon application of power to the ECM blower motor via the serial communication link between the variable speed furnace control and the ECM blower motor. This eliminates the need for a multitude of different ECM blower motors where each one is programmed for one specific furnace model. The ECM blower motor is energized with 115-vac whenever power is available at the variable speed furnace control, but operates only when the variable speed furnace control communicates the proper serial commands to the ECM blower motor at PL13. The ECM blower motor is first fed 115-vac power through the 5-pin connector PL14. The 115-vac power is then rectified to DC by a diode module. After rectification, the DC signal is electronically commutated and fed in sequential order to 3 stator windings. The frequency of commutation pulses determines motor speed. Setting up the desired airflow CFM for an ECM blower motor is done by setting the A/C or CF selections on the variable speed furnace control shown in figure 1. The ECM blower motor then delivers requested airflow CFM as defined by serial commands received from the variable speed furnace control. The ECM blower motor is configured via the variable speed furnace control to react to changes in system static pressures to maintain constant airflow CFM. The ECM blower motor delivers requested airflow CFM up to about 1.0 in.w.c. of static pressure for most airflow settings. The variable speed furnace control is pre-programmed and contains all the information relative to each furnace model for all modes of operation. Blower characteristics for each model (airflow CFM, torque, and speed versus static pressure) are known from laboratory testing. If any 3 characteristics are known, the fourth can be defined. The airflow CFM is known because of the A/C and CF selections, model plug, and the thermostat input signals on the variable speed furnace control. The variable speed furnace control then communicates the airflow CFM to the ECM blower motor. Torque is known by the ECM blower motor because it is directly related to armature current which is measured by the ECM blower motor control. Speed is measured from the generated back EMF by the ECM motor control. This information (airflow CFM, torque and speed) are entered into an expression which calculates torque from speed and airflow CFM numbers. If the calculation does not match stored blower characteristics, torque is adjusted every 0.8 seconds until agreement is reached. The ECM blower motor does not directly measure static pressure, but does react to changes in static pressure to maintain constant airflow CFM.

Nathan, I doubt you would hit even $500 gas bill where I am in Katy, Texas for the simple reason I am on electric heat and I don't come anywhere close to $500 electric bill. Last winter was colder than normal and I might have brushed $300 for the whole winter season. A furnace rated above 80% efficiency (for heating) for my area are simply not worth the expense, the money is better put toward a more energy efficient air conditioner. Air conditioning for my market (Katy, Texas) can be as much as 10 months of the year. It's more common to replace furnaces here in the summer than the winter, if you need some perspective. (IE: the AC can not cool without the furnace, but the furnace can heat without the AC.) What justifies sending 20 percent of your fuel out the exhaust? 20% of what? If you spend $300 for heat, that's a whopping $60 a season. x 20 = $1200 that's if you could find a furnace 100% efficient. But you won't do that... best case is 97% efficiency. So you will save $51 a season using the above logic. These furnaces come at a premium. You will never recoup your so called waste. 90% you save a whopping 10%. = again $300 for heat. You save $30. As pointed out by ionized there is the possibility of condensate water freezing. However, even this is not a problem in my area (It's almost always above freezing here). I have a long standing customer of mine with a condensing furnace. (Installed by builder of the home). There has never been an issue with condensate line freezing. (It has clogged up though, with slime) so there is yet another expense that you wouldn't have with a non-condensing 80% furnace. What you think you are going to save, depends heavily upon the climate in which you live. Low use appliance simply will not recoup what you think you are wasting.