Two, three and multi-stage vs. single stage furnaces

Old single-stage gas furnaces are being replaced by a newer type of gas furnaces: multi-stage furnaces, with air sealed combustion and variable speed motors. These new furnaces offer versatility and can increase comfort and energy savings. They operate very quietly and can reduce temperature swings and improve home comfort and energy-efficiency. The problem with them is that... they are a lot more expensive.

Two, three and multi-stage furnaces have several levels of burner function and modern variable speed fan motors, allowing high electricity savings. They run on natural gas or propane. Condensing furnaces
Most modern gas and oil furnaces are of condensing type, that is, they can recapture and reuse the water vapor and the combustion gases, increasing their efficiency to levels above 90%* (98% in the case of some new premium gas furnaces...)

The technology, how they work

While single furnaces operate like a car in a stop-and-go line, stage furnaces make slight adjustments. They have special valves and a variable speed air blower motor that allows them to operate in very small increments.

Multi-stage furnaces may begin at the highest blower level possible, but once enough heat is built up they will only push small amounts of heat.

Multi-stage furnaces are premium furnaces. Their efficiency (AFUE) is typically above 94%, and they are able to extract heat from the exhaust gases, by reusing them (they are of condensing type).

The really important, and what makes multi-stage furnaces special, is their flexibility. Single stage units have only a On/Full Blast operation mode; multi stage-furnaces can deliver different amounts of heat.
And they can be used to provide the optimum amount of hot air into each room. We can use them with programmable thermostats and ducts with dampers to have different temperatures in different rooms, responding to different needs, which is great for energy savings.

Two stage furnaces

Two-stage or three-stage units have some similarities with the more advanced modulating/multi-stage furnaces.

But they do not work in small increments. As their name suggests, they have intermediate stages: they can operate, say, at 40%, 65% and 100% of their full capacity.

Having a On/Low stage, for instance, can be advantageous in relatively mild weather conditions. Some two-stage furnace have a variable and efficient fan speed motor (with continuous fan option), which is important for electricity savings.

Do Multi-Stage Furnaces deserve their Higher Prices?

Energy performance is important, and you should choose a 90%+ AFUE (Annual Fuel Utilization Efficiency) furnace for comfort and performance. A 90% AFUE furnace will lose 10% of heat during the combustion process, while a 95% AFUE furnace will lose only 5%. See our own videos on Furnaces here:
Efficient Furnaces
Buying Efficient Furnaces

Anyway, do not overvalue the AFUE of the furnace: a small difference in the AFUE may not be relevant. The efficiency of a furnace system depends also on installation issues, ducts and venting. These last features can be more important than the differences in the energy coefficient.

Top furnaces – contrary to 80%+ and other older furnaces - have sealed combustion (important for safety and health reasons) and, as mentioned before, are able to provide more comfort, less drafts, less noise and a quicker heating, responding to thermostat settings. And that’s very important for zoning (heating the rooms at different temperatures) and for energy savings.  

Pay attention to circulating fans. The best furnace models work with a variable fan speed motor (with continuous fan option). These motors consume very low amounts of electricity, which is a significant advantage over traditional motors, namely for homes using extensively the furnace circulating blower to move air around the building. These fans can now use less than 100 W of electricity on low speed, allowing considerable electricity savings.

Advantages, Disadvantages and Payback

Multi-stage furnaces will cost you, say, $2,000 or $3,000 more other low and mid-efficiency furnaces.

But since variable stage-furnaces can deliver the right volume of heat into each room, quickly and smoothly, they can save large amounts of energy, namely when used in conjunction with different thermostats and special furnace dampers, to distribute the heat.

Zoning (room-by-room heating) is very important, and provide high energy savings if properly used and designed. Why should you have to heat rooms that aren't being used?

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