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

Multi-stage (or modulating) furnaces and three and two-stage furnaces are relatively recent alternatives to the more common single-stage furnaces. They operate very quietly and can reduce temperature swings and improve home comfort. But they are also more expensive and only marginally more energy-efficient.

Two, three and multi-stage furnaces have several levels of burner function and are often associated to modern variable speed fan motors, a way of getting high electrical savings. They run on natural gas or propane.

The technology

While single furnaces operate like a car in a stop-and-go line, new stage furnaces make slight adjustments without being always cycling on and off.

These features are largely based on special valves and on the variable speed of the furnace's indoor air blower motor. In the case of the multi-stage furnaces the valves operate in very small increments, according to the thermostat setting, regulating the amount of fuel and the temperature.

Multi-stage furnaces are premium furnaces. Their efficiency (AFUE) is typically above 94%, and their technology extracts heat from the exhaust gases by reusing them (in other words, they are of condensing type).

Two-stage or three-stage units have large similarities with the more advanced modulating/multi-stage furnaces. As their name suggests, most three-stage furnaces operate at 40%, 65% and 100% of total capacity, while most two-stage furnaces operate at 60% and 100%.

Below, we explain these differences in more detail.

Single vs. Two stage furnaces

The two-stage furnaces are "smarter" than single stage furnaces. They haven't just a simple On/Off operation; they have a On/High (100%) and a On/Low (60%) stages (and a Off stage).

The On/Low stage is a big advantage in situations like mild weather conditions, where the furnace can adapt the amount of heat produced to the situation: contrary to the single stage units, the two-stage furnace only switches to the 100% heat output when needed (very cold outdoors...).

Prefer a two-stage furnace with a variable fan speed motor (with continuous fan option). It's motor is much more efficient, and can provide large electricity savings and a much quieter operation.

Two vs. Three-stage furnaces

Three stage furnaces are more flexible than two-stage furnaces (and single stage furnaces). Their valves can stage (typically) at 40%, 60% and 100% of their capacity, increasing it to respond to the heating requirements. Most three stage furnaces have variable fan speed motors with continuous fan option and are particularly quiet.

Two and three vs. Multi-stage (modulating) furnaces

Multi-stage gas furnaces are the most energy efficient furnaces (rated up to 98% efficiency) and provide extreme comfort. But they are also those with the highest cost.

They share the versatility of three and two-stage furnaces, only in a higher degree. They operate in a very incremental way. Multi-stage furnaces may begin at the highest blower level possible when becoming to heat the house, but once enough heat is built up they will only push small amounts of heat into the house (in a very frequent basis).

Advantages of Multi-Stage (and 3 and 2 stage furnaces) over single stage Units

Multi-stage furnaces (including the two and three stage furnaces) do not necessarily offer more energy-efficiency than single stage furnaces. They tend to be slightly more efficiently, but not significantly.

What they do offer more is flexibility and comfort (less swings of temperature), electricity savings, and a faster response to the heating requirements..

Pay special attention to the circulating fans of the new furnaces. As mentioned above, the best furnaces work with a variable fan speed motor (with continuous fan option). These motors will 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 ventilation air around the building.

These fans can now use less than 100 W of electricity on low speed, allowing considerable electrical savings.

Disadvantages

Two and three stage furnaces are pricey - that's their big disadvantage. Installation can also be more expensive, mainly due to higher requirements or modifications on the venting system.

Payback

Like other high-efficiency furnaces, multi- and two and three-stage furnaces make little sense for small needs. The modulating/multi-stage furnaces are premium quality furnaces that can cost you $2,000 or $3,000 more than mid-efficiency furnaces, an amount difficult to recover if you live in a moderate climate or if you have small heating needs.

Anyway, for small heating needs, a central heating system (including those based on single-stage furnaces) is also difficult to justify - namely if your home is properly insulated, sealed or if it uses passive solar heating strategies. See: Alternatives to furnaces