Multi-stage, variable-speed (MS/VS) furnaces have grown in popularity over the years. For homeowners, the system is a quieter alternative that provides more even temperatures. For contractors, the system is a step-up installation with more profitability.

Tim Kensok, Honeywell product manager, recognizes the benefits of the MS/VS furnace, and says that these systems work well with some specific hvac accessories to create a more efficient home comfort package. These accessories include multi-zone temperature control, electronic air cleaners (EACs), ultraviolet (UV) air treatment, and a fan-powered whole-house humidifier.

Kensok asserts that recognizing and offering the benefits of these accessories with an MS/VS furnace can increase a contractor’s bottom line. “This can be successful in creating additional consumer pull for higher-end, higher-margin products, while at the same time giving homeowners the added features they are looking for in comfort.”

The appeal of multi-zone systems is that they offer homeowners the ability to better control the temperature and comfort of their home on a room-by-room basis.

Andrew Hulse, Honeywell zoning marketing manager, says that these systems are ideal for multi-level homes and homes that have hot spots and cold spots. The multi-zone system enables the consumer to have more than one thermostat to control the temperature conditions in separate rooms. For example, at night the heat in the bedrooms can be turned up without having to heat the rest of the home that is not being occupied.

This type of system, according to Kensok, works particularly well with MS/VS furnaces. On single stage furnaces, airflow cannot be adjusted and runs at only full force. With the MS/VS furnace and true multi-stage zone controls, the airflow will be reduced or increased based on actual demand, which can greatly simplify the installation of a zoning system in an application where the ducts may be oversized.

Cleaning The Air

Kensok says that the MS/VS furnace also works well with air-cleaning devices, specifically an EAC. Like the multi-zone control, the impact and efficiency of the EAC is greatly increased when used with an MS/VS system.

The EAC works by putting an electrical charge on particulates in the ventilation system. When these particulates are charged, they are drawn to an electrical plate and trapped. Kensok explains that the longer these particulates are in the system, the greater they will be charged. This in turn greatly increases the efficiency of capturing these airborne particulates.

This is why the EAC works so well with the MS/VS system. When the airflow is adjusted and slowed down, the particulates are in the system longer before having the chance to pass through. Kensok says that normal media air filters can capture only about 15% to 20% of airborne particulates in the 0.3-micron size range. The use of an EAC, such as Honeywell’s Enviracaire Elite Model 300 air cleaner, captures 70% of 0.3-micron size particles at a rated air velocity of 500 fpm, says Kensok. He adds that when airflow is reduced to 300 fpm through the use of an MS/VS furnace, the efficiency of the EAC jumps to 95%.

UV air treatment can also be used in the system to work with the EAC. Where the EAC collects particulates, the ultraviolet light can zap and kill living bacteria suspended in the air. With a more limited airflow, harmful particulates remain longer in the system, greatly increasing the chance that the UV light will kill them.


Finally, when choosing a whole-house humidifier for the MS/VS furnace, Kensok suggests using a fan-powered, flow-through model in favor of the bypass model. In either case, when the heat is pushed through the system, an evaporator pad provides the needed moisture to the airstream. But with an MS/VS furnace, the reduced amount of airflow can limit the amount of moisture actually evaporated.

That is why Kensok recommends the fan-powered model, which is not dependent on the amount of airflow through the system to provide the necessary moisture.

“There are many homeowners who simply don’t know that these products exist,” Hulse concluded. “It’s just a matter of awareness.”

Publication date: 11/26/2001