HOW IT WORKS

The variable-speed draft inducer is an ingenious device currently offered on Trane, American Standard, Carrier, and Bryant high-end furnaces. It’s offered on Trane XL90 two-stage and two-stage variable-speed gas furnaces; American Standard “Freedom 90” two-stage and “Freedom 90 Comfort-R” two-stage variable-speed gas furnaces; Carrier “WeatherMaker Infinity” (Model 58MVP) gas furnaces; and Bryant “Plus 90i” (Model 355MAV) gas furnaces.

Carrier introduced variable-speed draft inducers on its furnaces in 1987. The way it works, according to Melissa Sodo, senior product manager for Heating Products, Carrier Corp., Indianapolis, IN, is through the control board and the variable-speed draft inducer ex-changing signals. “Once the pressure switch is closed, the desired rpm for the inducer is locked in. This ensures that the proper ‘air/fuel’ mixture is achieved and that the variable-speed inducer will only run as fast as needed to close the pressure switches.”

Sodo notes that the variable-speed inducer is very adaptable, and the settings can change and adapt with every call for heat. Once the setting is established, the inducer rpm is constantly monitored to ensure proper operation and the proper air/fuel mixture for the furnace heat exchangers. If the rpm goes too far outside of the calculated value, then it will give a diagnostic code.

The furnace control board has parameters for low-stage, high-stage, and continuous monitoring, which once again guarantees the proper air/fuel mixture.

Tim Storm, furnace product leader for Trane and American Standard, Tyler, TX, notes that in the company’s high-end furnaces, the microprocessor-based variable-speed draft inducer also includes an adaptive learning routine, which allows the draft inducer motor to learn the optimum speed and efficiency for both low- and high-fire operation.

“When the furnace is first started after installation, the learning routine will slowly reduce the speed of the draft inducer while it learns the optimum combustion efficiency and operating speed for this furnace under the current installed conditions, which includes vent pipe size, vent length, line voltage, gas pressure, and climatic conditions,” said Storm.

The learning routine for high-fire operation takes place in a similar sequence, noted Storm. The learning routine is repeated periodically to ensure the draft inducer is operating at the lowest and quietest possible speed.

The variable-speed draft inducer continuously monitors pressure switch contacts during normal heating operation. If the contacts are ever sensed to be open, the draft inducer increases in speed, then checks the contacts again.

If the pressure switch contacts are still in the open position, the draft inducer will increase speed again until the contacts close or the maximum allowed time for proof of pressure switch closure expires. If the pressure switch contacts cannot be closed, the draft inducer sends a pressure switch error to the integrated furnace control. A pressure switch diagnostic fault code will be displayed on the furnace control.

“Operation of the furnace will not be allowed until the fault is cleared,” said Storm. “If the high-fire pressure switch cannot be closed, the furnace control will revert back to low-fire operation rather than shutting down, in an effort to provide heating until the fault can be corrected.”

For homeowners, the variable-speed draft inducer means a more comfortable, quieter environment. “Our furnaces with variable-speed draft inducers provide contractors with an opportunity to delight their customers with a unit that provides more uniform comfort, is quieter, more energy-efficient, and costs less to operate,” said Sodo.

Carrier’s 58MVP and Bryant’s 355MAV furnaces are extremely quiet and efficient. To give you an idea of just how quiet they are, it would take 24 variable-speed 90%-efficient furnaces operating at the same time to equal the sound of one standard 80%-efficient furnace, the company said. In fact, Sodo noted that the company has had customers who call to report that their furnace isn’t working, because they can’t hear it operating.

MORE SALES FOR CONTRACTORS

Contractors like Scott Boyer, president of Boyer Refrigeration, Altoona, PA, say that they like the variable-speed draft inducer, because it has helped increase sales through word-of-mouth referrals. He said that homeowners appreciate how quiet the furnaces are, both inside and outside.

“You can’t even hear the thing run. You can’t even hear it when you’re down at the furnace. You almost have to touch it to know it’s running,” said Boyer.

Something else Boyer likes is that the variable-speed draft inducer allows 200 feet of vent length and the capability to terminate the intake and exhaust vent in different pressure zones. For example, the intake can be installed on the side of the home while the exhaust exists through the roof.

“This allows us a lot of installation options,” said Boyer. “Older homes often were not constructed with the furnace sitting right beside an exterior wall, where we can just punch the flue outside. This allows us to terminate the flue at the homeowner’s desired location.”

Nothing special needs to be done with the variable-speed draft inducer during installation, noted Sodo. “The contractor should follow the installation instructions to properly size the vent system, just as he would for any other furnace.”

As for maintenance, the variable-speed draft inducer should be visually checked each heating season for any signs of lint or dust that may accumulate around the cooling fan on the electronic control board. Any accumulation can be removed with a vacuum or soft brush, said Storm.

The variable-speed draft inducer has upper and lower rpm speed limits for both low- and high-fire operation. To determine if the draft inducer is working correctly, Storm said service technicians can connect to the furnace control board, which has a test point that allows them to calculate the current draft inducer rpm. “This will allow them to determine if the furnace is operating within the factory-specified speed range.”

So the next time you’re trying to sell a customer a high-end Trane, American Standard, Carrier, or Bryant furnace, don’t forget to talk about the benefits of the variable-speed draft inducer — it just may induce your customers to buy.

Publication date: 11/25/2002