Fancy Furnace with two stage gas system and ECM motor

We are servicing many “fancy furnaces” with 2 stage gas system and/or ECM motors and we are finding many installed by competitors that are failing to utilize and set up the furnace features, in particular the dip switches to advantage of all these features.
An ECM (electronic commutated motor) is a brushless DC motor with a permanent magnet rotor and a built in inverter.
DC motors are more efficient than AC motors.

Lennox fancy furnace
Lennox fancy furnace

The ability to move air rapidly and efficiently in HVAC systems is essential to achieving a cost-efficient comfort system. The electrical motors that were traditionally used in HVAC applications were AC induction style motors that were either single-or multi-speed — they’d run at one speed for heating, another for cooling, and sometimes a third speed for two-stage systems.

The problem with these types of motors is that they use a lot of energy because they run at full amp draw at all times. Also, since the motors run on high speed all the time, the noise levels are also high.

To address these two concerns — efficiency and noise — the electronically commutated motor (ECM) was developed. According to GE, an innovator in ECM technology, the wide range of the ECM motor, high efficiency, and programmability gives it a virtually unlimited range of performance characteristics.

ECMs (Electronically Commutated Motors) manufactured today are the best they’ve ever been, and can provide unprecedented airflow delivery to match the required BTUH of a heating and/or cooling system.

There are, however, some serious downsides associated with improper application of these motors in new and existing HVAC systems.

The Good:
When a furnace or air handler with an ECM is installed on a good duct system, coil, and filtration system with static pressures within proper design parameters, it can perform flawlessly. These systems can deliver precise airflow to match two-speed condenser designs and multi-stage gas furnace firing rates to deliver quiet, draft-free comfort in virtually any configuration.

These issues can be avoided with good pressure and airflow testing, both before and after equipment is replaced, and on every system you service. When you measure, you know. When you don’t measure, you’re just guessing that it will turn out OK.

Additional benefits of these brushless DC motors include low electrical consumption when operating within design TESP (Total External Static Pressure). When installed properly, systems with ECMs can improve dehumidification, reduce register noise, save energy, and provide trouble-free operation.

The Bad:
In marginal systems with higher-than-design static pressures — for example a .5–in. TESP rated furnace with ECM on a .8–in. TESP system — the motor still may be able to maintain airflow close to design. The tradeoff is higher velocities often cause a noisy system and some coil blow-off. The extra torque often required to overcome the higher statics results in higher electrical consumption. This reduces actual equipment EER, which means your customer won’t get the savings of the higher efficiency matchup.

High velocities and pressures created by ECMs on systems with dirty evaporator coils can also blow moisture off the coil surfaces into the supply ductwork. This can reduce the system’s dehumidification capability and cause numerous moisture-related indoor air quality issues.

The Answer:
These issues can be avoided with good pressure and airflow testing, both before and after equipment is replaced, and on every system you service. When you measure, you know. When you don’t measure, you’re just guessing that it will turn out OK.

Of course, there is more to this in terms of troubleshooting and solving these issues. The bottom line is that while the ECM is a great motor, as the servicing or installing contractor it’s your responsibility to make sure its application does not cause unintended consequences detrimental to the health and well-being of your customers’ homes and HVAC systems.