Tag: Variable Speed

First, let’s cover the basics. X13 is a brand name for the Regal Beloit / Genteq brand of constant torque motors, there are other manufacturers who make them, but the term “X13” has become pretty much synonymous for the fractional horsepower HVAC constant torque motor.

Also, this article is specifically discussing the common residential/light commercial motors. There are other types of variable and constant torque motors and equipment not being addressed here.

Both variable speed and X13 motors are ECM or “Electronically Commutated Motors,” This means the DC power that drives them is electronically switched from positive to negative to spin the motor. Both are more efficient than the typical PSC motor with ECM motors commonly being about 80% efficient and PSC being about 60%.

Both X13 and variable speed motors are DC (or alternating DC if you prefer), 3 phase, permanent magnet rotor motors that use back EMF to determine motor torque and adjust to load conditions.

The primary difference is the type of inputs to the motor control. A variable speed motor is programmed for a specific piece of equipment to produce a set amount of airflow based on the particular static pressure profile of that system as well as based on the inputs from the air handler circuit board or system controller. In other words, a variable speed motor can ramp up down based on the static pressure as well as the staging of the equipment, pin/dip switch or controller settings for desired airflow output and “comfort profiles” that can be set up to allow the blower to ramp up or down for enhanced dehumidification and comfort.

An X13 motor is programmed to produce a set motor torque based on which input it is receiving 24v. This means that while an X13 motor is more efficient than a PSC motor and does a better job of ramping up to overcome static pressure increase it does not have the level of control that a variable speed has and it also does not produce an exact airflow output across the full range of static pressure.

This is why when you check the blower charts on a unit with a variable speed motor the CFM will remain the same over a wide range of static points, but when you look at an X13 system, the CFM will drop as the static pressure increases.

— Bryan

First, let’s give proper credit. Most of the best practices and tools for the diagnosis of ECM and X13 motors come from Regal / Genteq and their site thedealertoolbox.com and their app the dealer toolbelt. Your best bet is to follow the practices shown there and use their TECInspect diagnostic tool shown below.

Here is the general process to follow when checking an ECM and X13 motor that isn’t running. Most of it is very common practices you would follow with any motor.

  1. Check for proper line voltage and 24v calls to the proper terminals in the equipment
  2. Check for proper control signal entering the motor from the 24v field wiring or boards. The fan speed selections and programming will vary by manufacturer but the intent is to see if there is a proper control input signal. This can be a bit a challenging and is the primary purpose of the TECInspect tool.
  3. Disconnect power and remove the blower housing
  4. Check for abnormal sounds and side to side bearing play. Because these motors have permanent magnets on the rotor they won’t spin freely like a normal motor and you will get an “indexing” feel on the shaft as you turn it.
  5. Look for signs of overheating, burned spots etc…
  6. Remove the module from the motor and disconnect the plug that connects the motor to the module. 
  7. Measure winding to winding on the plug feeding the motor (called phase to phase below). resistances should be less than 20 ohms and nearly the same between all phases/windings.
  8. Measure from each winding to ground on the casing and you should see no less than 100k ohms to ground.
  9. If the motor checks out OK and the module is receiving inputs but the motor still isn’t running then it is the module that needs to be replaced

The diagnosis of ECM and X13 motors is actually pretty easy if you do a bit of reading and take a practical “process of elimination” approach.

— Bryan


Dehumidification features are common on residential systems ever since the introduction of variable speed blower motors. The system is set up so that the blower can produce less CFM per ton when latent (humidity) load in the space is higher than the setpoint relative humidity. Slowing the blower increases moisture removal by reducing the sensible load on the evaporator coil and therefore dropping the coil temperature and surface dewpoint.

Most variable speed fan coils and furnaces have a terminal designated for Dehumidification and it can be called D, dehum, DH or something else depending on the manufacturer.  In all cases I am aware of, this dehumidification terminal must be energized for the blower to go to full speed and when that terminal is de-energized the blower speed (usually) drops to 80% of full speed.

For years we have seen thermostats with designated dehumidification terminals to match up with the fan coil/furnace terminal, so it was just a matter of disconnecting a jumper from the dehumidification terminal to the R terminal in the unit and connecting a wire from the designated thermostat terminal to that dehumidification terminal in the unit. The diagram below is an example of this on an old Carrier Thermidistat with a variable speed Carrier fan coil.

We now have 24v control smart thermostats like Ecobee, Cor, Nest and Lyric with a lot more flexibility in how they can be set up rather than having a single, designated dehumidification terminal.

I am a big fan of EcoBee for many reasons including their Alexa integration, remote wireless sensors and application flexibility… but you need to be really careful with how you set them up, ESPECIALLY when setting up dehumidification.

The image above is a GIF and should show you the first part of the dehumidification setup. I am setting it up for a single speed compressor heat pump with a variable speed fan coil. EcoBee has contacts labeled acc+ and acc- that can be set up to do a wide variety of functions. For this typical dehumidification function using the system you would select Menu>Installation Settings>Dehumidifer >1 Wire ACC+>Open contact state to activate dehumidifier.

This setup uses 24V power from the R terminal to energize the acc+ terminal and therefore the dehumidification terminal in the fan coil/furnace when there is NO call for dehumidification.

Now for a controversial part. Go to the equipment menu and select Dehumidifier to “dehumidify with fan”= no. We have seen several occasions where the blower continues running with no cooling call if this setting is set to yes when there is a dehumidification demand and no cooling demand.. According to the EcoBee website HERE it appears to say the opposite, but we have confirmed on a few occasions that this occurs and there appears to be no adverse effects from setting it to off becasue the blower is still controlled by the thermostat for cooling operation and dehumidification without cooling is not possible without an external dehumidifier.

In order for the system to over cool below the temperature setpoint to dehumidify you need to go into the thresholds menu and set up AC over cool Max to the maximum temperature below setpoint that would be allowed during dehumidification by the equipment.

— Bryan



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