I have a multizone hydronic heating system that has a pump for each zone.

The Pumps are Taco 0015 three speed pumps like this one that I am to understand are permanent split capacitor motors.

The thermostat I am testing is an Aube TH401 like this one that uses a triac for switching.

(I have been using old style bi-metallic thermostats to hold the temperature in each zone but they are very inaccurate and allow large temperature swings.)

So, the new Aube TH401 is a little more expensive but it holds temperature better. It operates on 5 minute cycles on "Fan" mode for a baseboard heater with an integral fan, and 15 second cycles when it is set for straight resistive heat. I have it set on the "Fan" mode since the resistive mode cycles are too short to actually move any water in the zone.

The thermostat has one to four little heat waves on the display to indicate "heat intensity". That is what the manual says. I am assuming it actually controls the amount of electricity a baseboard heater would receive by controlling the duty cycle of the triac.

And now my question: If the duty cycle on the triac is being controlled to hold a tight temperature instead of just turning it full on and full off, can operating a PSC motor on a thermostat like this damage the motor?

EDIT: In the last half of this video the gentlemen shows what he believes is a Triac based fan speed controller. If ceiling fans use PSC motors and Triac speed controllers then it would seem that a Triac based thermostat and PSC pump would be able to work together. I have set the three speed selector to the highest setting so the minimum speed from the thermostat is less likely to be below the pumps starting torque/speed. So far, no black smoke from the motor so the experiment continues.

  • \$\begingroup\$ Regarding duty cycle of the triac see my answer to electronics.stackexchange.com/questions/232921/…. \$\endgroup\$
    – Transistor
    Jan 1, 2018 at 20:50
  • \$\begingroup\$ @Transistor Yes my use of the term "duty cycle" may be a misnomer. Rather than indicate the amount of full on vs full off time, I meant to indicate the percentage of time that the sine wave that is turned on. Should I use a different term for that? \$\endgroup\$
    – ArchonOSX
    Jan 2, 2018 at 10:17

1 Answer 1


It is difficult to confidently determine how the Aube TH401 works. The 15 second and 5 minute cycle mode selection terminology seem to indicate that both modes use zero crossing or integral cycle control as described in Transistor's answer to this question. That seems the most likely case, but the "heating intensity" indicator confuses things a bit. It looks to me like the "Std" setting results in a 15 second minimum on and off times in response to the thermostat. In addition there are 5, 10, 15, 20, 25 and 30 minute settings. That provide longer on and off times. The manual does not have a very good explanation.

The product information indicated the device is not rated for inductive loads. That seems to contradict the information about a "fan" setting. The device may be ok for an electric heater with a small fan. A load like that would be predominantly resistive. The product indicates it is not for driving a relay. There is also a minimum current that is higher than the current required for most relays.

The bottom line is that the device will probably not control the speed of a PSC pump. It does not even seem to be recommended for turning a PSC pump on and off. If the device is ok turning a pump on and off, the pump motor would probably be ok with 5 minute or longer cycles, but it would probably last longer if the cycles were a little longer than 5 minutes. If the device would actually modulate the speed by phase angle control, that would probably be ok (but no guarantee) if the lowest speed was not slower than the lowest switch speed. The speed switch would be set to the highest speed when using a phase angle control unit.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.