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ECM, variable-speed, multi-stage confusion

Whitney Larsen | Posted in Mechanicals on


I’m confused about the words ECM, variable speed, and multi-stage. If anyone can offer definitions, that would be very helpful.

If I buy a furnace that is labeled as having an ECM (according to the AHRI certificate), is it necessarily a variable speed motor? My understanding is no.

What about the reverse? Are all variable speed furnace motors ECM? Maybe…

The word “multi-stage” has nothing to do with the motor, correct? That has something to do with the number of burners on or amount of gas being burned, right?

I’ve framed all these questions in terms of furnaces, but do any of these answers change when talking about heat pumps?

Thanks in advance for your help!


  1. GBA Editor
    Martin Holladay | | #1

    I suggest that you read this article: All About Furnaces and Duct Systems.

    In that article, I wrote, "The simplest furnaces are single-stage furnaces with single-speed blowers. If the furnace is rated with an output of 60,000 Btuh, that is the furnace’s output whenever it is running.

    "More sophisticated two-stage furnaces can operate at two different output levels. Most of the time, these furnaces operate at a lower Btuh output; the higher output is only needed on the coldest days of the year.

    "Modulating gas furnaces are more sophisticated than two-stage furnaces. They include an automatic fuel valve that varies the amount of fuel delivered to the burner. Many modulating furnaces also include a variable-speed blower motor (usually an electronically commutated motor, or ECM) which (like the automatic fuel valve) ramps up and down in response to heating demand. Since modulating furnaces can match the heating demand precisely, they provide more even heat than single-speed furnaces which operate with a stop-and-go jerkiness."

    All ECMs are variable-speed motors. However, not all variable-speed motors are ECMs. ECM blowers include a control circuit that adjusts the blower speed to compensate for increased static pressure (which can occur, for example, when dust builds up on the filter).

  2. Whitney Larsen | | #2

    Thanks Martin! Very helpful.

  3. Charlie Sullivan | | #3

    From a practical point of view, I think that Martin has it nailed. From a more general engineering point of view, it's not quite true that all ECMs are variable speed. Inherently the technology has that capability, but not all ECMs take advantage of it. When it comes to commercially available HVAC equipment, I don't know of any equipment with ECMs that doesn't either automatically vary the speed or have some provision for the user or installer to vary the speed. So from a practical point of view, within that sphere, I think it's safe to say that all ECMs are variable speed. But if you are curious about the fundamentals of the technology, and what's possible beyond what's available, read on for the exceptions to that rule.

    Fundamentally, all motors are variable speed. It's just that they need some control electronics to vary the speed. An ECM is even worse than that. It can't run at all, even at constant speed, without its electronics. If you are using an ECM, you've already paid for the electronics to run it, so you might as well use the same power electronics hardware to control the speed as well. But you can also build the control electronics without that capability, or build it with the capability there, but leave it unused.

    An example of where constant speed ECMs are used sometimes is cooling electronics. Most computers now automatically vary the fan speed according to the cooling requirement, but many less sophisticated systems, as well as older computers, use constant fan speed even though they use ECMs, usually powered from 12 VDC.

    So why use an ECM if you aren't going to take advantage of its variable speed capability? Because ECMs are typically more efficient than typical induction motors for the same size. The difference gets more dramatic as you go to smaller sizes. But then again, if you have already paid for the electronics, why not take advantage of the variable speed capability.

    In a heat pump, there's a compressor motor as well as a fan motor. Some have variable speed fan motors but not variable speed compressor motors. Having the compressor variable speed too can give you gains in COP at partial load, and can help avoid cycling which hurts COP further.

  4. Whitney Larsen | | #4

    Thanks Charlie and Martin. From a program administration perspective, it's easy to make clear requirements for ECMs (since its a field on the AHRI database), but more difficult to account for other characteristics like variable speed or multi-stage/modulating systems. Knowing that practically, most/all ECMs are variable speed simplifies the situation.

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