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I'm in the market for a new heating and air conditioning unit for my home. Most offer variable speed blowers and the more expensive models are equipped with BLDC motors, which seem overkill. I am having a difficult time seeing the advantages to variable speed blowers other than perhaps to reduce noise.

I figure the goal is to heat up the interior as fast as possible by heating the coils as hot as possible and forcing as much air as possible over them. Of course, you don't want to turn your exhausts into hot air guns. Am I missing something here?

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  • $\begingroup$ This is very complicated. Why the mfg provides this, what the mfg thinks you can do with it, and what you can really do with it might be very different things depending on the details of the installation. The control logic of the blower can be based on comms with the outdoor unit, or sensors, or just be baked into the feedback of rpm vs amp draw. It may have humidity sensing, or not. It might try to meet duty cycle targets, or not. It may just be to game a part of the efficiency rating test for whatever rating agency controls the mfg. $\endgroup$
    – Phil Sweet
    Commented Sep 13, 2023 at 17:18
  • $\begingroup$ Add to that thermostats that think they know how to control the blower - but don't; and people that think they know how to control the thermostats - but don't, and you can have a fine mess on your hands. $\endgroup$
    – Phil Sweet
    Commented Sep 13, 2023 at 17:19

2 Answers 2

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Variable blowers are typically combined with variable speed air conditioning/heat pump compressors. The compressor is the major energy draw. Energy usage is proportional to the cube of the pump speed, but cooling is proportional to the speed (mass of refrigerant moved). A non-variable system must be sized for max load, and runs that way for a shorter time. So we reduce the refrigerant flow when we don't need full heating or cooling to save money by running slower for longer. There are also comfort improvements in the summer via better dehumidification running the system longer. We save a little on the blower fan speed too, but it's mostly the compressor. The pump/fan laws that govern this are known as the pump affinity laws.

The goal is to heat/cool the interior to the setpoint and keep it there.

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    $\begingroup$ So it's a closed loop PID control system with the thermostat and hygrometer as continuous inputs? $\endgroup$
    – user148298
    Commented Sep 14, 2023 at 19:29
  • $\begingroup$ @user148298 that's it $\endgroup$
    – Tiger Guy
    Commented Sep 14, 2023 at 21:16
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ordinary blower fans are AC induction motors with a separate winding for each fixed speed (low, medium, high). they are very quiet, cheap to build, efficient, and are well-established in practice.

However, it is possible to do the trick with a single set of windings if you have a variable frequency AC power source, for example, or with a BLDC-type motor. By building the motor with a single set of windings, the motor can be made smaller and lighter (which means less expensive). Then you trade off the cost savings for a lighter, cheaper motor against the cost increase of the VFAC drive board or the BLDC drive board.

Note that the air molecules themselves don't care what kind of motor is used to power the fan.

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