HVAC Electric Motors Matter to your Comfort and your Wallet. Here is why.

Motors in HVAC components have a direct bearing on your comfort and on how much you pay for heating and cooling. Therefore, it is essential that you know a little something about these types of motors. We will concentrate in this discussion more on fan motors for air handlers, but the same themes also play out for compressor motors and motors associated with modulating furnaces.

Let’s make sure we know why we should care

The choice of motor driven components relates to how comfortable you are in your home, the quality of your indoor air, and the ability of your HVAC system to serve you with the nuanced services you need to be happy, comfortable and frugal… or frugalesque. Motor choice can save you money.

Heating and Cooling Components encompassing ECM motors can cost 40% more – more or less and components are always a major investment. And salespeople will manipulate you and your aversion to spending a penny you don’t need to spend, and maybe manipulate you into a less beneficial solution.

So you need to know the fundamentals here. The point being made is that the extra upfront cost of components with ECM motors are a good investment in saving money and being more comfortable.

There is an important aside here: ECM and PSC motors  react differently to static pressure problems n your heating and cooling system. Problems with static pressure, air movement constraints-  are a fundamental issue that we deal with separately. here. LINK

There are two critical factors that make electronic commutated motors a much better investment than the traditional PSC motor even though the PSC motors are less expensive up front.

ECM motors save you money, but let’s talk about the comfort advantages of variable speeds, first.

First, ECM motors can vary their RPM by controlling the pulse waves of current. This allows important variability in how much conditioned air or just circulating air is being driven through the system.
Traditionally, PSC motors were single speed. This is what most heating and air conditioning systems have if you have not upgraded in 10 years, or you are proud of your 20 year old furnace. The blower comes on full blast until the thermostat senses enough temperature and the fan cuts out as soon as the heating or cooling cycling ends. You may notice a pattern of being quite toasty and then feeling a draft when the heated air quickly dies off. These are the ups and downs of temperature as created by a one speed PSC motor.
Check out the image below.

Caveat on Saving Money with ECM Motors

ECM motors, and we will explain the electrically modulated motor in detail separately– ECM motors like every other component in the system, depend on all other components to be working effectively. Ignored problems in air distribution, poor filter selection or clogged filters, kinked flex duct, problems with cold air return patterns can actually cause your electric bill with new ECM motors to go up, because ECM motors are programed to move a certain volume of air and will strain themselves using extra power, to move the air quantities the system calls for.

Static pressure properly balanced throughout the system is essential to saving energy and being comfortable. We will go into more detail later, but PSC analog motors don’t use any more power because they just move less and less air as the system static pressure increases. Static Pressures across your air distribution systems are critical. LINK Again, we will explain permanent split capacitor motors separately, but the fundamental caveat needed to be shared up front. More on the costly travesties of defects in duct systems here. End of caveat.

To compete in the market with competitive pricing, PSC motors are often connected up in multi-speed variations using circuitry to change the analog dynamics of the permanent split capacitor motor. Multi-speed here means a single speed for various types of conditions; one speed for heating, another speed for cooling; a special speed for just air circulation. These are set speed settings but when viewed at a heat and cooling system, these are thought of as multi-speed units.  We showed a multi-speed PSC schematic earlier.

But ECM motors allow much more flexibility and comfort to your home, and this really helps with our fickle North Texas weather that can have us heating in the morning and cooling in the afternoon. And you don’t really need a lot of heating as the day warms and you don’t really need a lot of cooling as the day cools, just enough, not a full blast boost of heat or cooling. ECM fan motors can make a real comfort improvement for North Texans.

Let’s talk about how these variable speed advantages can impact your comfort.

With true variable speeds, your system can dial in the amount of cooling and heating to modulate your home environment to the needs you are experiencing. While a single stage system is pushing up the temperature to maybe you are too warm and then cuts off until the temp cools to the set point and by then you might be chilled. A true variable speed system can deliver just enough heating or cooling to maintain an even and comfortable temperature in your home. Now what is that worth in comfort?

temp changes over time illustrated

No one deserves to be tortured by full blast heating with wide temperature fluctuations.

There is also a noise advantage.

The ability of ECMs to ramp up and ramp down slowly adds to their quiet operation. And of course, all this smooth ramping of speeds is much easier on switches, even a “small” 200% current surge from a PSC motor to ramp up speed is a jolt to the system. Anybody have an ratcheting breakers when the A/C comes on? It happens. A lot. Do you hear your heating or cooling system come on? Does it annoy you? Does it wake you up at night? Here is a solution to discover, next time you are making upgrade decisions. LINK

We have issues with humidity in North Texas. The size of your issue with humidity is a direct function of how much air leaks into your home and how well you home handles the moisture the human inside generate. This is an important but separate issue discussed here. You do realize that the humidity of the air in your home has a huge impact on air quality?

In our seasonably humid climate here in North Texas, ECM motors can be programmed to run slower and cool slower, when appropriate, to better handle the latent heat generated by condensing moisture in your evaporator coil, as compared to rapid cooling and the system cuts off and leaves you feeling muggy because the cool temperatures have been achieved but too much moisture has been left in the air. Too many people have too big an A/C because most people think more comfort is a function of more cooling power. This is not true and, and too big can work against you. We talk about too big here.

Using ECM motors and the right controlling systems, humidity levels can be set at a precise range and the system with dehumidify as needed to keep you in a safe and comfortable state. We always need to control for moisture at the sources as described in the above links, but precise control of humidity can really keep you comfortable by making sure the conditions for comfort are in your home, before your body and brain start whispering to you that you are not comfortable.

Is proper air filtration important to you? How often is your filtration system working? When the heating or the air conditioning is occurring? Or are we running a system full blast in air circulation mode and running up the electrical bill?

Now many of us like clean indoor air and use filtration systems to help clean the air. We discuss Filtration here in depth. LINK but if your system is not moving air, there is no filtration going on. Variable speeds allow circulation stages that allow constant filtration to a level needed, and saves you money.

Analog one speed systems set on air circulation, are loud and expensive to keep on at all hours, and without considerable system control you are manually selected whether you are heating, or cooling or circulating air. These particular aggravations are determined by your own system you live with. These are what you consider when you go through the process of evaluating new investments in heating and cooling components. We discuss the wide spectrum of issues to consider in a major upgrade here. LINK

Let’s pause and look at another critical difference in PSC and ECM motors. They each respond differently to changes in the static pressure in the system.

Static pressure is the basic resistance of the air passages in any particular air distribution system. The basic static pressure system is set up by the installation contractor. So many tons of conditioned air equals a cross section of the ducting system and the speed of the air in circulation. The speed of the PSC motor is set according to design and the system works and the system provided the prescribed airflow.

Comment: This is a bit theoretical to make the point which is coming, but there is much less duct design and attention to ducting during upgrades to major components than one would wish for, and this is a not good. This is not an abstract issue, but depends on the situation in your own home. We discuss possible mechanical defects that can impact and erode performance here. We mentioned defects like this can cause your ECM motors to run hard and fast and not save you money. Well all kinds of comfort problems, and issues which strain and destroy your heating and cooling components prematurely, start with ducting defects. Make sure you check these issues out. You will not be comfortable, you will be tossing money into the disposal, until these issues are diagnosed and solved.

Back on point – with PCS motors, a change in static pressure- a dirty filter can reduce air flow; or human disruption of the return air system, or a very wet evaporator coil, can impact the static pressure, and reduce the actual air flow. PCS motors run just fine at rated speed, but air is not moving as well as the static pressure increased. Your air flow can be choked down, until your system can actually freeze up which is not good. Low are flow can also starve your furnace heat exchanger and cause serious damage. Other issues can arise. Pressure issues in HVAC systems is discussed here.LINK Have you changed your filters lately? This is a big deal. LINK

On the other hand, DCM motors are programmed to deliver a given quantity of air through the system and will increase in velocity and effort to push through changes in static pressure. They can actually push so hard as to damage the motors- denature a permanent magnet or melt insulation on wiring in the motor… ECM motors can actually deform a clogged filter pushing on it and then all the crap just bypasses the filter… money, money, money.

The point here is that we need to be aware of problems which can impact the static pressure in the system and that all components are impacted, each in different ways. If you hear your blower motor sounding strained, check out what is happening. If you seem to be lacking air flow in the system, this is a sign of a static problem. Depending on the motor types you have in your system.

Secondly, we need to talk about actually saving you money.

You are paying more up front, how are you going to get your money back. Actually, the payback period is short. About 3 years. This is a generalization, of course, what is important to you is what is happening at your home.

If you want to run your filtration system all the time, you will save even more money. A typical PSC motor running all the time can use 500 watts or more, while an ECM would run less than 100 watts. The ECM motor is adjusting its speed and effort as the system calls for services. We do remember what the PSC motor is doing. Running full blast at rated speed.

If you have a mechanical system to drive out stale air and bring in filtered outside air- I am describing a mechanical ventilation system, you will save more money. More on mechanical ventilation here. LINK Where is your fresh indoor air coming from when the doors and windows are closed? Worth thinking about. LINK

ECM motors are just more efficient. They are about 80% efficient as compared to PSC motors approach 60% efficiency. Efficiency here means electrical energy doing the good work and just not creating heat. The precision of the commutation and the lack of friction just basically save you money every time the system comes on.

Miscellaneous Advantages of ECMs over PSC Motors.

A Comparison list between PSC motors and ECM

This useful diagram swiped from the Dept of Energy.

The elimination of the sliding contact between brushes and armatures allows brushless motors to have less friction and longer life. The working life of an ECM motor is basically limited by the lifetime of their bearings.

Brushed DC motors- the PSC motor in our discussion- develop maximum torque when stationary, linearly decreasing as velocity increases. Electronically commutated motors- ECM motors, can create as much torque as needed (within design limits)  at whatever speed is desired because the controller system is dictating the driving currents whereas, an analog electrical motor with brushes is using the reactance of the circuit created by the analog design to control speed and torque using the natural electrical physics of the capacitor and inductor components and in relation to the load on the motor.

Brushless motors are more efficient. Higher torque to weight, higher torque per watt, which is what you pay for each month measured in kilowatts. Electronic commutation allows more precision to the electrical currents and more efficiently applies the torque developed. You also have no friction losses from the brushes dragging on the armature.

Brushless motors are more reliable. There is no erosion of the brushes because there are no brushes. The components are cooled by being attached to the shell of the motor and do not need airflow for cooling; therefore, brushless motors can be enclosed and kept clean.

Components using ECM motors are the future.

PSC motors are the legacy systems which hang on because it is what people are used to, and quite frankly, I do not have to tell you the power of saving a buck right now.

But I have laid out a case in much detail, maybe even too much detail for investing in ECM motors. There are other benefits to using ECM motors in compressors, in furnaces, but this story has carried on too long already.

Make sure you figure out how to best increase your wealth of time rather than save a buck right now. Save bucks right now by not ordering pizza out. Save for the future by saving money day in and day out.

Any questions? Any comments? Anybody want to add their two cents. Feel free.

Stu Langley

meet the author at StuLangley.com

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