When it comes to discussing the pro's (or perhaps the lack thereof) of ECM motors for use in HVAC air handler systems, I've found that this document is very informative:
Perhaps someone else can explain why the flow characteristics of a duct system is described in terms of the back-pressure it generates instead of speaking directly about the RESISTANCE of the ductwork, even perhaps putting a number on it (which surely can be done, given that we might know the CFM and the air pressure at the input side of the duct, and assuming the pressure at the far end is zero). When someone is talking about high static pressure, they are essentially saying that the duct system has a high resistance to flow (caused by any number of reasons - closed or blocked vents, small-diameter ductwork, long runs of small-size ducts, turbulence caused by right angles, filter too small or too dense, etc).
Which could be why my idea that the gating of furnace output around the A/C coils is not appreciated as something that can reduce airflow resistance (ie - reduce static pressure).
As someone who's been living with and has experienced HVAC systems with single-speed AC fan motors, I really can't appreciate the need for a variable-speed fan motor. All this discussion about how PSC motor efficiency drops to 15% - 30% when used at low speeds is a real mystery to me - are there really furnaces out there that have the necessary electronic controllers that will use PSC motors in such a variable-speed capacity? Why no real discussion about the efficiency of 2-speed AC motors?
I also don't understand how running a fan at low speed is better at humidity removal when the HVAC system is in A/C mode - yet ECM makers make that claim.
Claims that ECM motors are just as reliable as PSC are also a crock, given that none of them could possibly be in service yet for 30 to 40 years as is the typical PSC motor to even begin such a comparison.
Youtube video showing badly-behaving ECM motors:
Apparently they are somewhat succeptible to lighting strikes (lightning doesn't have to hit your house directly to dammage the electronics in your house). I guess your entire furnace is more succeptible to lightning when it's got it's own computer. Another strike against the modern furnace.
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Conclusions:
The main conclusion that we would draw from this study is that although the use of an ECM has the potential to reduce fan electrical power draw, much of the benefit is lost in systems with excess static pressures. A full analysis of this problem was done by Lawrence Berkeley national Laboratory (Lutz et al., 2006). In other words (as in many cases in the building industry) the benefits of high technology can be defeated by poor design and faulty installation or implementation. Problems can include excessively constricted duct designs and installations, restrictive return plenum fittings, or excessively restrictive filters (see ?Is There a Downside to High-MErV Filters?? HE nov/Dec ?09, p.
32).However, with better designs, air handler efficiencies can be improved?significantly beyond the typical values assumed in previous work (that is, 2.5 CFM/W or 0.4 W/CFM). This is especially true when a given air handler is used at the lower end of its speed range. For instance, a 1.5- to 3-ton unit being used at 2 tons air flow at 0.5 IWC static pressure has an efficiency in the range of 3.7 CFM/W (0.27 W/CFM). Of course, reducing air flow for a given size of outdoor unit can have negative consequences, such as reducing overall efficiency (SEEr and EEr). But this factor can provide additional ammunition when arguing for tighter sizing of cooling equipment, and/or two-stage equipment with a variablespeed air handler.
In other words, if you can keep the air flows down (all other things being equal), you are giving your ECM a better chance to achieve high CFM/W efficiencies. The measurement of air handler efficiency is relatively simple; it can be done mostly with gear that a home performance contractor is likely to have. an air handler powered from an electrical receptacle can be quickly measured with a plug-in power meter such as a Kill-a-Watt. However, power measurements are more time-consuming if the air handler is hard wired. But overall, increasing the data set of installed ECM air handler efficiencies could be very informative, as would measuring and recording the operating external static pressures for these units.