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  1. #1
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    Dec 2011
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    Questions about a Rheem

    Pardon me for hijacking the thread, but I have a similar question regarding variable speed, vs mult-speed as well. I am looking at getting a 2-stage Rheem furnace (see my previous post asking about dual-fuel and such) I see on their web page, that they have 2-stage furnaces with variable speed ECM motors, and with PSC motors. Now I am of the thought that a PSC motor is the regular old-school AC induction motor that has been used in appliances and such for the past 70 years or so. They have multiple windings for different speeds. Now, the big question is that on a PSC equipped furnace, does the furnace run the blower on a lower speed winding when it is on a lower heating stage, or does it blow at the designated speed at install all the time? Also, I have heard that the ECM-equipped furnaces are a bit more unreliable because the circuit boards have semiconductors on them regulating the high power requirements of the blower to regulate the speed, versus a PSC motor equipped furnace would merely have relays to switch on and off the different windings, and only be carrying low voltages through the circuit boards.

    Am I correct on these assumptions? If not, please correct me.
    Last edited by stevenw; 01-18-2013 at 11:22 AM.

  2. #2
    Join Date
    Dec 2011
    Posts
    9
    I have another question concerning backup power to throw into the ECM vs PSC debate... PSC motors tend to require a large amount of startup amperage. A generator or inverter that is powering the motor device must be sized large enough to handle this startup "surge". This causes the backup power device to be inefficient because it is oversized for the continuous load. Do ECM motors have the same large startup power requirements that can be sometimes 4X or more the operating power requirement?

  3. #3
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
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    2,645
    Regarding your question about the PSC motor changing speeds with the high/low firing of the furnace. Some manufactures will have that happen and others (the Goodman 2-stage furnace comes to mind) do not. I can not speak to the Rheem model.

    PSC motors are "old school" and are just very limited in what they offer verse the ECM motors.

  4. #4
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    5,520
    Quote Originally Posted by stevenw View Post
    Pardon me for hijacking the thread, but I have a similar question regarding variable speed, vs mult-speed as well. I am looking at getting a 2-stage Rheem furnace (see my previous post asking about dual-fuel and such) I see on their web page, that they have 2-stage furnaces with variable speed ECM motors, and with PSC motors. Now I am of the thought that a PSC motor is the regular old-school AC induction motor that has been used in appliances and such for the past 70 years or so. They have multiple windings for different speeds. Now, the big question is that on a PSC equipped furnace, does the furnace run the blower on a lower speed winding when it is on a lower heating stage, or does it blow at the designated speed at install all the time? Also, I have heard that the ECM-equipped furnaces are a bit more unreliable because the circuit boards have semiconductors on them regulating the high power requirements of the blower to regulate the speed, versus a PSC motor equipped furnace would merely have relays to switch on and off the different windings, and only be carrying low voltages through the circuit boards.

    Am I correct on these assumptions? If not, please correct me.
    Yes, they usually change speeds. But the speed range of a PSC is much, much narrower. A 2 ton capacity blower on a small 40-50k BTU furnace for example unit for example, with good ductwork, can deliver 800CMF on Hi, but then only drops to around 700 on med-hi, 650 on med-lo and 550 on lo. The speed range might be maybe 700-1000RPM (not sure what it actually is). A ECM motor can usually run about 300-1300RPM. Keep in mind that a ECM might be 1/2 HP in that application, while the PSC might be 1/3HP. The PSC power consumption doesn't drop much in the lower speeds or as ar resistance increases... less load. A full vairable speed ECM motor will run at whatever speed it needs to reach the selected airflow level.... even if it needs 1300RPM and the full 1/2HP. SO a PSC on undersized ductwork might draw 400Watts, but only deliver 650CFM. A VS motor might deliver hte full 800CFM but be consuming 600Watts to do it.

  5. #5
    Join Date
    Dec 2011
    Posts
    9
    Thanks for the answer. I would prefer a furnace that actually does NOT do this. My house is a tri-level and when the climate control system is not in operation, the living room / kitchen end of the house gets much cooler than the den/bedroom end of the house (Where the HVAC plant is located). With the system running, temps stay very balanced at both ends of the house. I am worried that a VS or speed-switched fan will not get the necessary amount of air to the living room end of the house when in low-fire mode to maintain balance. My current furnace I think is way over-sized because it runs for only very short durations even during the coolest weather. I am thinking a 2 stage furnace, running at a high fan speed all the time, even in low-fire mode will help with air distribution.

  6. #6
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
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    2,645
    Quote Originally Posted by stevenw View Post
    Thanks for the answer. I would prefer a furnace that actually does NOT do this. My house is a tri-level and when the climate control system is not in operation, the living room / kitchen end of the house gets much cooler than the den/bedroom end of the house (Where the HVAC plant is located). With the system running, temps stay very balanced at both ends of the house. I am worried that a VS or speed-switched fan will not get the necessary amount of air to the living room end of the house when in low-fire mode to maintain balance. My current furnace I think is way over-sized because it runs for only very short durations even during the coolest weather. I am thinking a 2 stage furnace, running at a high fan speed all the time, even in low-fire mode will help with air distribution.
    That is the wrong way to look at it. High speed fan only means high noise levels. You want a VS motor and let it run all the time. You can let it run at a higher speed (if desired) when it is on continuous fan mode. Then when the fan runs on high or low fire it'll run normally.

  7. #7
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    Sep 2008
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    Western PA
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  8. #8
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,077
    I'd think about all 2 stage furnaces use 2 fan speeds so the air doesn't get too cool in 1st stage. You can defeat this in a PSC furnace by wiring a high speed to low fire but if temp rise gets too low, you risk furnace damage from condensation in the primary. We've sold 2 stage furnaces forever without complaint.

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