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  1. #1
    I'm trying to choose a new furnace. I'm struggling because each HVAC company tells me something different. Based on one company's recommendation, I was thinking of getting an 80% 2 stage furnace with a variable speed blower, but the other company's rep said that with my "average" ductwork, a variable speed blower would not work properly. He said that a VS blower would be noisier than a standard blower. Does he have a point? Is a VS blower a mistake with ductwork that's not perfect? Is a VS blower worth the extra cost?

  2. #2
    Join Date
    Jun 2002
    Actually, and the pro's here can correct me if I'm wrong, but a good VS motor can move almost twice as much air through the same ductwork as a non-VS motor of the same size.

    (Or more specifically, a VS motor can deliver the same air flow at 0.8" W.C. that a non-VS one delivers at 0.5").

    Within these limits (e.g. up to the 0.8" max), the VS motor can determine the resistance to air-flow that your duct system provides and automatically deliver the correct air-flow. They are also very quiet and use a fraction of the electricity -- 60 watts? -- that a conventional blower uses.

    Only downside I can think of is the electronics are more expensive to deal with if they go bad.

    The variable units ramp up and down instead of an instant WHOOSH, and if you get one that is the right size for your house (e.g. be very careful not to get one that is too big, especially if it's a 2-stage since you're wasting your money if you do), then they are so quiet it can be hard to even tell they are operating if the air handler is in your basement or attic, especially in cooling mode. I can be standing right next to my Trane XV80 in the crawlspace and almost not even hear it.

    In the summer, you can get extra dehumidification if your VS blower is set to run slower for a period of time at the start of the cycle (or you can get a dehumidistat that does this only when needed).

    Variable speed blowers are wonderful.

    [Edited by JoeSix on 07-29-2005 at 12:27 PM]

  3. #3
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    If your duct system has a "high" static,resistance to air flow,a variable speed will move more air,this could make a little more noise,depending on the duct system.

    I'd go with the first guy and ask him to test the static,Toatol External Static Pressure,of the duct system ,Now,and see what if any modifications are needed.

    The above post tells you most everything else,as to why a VS is better.

  4. #4
    Thanks for your suggestions.

    Does anyone really do the Manual J and D calculations anymore? It seems to me that HVAC salesmen only make educated guesses. When I asked one salesman if he did the load calculation, he said that he stopped doing the Manual J's because he always had to fudge the results. He said that there are too many unknown variables with older houses to get the correct answer. Should I continue to ask for a load calculation?

  5. #5
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Yes ,do it it yourself if needed ,click on the "bullseye" above to do your own.

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