Variable speed furnaces with shared return duct work - should it be done?
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  1. #1

    Variable speed furnaces with shared return duct work - should it be done?

    Hi,

    As we continue to get quotes for replacing our 2-speed 80% AFUE furnaces (15 years old and becoming problematic), we have had 5 different quotes, mainly for Carrier and Lennox systems. Were ready to commit to Carrier 96% AFUE Infinity systems (variable speed) but the latest vendor was wrapping up a quote with the same equipment (Infinity with variable speed) and noticed that although we have 2 furnaces, we have shared return duct work from the first and second floors. He stated that without adding dampers, Carrier does not support using variable speed furnaces with shared return duct work - something about when one unit is on, it reverses the motor on the other because it spins freely, so when the reversing motor does kick on, it literally slams on because it goes from one direction to the other. Apparently the 2-speed motor (non-variable) in the lesser Performance series will not move backward passively in the shared return duct configuration, so that is the way we should go. Although nobody else raised the issue, I wouldn't expect someone to try to sell me less expensive equipment unless this was true. Additionally, he called the Carrier rep who confirmed that the variable speed Carrier product should not be used with shared duct work.

    1) Is it correct that with shared return duct work, we cannot go with variable speed without adding dampers to the ductwork (which apparently isn't worth the comfort gain for the cost involved)?

    2) Is this a Carrier-specific issue (passive reverse of variable speed motor) or is it the case with all variable speed furnace brands?

    Would really appreciate a professional opinion so I put the right equipment in.

    Thanks!

  2. #2
    Join Date
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    "Carrier does not support using variable speed furnaces with shared return duct work - something about when one unit is on, it reverses the motor on the other because it spins freely, so when the reversing motor does kick on, it literally slams on because it goes from one direction to the other. "

    I've only seen this once, and it wasn't carrier equip..but it sure did happen.
    never in 15 years have I seen it since.

    Dampers can make a lot of differenc in comfort and being able to
    adjust the air flow. I shut off the damper in winter to my bedroom
    and the air flows more into other rooms. much better than just
    closing the register..which makes it whistle & doesn't force air into
    other ducts.

    someone more carrier experienced will be along soon to give
    you better info.

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  3. #3
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    It shouldn't be done with any variable speed blower equipped equipment, but not for the reason stated. If anything, a standard blower will spin backwards even more freely than a VS blower...

    The reason two VS blowers should not be on a common return is that they can/will be constantly ramping up and down as they fight each other to maintain their respective airflow settings.
    If more government is the answer, then it's a really stupid question.

  4. #4
    Quote Originally Posted by mark beiser View Post
    It shouldn't be done with any variable speed blower equipped equipment, but not for the reason stated. If anything, a standard blower will spin backwards even more freely than a VS blower...

    The reason two VS blowers should not be on a common return is that they can/will be constantly ramping up and down as they fight each other to maintain their respective airflow settings.
    One contractor responded to us by saying as long as the ductwork is not undersized the system would work just fine.

    Why are there so many different opinions on this when it sounds like it really should not be done?

  5. #5
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    Quote Originally Posted by mark beiser View Post
    It shouldn't be done with any variable speed blower equipped equipment, but not for the reason stated. If anything, a standard blower will spin backwards even more freely than a VS blower...

    The reason two VS blowers should not be on a common return is that they can/will be constantly ramping up and down as they fight each other to maintain their respective airflow settings.
    I would agree. You have a bad design, seperate the returns

  6. #6
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    Quote Originally Posted by jmehrlich View Post
    One contractor responded to us by saying as long as the ductwork is not undersized the system would work just fine.

    Why are there so many different opinions on this when it sounds like it really should not be done?
    Unfortunately for the consumer the ratio of contractors that know the correct answers are far fewer than the ones that do not know the correct answers.

  7. #7
    They told me that separating the returns would cost way too much and not be worth it. Should I just put in 2 stage furnaces without Variable speed? I do not want to get talked into something if it is not the right equipment and it will not perform correctly.

  8. #8
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    Oct 2004
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    So Cal
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    Only way you can use a common return is to twin the furnaces (if PSC).
    2 furnaces operating as one.

    Pretty rare to find twinned furnaces in residential, not so rare on commercial.

    You would be using a single thermostat to control both furnaces. Both need to run at same time.
    Not sure how efficient that would be in a ressy situation.

    This would be for a standard PSC motor, NOT for a VS motor.
    VS motors can't be twinned.

  9. #9
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    perhaps a check valve of sorts could be built. a spring loaded damper on each blower, like a zone bypass... that would only solve the reversing issue... it would not solve the ramping to balance between the two systems...
    The TRUE highest cost system is the system not installed properly...
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  10. #10
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    Quote Originally Posted by mark beiser View Post
    It shouldn't be done with any variable speed blower equipped equipment, but not for the reason stated. If anything, a standard blower will spin backwards even more freely than a VS blower...

    The reason two VS blowers should not be on a common return is that they can/will be constantly ramping up and down as they fight each other to maintain their respective airflow settings.
    What he said^. Separate the returns and avoid possible issues in the future.
    Climate Control Solutions for your Home or Office

    Serving Northeast Philadelphia and Surrounding Areas

  11. #11
    Join Date
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    Quote Originally Posted by mark beiser View Post
    It shouldn't be done with any variable speed blower equipped equipment, but not for the reason stated. If anything, a standard blower will spin backwards even more freely than a VS blower...

    The reason two VS blowers should not be on a common return is that they can/will be constantly ramping up and down as they fight each other to maintain their respective airflow settings.
    That makes alot of sense. Is there any documentation that you know of on this. I know a guy who likes to use shared returns, it just doesn't seem right to me. He argues that it fine.
    "The only way in which a human being can make some approach to knowing the whole of a subject is by hearing what can be said about it by persons of every variety of opinion and studying all modes in which it it can be looked at by every character of mind.
    No wise man ever acquired his wisdom in any mode but this."
    John Stuart Mill

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