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  1. #1
    Join Date
    Feb 2004
    Posts
    110

    Furnace Replacement

    Hi Everyone, I am getting ready to replace my furnace and had a few questions about what I would like. I have 3 contractors comming over but wanted to get the pro's opinion. Did a manual J and have a total heat loss of 70KBTUH and a heat gain of 40KBTUH. It appears that based on manual J 75KBTUH at 95% AFU would be great at a 3.5 ton coil. On the heating size the first floor is 68% of the total load and the second floor is 32%. This seems to line up perfectly with a 2 stage furnace output setting.

    This is what I am thinking, not sure if it is possible. All main trunk lines are in basement and easily accessible. Is it possible to zone a 2 stage furnace for the above loads? If the furnace kicks on low, 52KBTU for the second floor, and it only needs 22KBTUH, will I be overfiring the furnace? Also, how are the ducts sized for the varios CFM outputs of the furnace? If they are sized for the max, and it is on low, static will be very low?

    Sorry the question, just want to get my head around how a 2 stage multi speed operates and if you can zone them and if a static pressure bypass damper is needed in the setup.

    Any recomendations on brand would be welcome and appreciated.

    Thanks for your help.

  2. #2
    Join Date
    Jan 2008
    Posts
    2,118
    It looks like you have got a good start to a new system.
    The problem with zoning comes in when the unit can't move enough air in that small zone, not how many btu's it actually needs.
    Properly sized ducts are imperative.
    Make sure your contractor knows how to do it right.
    I personally don't like by-pass dampers - it can increase the return temps, and make a furnace run too hot.
    "Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler

  3. #3
    Join Date
    Feb 2004
    Posts
    110
    Thank you for the reply. So it does sound doable with a zone controller and the proper 2 stage stats? Sound like the key is sizing the main trunk for that zone to the cfm of the blower on low fire and not the cfm of the zone branches? As you indicated proper duct sizing will be key.

    Any input on good quality manufacturer that have multiple stage furnaces?

    Again, thanks for the help

  4. #4
    Join Date
    Aug 2011
    Location
    Eastern Shore, MD
    Posts
    799
    Quote Originally Posted by lkaufman View Post
    Thank you for the reply. So it does sound doable with a zone controller and the proper 2 stage stats? Sound like the key is sizing the main trunk for that zone to the cfm of the blower on low fire and not the cfm of the zone branches? As you indicated proper duct sizing will be key.

    Any input on good quality manufacturer that have multiple stage furnaces?

    Again, thanks for the help
    ill get yelled at for this, but there really isnt much difference between brands. all blow hot air, all will run less efficient in low stage.

  5. #5
    Join Date
    Sep 2002
    Location
    Northeast Ohio
    Posts
    4,855
    Most if not all major manufacturers have modulating or multi staging units. On top of this are differing degrees of complexity and features. The most important thing is the installer. A bad install of the most sophisticated equipment will under perform or worse. Add in zoning and duct design and it becomes imperative to choose the proper company. Choose the contractor and work with his recommendations. Come back here if you have questions.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  6. #6
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,599
    I would not use 2 stage stats, I would want the panel to control high & low fire based on number or percent of zones calling. We use Jackson's Z600 and tell the panel the minimum number of zones that must be asking for heat before the furnace can switch to high. Honeywell's 322 and 432 panels do it based on % of zones calling. Zonex times to high but only if the discharge air sensor says it is OK and won't overheat the furnace.

    If you do use a 2 stage stat, only put them on zones that can handle 100% of the heat when they are the only zones calling. May only be a large area, the rest of the zones should not have high fire capability.

    Harder to get it right with existing ducts than with starting from scratch and sizing for zoning.

    Carrier Infinity zoning is the best system. Couple that with their 3 stage MVC or the new modulating furnace and you'll have the ultimate. Bryant Evolution is the same.

  7. #7
    Join Date
    Oct 2011
    Posts
    10
    Hi Gross,

    When you say it will be less efficient in low stage, just how much does the efficiency go down on a 2 stage 95% furnace - 85, 80%? On a 2 stage furnace how much of the heating time does it spend in low stage, typically?

  8. #8
    Join Date
    Jun 2011
    Location
    Wake Forest, NC
    Posts
    352
    Zoning can be a problem with furnaces if the small zone is so small that the furnace cycles tripping the high limit. Proper duct sizing is critical for the smaller zone.

    I also am not a huge fan of bypass dampers, but if one muse be installed make sure the bypass is run to a return box away from the unit with the other return(s) from the home also running to that box. Then have one large return to take all return air and bypassed air back to the return plenum on the furnace. That allows the hotter bypassed air to mix with regular return air before entering the furnace which will help to prevent over heating.

    It is also possible to run a "dump zone" that terminates in the crawl or attic that only operates when only the small zone is calling and only in heat mode. It requires creative wiring, and an extra damper, but it is possible. I definately would not recommend this, but can be used as an option of last resort.

    Good luck.
    It's not rocket-science...

    It's electromechanical thermodynamic engineering

  9. #9
    Join Date
    Feb 2009
    Location
    Portland OR
    Posts
    2,029
    Quote Originally Posted by Gross View Post
    ill get yelled at for this, but there really isnt much difference between brands. all blow hot air, all will run less efficient in low stage.
    Let the yelling begin, JK
    Actually Trane is different in this regard, they use a fully variable speed inducer motor on their two stage and modulating furnaces so that it is 95% efficient or higher in all stages of operation unlike other brands that do not use an variable speed inducer motor so you don't get proper air and fuel mixtures and the system leans out.
    Check out my YouTube channel - http://www.youtube.com/user/skyheating1 We have customer testimonials, product reviews and more!
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  10. #10
    Join Date
    Feb 2009
    Location
    Saskatchewan
    Posts
    14
    The new Carrier "59 series" is supposed to be variable inducer also, is supposed to be available at start of the year.
    To quote Carrier:
    "The 59MN7A Multipoise Variable--Capacity Condensing Gas
    Furnace features the modulating InfinityR System. The innovative
    modulating gas valve is at the heart of this furnace’s quiet
    operation, along with the variable--speed ECM blower motor and
    variable--speed inducer motor."

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