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  1. #14
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,064
    Not a good idea to twinn 2 different size units.
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    How many times must one fix something before it is fixed?

  2. #15
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Originally posted by beenthere
    Not a good idea to twinn 2 different size units.
    Yeah, having a 5 ton blower and a 2 ton blower sharing the same return and supply is BAD mojo.
    If more government is the answer, then it's a really stupid question.

  3. #16
    Join Date
    Jul 2004
    Location
    West TN
    Posts
    983
    I also agree with putting the 2 ton on a zone by itself.

    Lemme explain twinning to you to be a little more edumacated hehe
    by twinning, we mean putting two furnaces parallel to each other. The furnaces share the same return and share the same supply.
    I've seen this alot in commercial applications.
    The furnaces have to specially set up for twinning though.
    Most furnaces have a 'Twin' terminal that you connect from one furnace to the other, and there are also special kits that provide the additional safeties required for problems that can arise in the future.
    All twinning that I've ever seen has had two furnaces that match sizes.

    In the application you are describing...
    Using the 5 ton to come on first then bring on the 2 ton as a second stage if needed...
    The 5 ton can 'recirculate' air through the two ton equipment. This can cause the blower wheel of the 2 ton unit to start rotating backwards. When the 2 ton unit does come on... the motor will have a hard time reversing and going the other direction. There's a chance that the motor will actually start up and run backwards too.
    Not good on the furnace, blower, or AC equipment.

    Also, by recirculating the air... the air will get colder and colder and eventually freeze the unit up.
    Or if your in the heating season, the air will get hotter and hotter and will end up tripping safety switches.

    I would re-figure the loads on the house again.
    Once using zoning and then a regular load calculation on each zone. There are different considerations when figuring the heat loads with a zone system, so the numbers will end up different.

    (havent' payed much attention to the numbers you give so take it for what its worth)
    I'd try to put the downstairs as a zoned system and the upstairs as an individual unit.
    The upstairs would need a filter grille of its own upstairs.
    The downstairs would need a filter grille of its own downstairs.

    Wished I was there to really diagnose the system though.
    There's alot I would check for.
    Load calculations
    Duct sizing calculations
    House design considerations
    etc


  4. #17
    Join Date
    Sep 2003
    Posts
    1,311
    Wormy described the twinning well. It looks like the configuration is basically as a twinned unit; I agree that it isn't a good idea to mix different sizes of units in a twinning setup.

    The ductwork was apparently not sized according to the International Mechanical Code, either. The IMC requires a Manual D calculation to be done (or other recognized method) and a Manual D calculation usually gives different pipe sizes, not just two. Granted that your house may be a rare exception, but I doubt it. Zoning is not intended to make up for improper duct design and zoning will only go so far in compensating for improper duct design.

    Options vary and some were already suggested. A whole system analysis by someone else would be the best bet so that all options could be explored and actual system sizing can be analyzed. Fixing the system so that the proper airflow can be achieved so as to maximize the efficiency of the cooling equipment may require something that all of us contractors hate to do.

  5. #18
    Join Date
    Sep 2004
    Posts
    64

    Thumbs up

    Thanks again to everyone. I think the next step for me is to get an professional out to evaluate what is here and tell me what should be here.

    I would guess the best person would be an independent Mech. Eng. firm?

    Someone had mentioned there are firms in the DFW area that do this?

    If my contractor would only tell me what/why he is doing things this might be easier. They are happy playing hit or miss with the system and my money.

    Thank you


  6. #19
    Join Date
    Sep 2003
    Posts
    1,311
    One place to check for a contractor is on this website. Otherwise when checking around ask them how they determine duct sizing. Don't go with a contractor that doesn't know the Manual D or sets it aside for an easier method.

    [Edited by sadlier on 09-08-2004 at 12:03 PM]

  7. #20
    Join Date
    Sep 2004
    Posts
    64

    Question

    Sadlier,

    Looking for a third party (nutral party) not sure where to look.

    I have requested general design info from current, will see what I get.

    All in all, the system is not doing the job but the season has turned. Not sure what I will manage to achieve.

    Did you see my zone breakdowns on sensible & latent loads, how would you recommend I use the existing 5 ton and where should I request a seperate system ?

  8. #21
    Join Date
    Feb 2004
    Location
    Wadsworth, OH
    Posts
    316

    Hmm

    You need to make sure that they provided some type of safety interlock between the two units or that they installed a backflow damper in the 2 ton duct. When the furnace operates with out the 2 ton air handler running the hot air will flow backwards through the 2 ton AH and be returned to the furnace. This will cause the furnace to become very inefective and HOT.
    Are you confident that the zoning system is functioning properly? Dampers open and close when appropriate, equipment operates when thermostats are calling?
    "When you perceive zoning not as a bandage but as an enhancement, you truly understand the dynamics and limitations of forced air heating and cooling"

  9. #22
    Join Date
    Sep 2004
    Posts
    64
    Joe,

    At this point in time, the answer would be no. I do not know if the zone system is working or not.

    I don't think there is a back flow damper installed and this is my main concern. I feel the 2 ton air unit is a band aid to the bigger issue. The system as designed was never able to do the job.

    I have lost faith in the orignal installation group and don't know where to turn.

    This was suppose to be a 10 year investment in the new HVAC system that has turned into 5 months of trail and error.

    I am meeting with the owner Friday to see what we can do...

  10. #23
    Join Date
    Feb 2004
    Location
    Wadsworth, OH
    Posts
    316
    I have had many contractors successfuly downsize equipment and add zoning to control comfort in a two story home.
    To be honest the load calculations don't seem that far off.
    Your mention of low air flow in some areas and noise in others has me concerned that the zoning system is not functioning or there are manual volume dampers closed in some ducts.
    If the 5 ton system is truley too small, you would not be getting air noise out of any registers.
    "When you perceive zoning not as a bandage but as an enhancement, you truly understand the dynamics and limitations of forced air heating and cooling"

  11. #24
    Join Date
    Sep 2004
    Posts
    64

    Hmm

    Joe,

    Your answer is just what I am hoping for. I don't know who to believe anymore but 5 months have gone by already.

    Your questions about a back flow damper is at the front of my list of concerns.

    All I know is I want it to work and pay itself back over time with little to problems.

    I hope you are right and this can be worked out.


  12. #25
    Join Date
    Sep 2004
    Posts
    64

    Hmm

    Joe,

    Your answer is just what I am hoping for. I don't know who to believe anymore but 5 months have gone by already.

    Your questions about a back flow damper is at the front of my list of concerns.

    All I know is I want it to work and pay itself back over time with little to no problems.

    I hope you are right and this can be worked out.


  13. #26
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    carestx,

    I don't remember the name at the moment, but tomorow I'll dig up the name of the company we have used in the past to do load calcs for us on wierd houses, like the Pollysteel constructed house we just did. They do consulting and design work, but don't contract at all.

    It may be worth talking to them, they are out of Arlington, but they may at least be able to refere you to someone that can help you with some independant consultation if they are not able to.
    If more government is the answer, then it's a really stupid question.

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