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Thread: Return capacity

  1. #1
    Join Date
    Oct 2005
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    4

    Question

    I have recently had some renovation work done in my forty-plus year old rambler. It involved moving interior walls in one end of the house. In the process, the contractor (not an HVAC contractor) had to reroute supply and return ducts. The house originally had a supply and a return in each room (no returns in the bathrooms).

    My concern is that instead of moving the return ducts, he simply capped them off. Instead of three returns and five supply vents in that end of the house we now have five supply and one return vent in an area of the house which is about 600sf. In the whole house we have thirteen supply registers and four returns.

    All of the return vents in the house are about 5" x 13". Most of the old supply registers were that size too but some of the new ones and the bathrm ones are smaller.

    When I spoke to the contractor's "Air Conditioning guy" about it he said we had plenty of supply volume. He said that if there was a supply shortage we would hear the ducts popping when the blower came on. He also said the heat exchanger would overheat and shut off automatically.

    Do I have any reason for concern?



  2. #2
    Join Date
    Sep 2001
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    Vancouver Canada
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    996
    You have plenty of reason for concern, Number 1 fire the HVAC guy, you are not concerned about the supply end of the system but the blocked off returns. If the system was properly designed according to the number of return airs in the home then you now don't have enough return air. As far as the furnace shutting off automatically, that is the job of the thermostat not the high limit on the furnace, yes the burner will shut down if it overheats but you can't leave a furnace operating in that manner, have this clown check the temperature rise accross the heat exchanger and make sure it is within the manufacturers guidlines, if not you will definetly need more returns or the existing ones made larger.
    "Go big or Go Home"

  3. #3
    Join Date
    Mar 2002
    Location
    Gaylord, Michigan
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    729
    Originally posted by Black Adder
    You have plenty of reason for concern, Number 1 fire the HVAC guy, you are not concerned about the supply end of the system but the blocked off returns. If the system was properly designed according to the number of return airs in the home then you now don't have enough return air. As far as the furnace shutting off automatically, that is the job of the thermostat not the high limit on the furnace, yes the burner will shut down if it overheats but you can't leave a furnace operating in that manner, have this clown check the temperature rise accross the heat exchanger and make sure it is within the manufacturers guidlines, if not you will definetly need more returns or the existing ones made larger.
    Ditto!

  4. #4
    Join Date
    Oct 2005
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    Thanks for the input. It confirms what I was afraid of.

  5. #5
    Join Date
    Nov 2004
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    2,927
    Originally posted by kayess
    Instead of three returns and five supply vents in that end of the house we now have five supply and one return vent in an area of the house which is about 600sf. In the whole house we have thirteen supply registers and four returns.

    All of the return vents in the house are about 5" x 13". Most of the old supply registers were that size too but some of the new ones and the bathrm ones are smaller.


    Do I have any reason for concern?


    What do you have now.I didnt understand.
    Do you have one central return now?How big is it?What size and type of equipment do you have? Is it being used now,if so how does it perform?

  6. #6
    Join Date
    Oct 2005
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    4
    As I said in the original posting, we now have four return vents each of which is 5" x 13". The equipment we have is a Trane XE1000 which I think is a 36000btu AC unit outside. The gas heater is a Carrier unit rated at 114 btu/hr.

    We are using the system and it does provide adequate cooling (we havn't used it in the winter yet as it is currently configured). My concern is whether it will operate safely and efficiently with the proportion of return vents to supply vents being 4 to 13.

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    It depends on the sizing of all the duts,return and supply,plus the turns and fittings.

    With that said,closing off existing returns ,probably reduced the air flow,and should be corrected.

    Call around and find someone who can,test the TESP(Total External Static presure),of the return and supply,and has the fan chart for the furnace,so they cn determine the cfm(air flow) being delivered.Then if it's low,insist the contractor correct it, by reconnecting the returns.


    Note that even before they disconnected the returns,air flow may have been on the low side.

  8. #8
    Join Date
    Feb 2005
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    Dothan, Al
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    incorrect information given - deleted so no one else sees it.

    My apologies,
    Richard

    [Edited by bornriding on 10-08-2005 at 11:05 AM]

  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Originally posted by bornriding
    5" x 13" X 4 = 20" x 54" or 1080 sg in - for three ton system - plenty. I see no problem
    Besides, as I have been told before, its not the size that counts. ( LiL pun )
    Its the amount of CFM that counts.
    Standard size return that we put in is 20"x32" for three ton system @ 1200 CFM.

    Hope this helps,
    Richard

    5X13 =65

    65 X4=260 sq. in or a 20X13 Grille,I see a problem.

  10. #10
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    Feb 2005
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    Dothan, Al
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    OOPS - and I thought I was good at math, please forgive my previous post.
    I stand corrected by 'Dash'

  11. #11
    Join Date
    Oct 2005
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    Smile

    Thanks to all who jumped in with advice. I will contact a local HVAC contractor and have them do an assessment of the system.

    What a great web site.

  12. #12
    Join Date
    Nov 2004
    Location
    SW FL
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    6,248

    Thumbs down Heater Size & Temp Rise

    Originally posted by kayess
    As I said in the original posting, we now have four return vents each of which is 5" x 13". The equipment we have is a Trane XE1000 which I think is a 36000 btu AC unit outside. The gas heater is a Carrier unit rated at 114 btu/hr.

    My concern is whether it will operate safely and efficiently with the proportion of return vents to supply vents being 4 to 13.
    Six 5" x 13" returns might be marginally
    sufficient for 1,200 CFM.

    Heater ( Input ? ) rated at 114 ( K ) btu /hr
    _________ 114,000 BTUh
    Q= CFM * dt * 1.08

    I would check that rated size and actual temperature rise.

    Is the output capacity ~ 83,000 BTUh?

    Seems like with ~ 1000 CFM,
    which is now probably your maximum,
    the temperature rise would be EXTREME ( 77'F).
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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