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  1. #1
    Join Date
    Apr 2014
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    Price VAV boxes not heating

    Hope this is an easy one. Have some SDVQ 5000 series units installed. The units with scr electric heat work great. The on/off units do not.

    They came bare from Price, added VAV controller later. Sending 24Vac signal successfully to the reheat contactor, and I'm seeing voltage to the contactor.

    Heat won't kick in and I'm wondering if it's because airflow switch is not making. These are 4 and 5 inch boxes so the engineer spec CFM are low. They initially called for 30 and 50. Anyone run into this scenario?

  2. #2
    Join Date
    Sep 2007
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    edmonds wa
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    Your meter may have the answer.
    UA Local 32 retired as of Jan 2020

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  4. #3
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
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    The manufacturers want 70 CFM per KW for electric heat in the FPB'S. You can get by with a little less but if you have 30 and 50 CFM the heater has to be smaller than 1 KW. If not the heat will not energize.
    No man can be both ignorant and free.
    Thomas Jefferson

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  6. #4
    Join Date
    May 2004
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    SE Pennsylvania
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    If the air flow switch isn't closed then you wouldn't have 24vac to the contactor. If you have 24vac the contactor should be pulled in. If it is and there is still no heat maybe there is a limit that breaks the high voltage to the elements

  7. #5
    Join Date
    Jul 2009
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    Wa
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    Volt meter is the answer like stated. Is the contactor pulled in? If your sending voltage to the contactor coil successfully liked stated then the air flow switch is made, and not your problem. Do you mean your controller is sending out 24VAC? The safeties will be in series with your contactor coil common mostly likely. So air flow switch or manual reset hi limit?

  8. #6
    Join Date
    Mar 2009
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    Several Miles from Sane
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    controls21,
    Is your "VAV Controller" equipped with on board relays or does it have Triac's ? Quest for knowledge here; what is the required inrush and holding VA of the Heat Contactor ? Most (contactors) are fairly low inrush/holding va but some can be as high a 35VA inrush and 9VA hold. Unless the Triac's are rated for 1.5 to 2 amps (36-48VA) you may need to install Isolation relays and switch the 24VAC straight from the transformer to the contactor.

    As others have said, check for safeties on th e"common" side of the circuit.
    If sense were so common everyone would have it !
    Stupid should Hurt !
    If people took care of their cars like they do their HVAC Systems you'd see a lot more people walking
    !

  9. #7
    Join Date
    Jul 2009
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    Wa
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    Cagey has as good point! I most always install iso relays for the heating stages. To prove his point it would be simple enough to jumper source power to the contactor coil and see if it pulls in. That would prove the safety circuit and point to the Triac issue.

  10. #8
    Join Date
    Aug 2014
    Location
    DMV
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    75
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    If flow switch isnt making check to make sure high and low tubing is connected correctly on the dp switch. Had several price boxes come from the factor with the tubes on the wrong side of the dp switch.

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