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  1. #1
    Join Date
    Jul 2011
    Location
    NEW YORK
    Posts
    58

    Confused Freezestat Questions Pro/Cons

    Dear Sir,

    Commercial Manual Reset Freezstat application, unit shuts down and preheat valve full open. But some say chill water valve shall open too? Skeptical.

    Please help feedback appreciated. Pro/Con's for the units mentioned, they have preheat coil (some steam, some hw coils) and CC in main unit, no reheat at unit:

    1) Rooftop AHU's that have SF and RF that serve VAV's.

    2) AHU's that have SF and RF that serve VAV's in mers.

    3) 100% OA Units. Supply Fan no return. Just exhaust in space to supplement for not having return.

    Respectfully need your expertise and experience,
    Bob C

  2. #2
    Join Date
    Feb 2008
    Location
    Jerzee
    Posts
    38
    Chilled water valve should open also. Be sure both hot water and chilled water pumps are running.

  3. #3
    Join Date
    Sep 2007
    Posts
    203
    In freeze condition you should be circulating your chilled water or coil is drained completely. Ideally adding a freeze pump to circulate the water in your chilled water coil.

  4. #4
    Join Date
    Mar 2008
    Location
    Kitchener, Ontario
    Posts
    508
    I would typically open both valves to verify water is circulating, Hopefully do to the risk of freezing the system that they have added glycol, some don't prefer to add due to the loss of heat transfer.

  5. #5
    Join Date
    Oct 2005
    Posts
    321
    Like they said, opening the cooling valve is good but doesn't help without a freeze pump or unless the main pump runs in the winter. You could also command the main pump on when there is a freezestat alarm if there is a BMS and not just local controls.

    For what it's worth, Honeywell lawyers (and there were lots of them!) always insisted the device be called a "safety low limit". "Freezestat" could imply it can prevent freezing, which it can't.

  6. #6
    Join Date
    Jan 2003
    Location
    USA
    Posts
    1,656
    Quote Originally Posted by tlp261 View Post
    Like they said, opening the cooling valve is good but doesn't help without a freeze pump or unless the main pump runs in the winter.
    Ditto. It’s rare to see coil pumps on CW coils and even less likely to see the CW pumps running in the winter. Sure you could kick everything on during a trip, ASSUMING your actually monitoring the fstat verses just hardwiring them.

    If the HW system has hot water flow and the HW valve fails open, fans off, econ closed….it should make little difference if the CW valve also opens. With no HW flow, the CW coil is likely to freeze regardless here.
    Compared to the energy costs of running the CW pumping system all winter, I would think glycol would be a more cost effective solution. Isolating and draining the CW coils is also not an uncommon solution.

    At a minimum, we use fail safe actuators on all HW coils with possible exposure to OA and economizers. Hardwire the fstat to the actuators and fans. If monitoring of the fstat is required, it’s a DPDT fstat or a relay on the main shutdown circuit fed back into the BMS. Through software, we open the CW valve if equipped. IMHO this will cover all but the most extreme failures and prevent the coils from freezing.

    If they want more protection, we certainly provide it.
    Propagating the formula. http://www.noagendashow.com/

  7. #7
    Join Date
    Jul 2011
    Location
    NEW YORK
    Posts
    58
    Thank you all, we have glycol in the system and due to units without OA and IT rooms we have chill water pumps 24/7 all year long.

    With that being said, with a standard rooftop AHU with preheat, humid and chw coil. Do you recommend to have the Freezstat open the Preheat full open, and the CHW Valve full open, when the fan shuts down?

  8. #8
    Join Date
    Jul 2011
    Location
    NEW YORK
    Posts
    58
    If you have a chw pump that is running all year long. Do you still reccommend a freeze pump? Maybe on 100% OA units that have Preheat Coil, Hum Coil and Chill Water Coil. No reheats only in space.

    Any Negs about opening the CHW Valve too? At all that you can think of?

  9. #9
    Join Date
    Jan 2003
    Location
    USA
    Posts
    1,656
    Quote Originally Posted by CHAMBERS87 View Post
    Any Negs about opening the CHW Valve too? At all that you can think of?
    The only con I can think of is cost. There should not be much need for CW freeze pump if you have a properly installed fstat and glycol in the system already. If the existing CW valves are spring return currently, I would spring them open. Otherwise I would leave them as is and worry more about the HW valve, coil pumps, fans, and dampers.
    Propagating the formula. http://www.noagendashow.com/

  10. #10
    Join Date
    Jul 2011
    Location
    NEW YORK
    Posts
    58
    Have some steam preheat coils too.

  11. #11
    Join Date
    Jan 2003
    Location
    USA
    Posts
    1,656
    If it makes hot (not gas fired burners), we open it on fstat trip.
    Propagating the formula. http://www.noagendashow.com/

  12. #12
    Join Date
    Oct 2005
    Posts
    321
    Quote Originally Posted by CHAMBERS87 View Post
    Have some steam preheat coils too.
    Be a little careful about having them fail open. We melted some controllers once when the medium pressure steam valve opened on the Unocc cycle.

  13. #13
    Join Date
    Oct 2005
    Posts
    321
    Quote Originally Posted by CHAMBERS87 View Post
    If you have a chw pump that is running all year long. Do you still reccommend a freeze pump? Maybe on 100% OA units that have Preheat Coil, Hum Coil and Chill Water Coil. No reheats only in space.

    Any Negs about opening the CHW Valve too? At all that you can think of?
    If both valves are open for an extended time like overnight, the CHW coil can pick up heat from the HW coil and cause problems.

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