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  1. #1
    Join Date
    Jul 2007
    Posts
    65

    freeze stat on mua

    Stupid question but, whats the purpose of a freezestat on mua?

  2. #2
    Join Date
    Mar 2003
    Posts
    7,321
    to prevent freezing, to start a freeze protection pump, or sound an alarm, to close a damper if say water stops moving and temp is dropping, in short to protect assets. how they are used varies, but for protection.

  3. #3
    Join Date
    Dec 2010
    Location
    Port Orchard, Wa.
    Posts
    3
    I would assume that there is a cooling/heating coil on the make up air handler. It is there to protect the coil from freeze damage.

  4. #4
    Join Date
    Jul 2007
    Posts
    65
    In this mua, its purpose is to heat outside air with a profile burner while old air is is exausted out by 2 fans interlocked with unit. No a/c coil or fluids to protect from freezing thats why i was wondering because i will need to set it.

  5. #5
    Join Date
    Aug 2006
    Location
    Columbus Ohio
    Posts
    1,980
    Sounds like it's not a freeze stat but just a stat to set your preheated air temp?

    Does it have a manual reset button?

    Is the bulb located upstream or downstream of the burner?

    Where is the air going to? Multizone air handler?
    UA LU189

    10mm, because it's better than .45acp

  6. #6
    Join Date
    Jan 2003
    Location
    Kansas City
    Posts
    1,482
    Make and model of the unit would help, but it sounds like it is a discharge air safety that shuts the blower when the burner fails to light. Is this stat in an electrical circuit that incorporates a time delayed bypass? Newer mua units incorporate this function in the main control board utilizing the discharge air temp sensor, but I have seen some older ones with a penn remote bulb stat with bypass timer that will energize a lock out relay when tripped.

  7. #7
    Join Date
    Aug 2006
    Location
    Vancouver , Canada
    Posts
    297

    Freeze stat

    We had a large fibre optics plant that had a 750,000 btu DFMUA for the fibre storage room. They changed the air over several times per hour as the finished, coated fibre was in 1 km lengths on styrofoam spools - there were well over 1000 spools in the room. I can't remember how much each spool was worth, but this stuff was being made into cable being sent all over the world. Pretty neat - the spools were about a foot in diameter and 8 inches wide - didn't weigh a pound or two ( from what I recall )

    They had big changeover as the coating off gassed for several days after production - basically a 70 F curing room. On big exhaust fan sucked air directly out of the room.

    Welllll, at - 40, the burner failed, but there was no freeze stat - fan kept running - froze the sprinkler heads and sprayed all the fibre spools with water - made them NFG.

    I was putting a manual reset freeze stat in that morning. At - 40 BTW.

    My story - FWIW

    Freonguy
    Superheat and subcooling tell it all !

  8. #8
    Join Date
    Jul 2001
    Location
    Canada
    Posts
    1,808
    Up here MUA units have a FREEZE STAT , shuts unit off if burner fails to operate , keeps it from dumping -40 'F air into the building.

  9. #9
    Join Date
    May 2007
    Posts
    732
    Quote Originally Posted by Control Man View Post
    Up here MUA units have a FREEZE STAT , shuts unit off if burner fails to operate , keeps it from dumping -40 'F air into the building.
    Units here in semi-tropical Arkansas should have them also. My best example of why is the Tyson Foods plant where several MUAs quit heating due to a gas pressure problem and water lines in the plant froze and burst on Sunday, preventing production from running on Monday. If the units had shut down, there would have been enough internal heat to prevent the freeze.

  10. #10
    Join Date
    Oct 2002
    Location
    Lady Lake, Florida
    Posts
    799
    I know from experience here in new england that all direct fired mua units I've started up or serviced have a freezestat and time delay to prove light off and a flame failure alarm in the event of flame failure which shuts down the fan, closes oa damper and powers flame failure lamp. Reset must be done in unit. Those units are some I actually like working on.

  11. #11
    Join Date
    Apr 2007
    Location
    T.C minnesota
    Posts
    12
    The purpose is to save the building from freezing. You can not let the unit run if the flame does not ignite. i've found them jumpered and the sprinkler pipes froze and destroyed the interior. 35-40 is average setting

  12. #12
    Join Date
    Oct 2001
    Posts
    6,945
    depending on where the Cap tubing is strung out with the range being narrow on settings i would say they use it on the inlet air .if the air is below the setting the heat is cycled to mantain minimum air temps just above freezing..especially if there is a large CFM fan.....
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  13. #13
    Join Date
    Dec 2010
    Location
    Canada
    Posts
    23
    Its usually called a low limit, wired with a bypass timer to allow temp to rise within a given time. If the burner fails this control will prevent cold air from causing damage. They're a pain if there solid state and it's 40 below (can't heat the sensor fast enough)

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