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.
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.
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.
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.
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.
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.
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
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.....
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)