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Trouble shooting freeze stats ?

27K views 14 replies 14 participants last post by  HVAC9900  
#1 ·
I'm working on a Trane AHU and I belive that the freeze stat is going bad. Is there some way that I can test it to make sure that is the cause of my problems? I have a customer that has to reset it every so often in the morining to get it to run. I've checked the outside dampers and the dampers are sealed tight so there is no outside air coming in. So that will not be the case.
Any suggestions?

Thanks!:confused:
 
#2 ·
should have OHM rating check with Trane.
 
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#3 ·
> I have a customer that has to reset it every so often in the morining to get it to run

Is this a rooftop? What is the heat source?

I have seen them trip on rooftop units with no hot water in the unit. Not enough heat migrates into the unit and it cools down enough to trip the fstat during unocc times.

Also check and make sure no part of the capillary touches the outside panels and frames. Same thing, they can cool down and cause it to trip during unocc.

Testing the trip point would be a PITA for a capillary type, replacing it is easy enough.
 
#4 ·
What type of control system is operating the building? The freeze stat is doing it's job protecting the HW coil. I wouldn't necessarlly blame the freeze stat for this problem. Check systems operations.
 
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#7 ·
Your freezestat should be an averaging element type safety. If any 12" section gets to setpoint, you trip. Pretty easy to test with a rag soaked in ice water (remember, you're not building a watch). It needs to be within a couple of degrees or so, with a setpoint of 36. Lots of good advice already given here about things to look at, also. The freezestat may be defective, but as already mentioned, it's more than likely just doing its job. You'll have to look at it and from an operational sequence standpoint, decide what could be causing it to trip. Tripping on the off cycle? No 100% shutoff damper shuts off 100%, and cold air likes to fall to the low point and force the warm air out. Running? OA/RA dampers not operating properly, low limit controls not functioning as needed. Bazillion different answers to this one, and a lot of them depend on the setup of your system. Look at the situation as a puzzle and figure out how the pieces go together. Good luck!
 
#8 ·
Yeah is this a freeze stat mounted to a suction line? If so maybe you dont have a head pressure control, and it is cooler at night and you start to freeze up there. And what is the customer doing to reset the unit, the freeze stat i am thinking of, or pressure switch should not need reseting. again i could be off on the type of saftey you meant but....
 
#9 ·
Had same problem on an Intellipak. Unit would trip low air temp limit otherwise called a freeze stat and customer would have to reset it at the unit. Checked all dampers, even bumped up the min voltage to the actuator so valve would not close all the way..no help. Found unit supply air temp reading 8 degrees different than Tracer Summit was seeing. Had to tell customer to get the people out who programmed the system to coorect the problem. Problem solved.
 
#10 ·
Assuming hot water coil in this unit,and that it is tripping overnight.
Would suggest that heating valve to coil be fully opened for a given time before OD damper is allowed to open.

If not happening during unnoccupied times - could be the heating valve is modulated closed,or is losing flow during operation when it should not( variable speed pump in the loop slowing down too much,valve shutting off somewhere,etc?).

Things to look at...
 
#11 ·
Sounds like the freeze stat is doing its job. Have you looked at the mixed air control to this unit? Re-post this in the controls section. Have some more info ready too, like discharge air temp, mixed air temp, location of freeze stat cap tube and what input/s control the discharge air.
 
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#12 ·
We had one similar to this a few weeks ago, set a trend and found that it would not always trip on the same temp (intermittent). Replace it, problem solved. Not expensive and easy to replace.


Didn't know about the 12" of cap tube wrapped with an ice cold rag. I will certainly use this in the future.
thanks
 
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