Freeze Stat trips, when OA temp drops to 30F, but zone is need of cooling.

[access_max]

Hi, I looked all over this forum and didnt find my answer, so here is my question.

My equipment (AHU) trips on freeze stat and shuts down the unit, I do have a few AHU's that back each other up, but here's what going on. I have a zone that is always hot and needs cooling, OA temp is in the low 30's, and OA damper is modulated and is open 100% to try to satisfy the zone after a few minutes or so, the freeze stat trips and unit is shutdown. Freeze stat is located after the HW coil, but before the CHW Coil. Freeze stat is set to trip at 33F

What can be done to prevent the trips in the future? I have full admin rights to the BMS, and can change the coding as I please, would it be logical to override the OA dampers to go to its minimum position, if the OA temp dropps below 35F?

But then thinking about this, wouldnt it start to open the CHW valve, and in doing so, it sends a start signal to the Chiller plant and start up the chillers when OA temp is 30F? BMS has chillers to be locked out if the OA temp drops below 45F. So chillers wouldnt start anyway. I guess what im asking is, what is the right thing to do, I know the Freeze stat is to protect the coil, but if the space is in need of cooling, then what do i do?

Thanks in advance.

[Nuclrchiller]

Check the control drawing's "sequence of operation". Your answer should be there. If my buildings had air side economizing I would tell you what mine said, but I have water side economizing.

[Chris_Worthington]

Control your dampers via the mixed air temp when economizing.

[bdivell]

Control your dampers via the mixed air temp when economizing.

[Trane Tech NYC]

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[Trane Tech NYC]

This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

You can find the rules for posting and qualifications here.

Additional infractions may result in loss of posting privileges.

No problem.

What do us non AOP member know anyway right, wont happen again.

[rscamaro]

No problem.

What do us non AOP member know anyway right, wont happen again.

You could always become a professional member with the star and help these people out. Not many posters from NYC here. I bet you have tons of equipment to talk about.

...Ron

[hugo]

Typically a preheat coil will be used to prevent the the cooling coil from freezing in cold weather

[alex1960]

Mixed air setting could be the problem. Check and reset above 500F

[maxster]

most of the hi rise Manhattan buildings are running a mixed air temp before the filters if your trying to control from a space sensor that is why your tripping the FS.you should be "mixing" return and OA to make a 55F discharge air into the space..return dampers should be 100% opposite the OA in action.

[pecmsg]

No problem.

What do us non AOP member know anyway right, wont happen again.

True but you never know

http://hvac-talk.com/vbb/showthread.php?t=116113

[pecmsg]

Hi, I looked all over this forum and didnt find my answer, so here is my question.
I guess what im asking is, what is the right thing to do, I know the Freeze stat is to protect the coil, but if the space is in need of cooling, then what do i do?

Thanks in advance.

My guess is the unit was never designed for this use.

[Chris_Worthington]

My guess is the unit was never designed for this use.

Agreed and one would like to hope that there is glycol in these loops as well, my guess notta.