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

[meplumber]

can you set the discharge air temp? if you set it at 45 that should cool the space and the heating coil should open to bring the temp up to keep from tripping. check to make sure your hw valve is opening.

This is what I was thinking. Your HHW Valve should be starting to open. No way you want to send 33 F air to the space, unless this is process cooling and if it is, then it wasn't designed correctly.

[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

[ryan1088]

X3

I'm surprised it doesn't have an OA low limit on it?

[ryan1088]

One other thing I was just sitting here thinking about..... Is the zone sensor/thermostat in question in a good spot? Away from coffee pots, copiers, printers, space heaters, etc? Not dealing with a false load here are we?

[behappy]

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.

You must have a pre-heat coil and I assume the freeze stat location is normal??


Freeze stat is set to trip at 33F

logic:

1) Yes - Modulate the OAD's slowly to maintain a MAT above freeze stat trip level.
No - go to 2

2) Yes - Close OAD's and open CHWvlv to cool the area.
No - CHW system is locked out on OAT stat. See #3

3) install stand along supplemental cooling in problem zone.

TaDa!

[access_max]

That's essentially what I did, I set the stpt of the MAT to 45 and controls is backing off of OA before the freeze trips, essentially maintaining 45 SA. And unit works great. Thx for all who helped.

Sent from my SCH-I535 using Tapatalk 2

[firecontrol]

Sounds if possibly the space's cooling load has outgrown the ability of the unit to handle it as installed. Do either the heating or cooling loops contain antifreeze? If so to what protection?

If you do the math and maximum outside air will handle the cooling load of the space then you'd need to find a way to eliminate the need to have freeze protection. 1) Isolate the coil(s) with their own HE, pump and antifreeze circuit. 2) Add additional means to cool the room that doesn't move air past the coils. 3) Possibly a bypass around the coils triggered by the freeze stat 1st stage with the 2nd stage shutting the unit down to protect the coils as is now the case.

If the building has use for the excess heat, maybe a sales pitch could be to "move" it somewhere it's needed. Making an expensive job look as green as possible sometimes shakes loose money that otherwise isn't available. Sell it as FREE heat.

Posted between your figuring a way to make it work. Congrats!

[mark_ballard]

You will have to control it by mixed air temp., ensuring that the mixed air sensor is not situated close to the heating coil (if it has one). Also, you will have to make your minimum 0% when economizing because even at 5-10% it can be too much cold air to keep it above freezing.

[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.