ReplyCheck 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.
Regular Guest
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.
Professional Member*
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.
Regular Member
Control your dampers via the mixed air temp when economizing.
Regular Guest
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Last edited by jpsmith1cm; 01-18-2013 at 06:40 PM. Reason: non AOP member
Professional Member*
My guess is the unit was never designed for this use.Originally Posted by access_max
R I P Icemeister
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Regular Member
Agreed and one would like to hope that there is glycol in these loops as well, my guess notta.
Professional Member*
Please realize that just because you CAN access the BMS and make changes, that doesn't mean you SHOULD.
Professional Member
Trane Tech NYC
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Professional Member
Professional Member*
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.
Professional Member*
This is where I'd be looking to go with it.
If the freeze-stat is tripping then the air is obviously too cold and will freeze the heating coil if OA is allowed to continue to be used for any length of time. I see it all of the time.
No one wants to be breathing air that is that cold anyway, makes for an uncomfortable time.
...Ron
Roof Rat
Regular Guest
No problem.
What do us non AOP member know anyway right, wont happen again.
Professional Member*
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
Roof Rat
Professional Member*
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.
Professional Member*
True but you never know
http://hvac-talk.com/vbb/showthread.php?t=116113
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X3
I'm surprised it doesn't have an OA low limit on it?
Professional Member*
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?
Professional Member*
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!
Life is too short, Behappy!
TFMM
Regular Guest
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
Professional Member*
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!
Last edited by firecontrol; 01-31-2013 at 01:17 PM. Reason: Slow typer
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.
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