Steam Valve

[noclaf]

Hey,

I have a constant volume AHU with a steam preheat coil...The existing DDC system keeps the valve closed during unoccupied mode with the OA dampers closed and RA dampers open of course. The only time the steam valve fails open is if there is a freeze stat trip. DO you think it's better to just fail the valve open whenever the fan is off? The freeze stat is interlocked with the fan starter. Is it ok to leave the steam valve open all night with the fan off?

[maxster]

if the coil is below ground level i would let it be opened with the fan off,that cold air will drop right into the return duct say if it is in a basement and bust the stagnant water condensate!the other side is the temp.will rise bigtime in the air handler during unoccupied.

[noclaf]

unit is up in the attic...OA damper closes tight, return damper opens. Just letting the valve stay open all night sounds like a waste of steam and it probably will cause all types of starup control problems. On the other hand, I would hate to see the thing freeze. However, the freeze stat does input to the DDC and it would cause it to open.

Any opinions would be great.

[jslimjeff]

I usally program two senarios to preven nusiance freeze stat trips.
#1 - When the OA temp is below 25 I crack the valve to 25% the allow for a small flow or if the MA temp sensor in the unit sees a temp below 40 when off it cracks the valve for a period of time.

This has been a great help to eliminate nusiance tripps and helps control energy cost by preventing run wild situations. I have had problems on certain jobs where we had the valve @ 100% and natural convection over the course of the weekend caused the space temp to rise and start the ahu in unocc cooling mode...

[qtip]

If the OA is below 32 control the valve to a 70 degree mixed or discharge air temp, Freeze stat trip valve goes open, power failure valve springs open.
The biggest problem I have seen with the valve open and the fan off is melting the wires inside the AHU.

[andserco]

A lot of scenerio can happen on the setup, one thing to look at is the steam traps,they need to be a F&T style so they will gravity drain,if they are a disc or inverted bucket that takes pressure to operate, just cracking the steam valves might not operate them, it will stack the condensate and freezing can happpen. What size unit is this, in extreme conditions could you start the unit and maintain a preheat air temp (not a mixed air ) because stratification and return air can give you problems..

[jslimjeff]

If it is Mod. Steam you should only be using F&T. What kind of wire are you using that is melted easily. I only use the MA sensor when the fan is off since it is the closest to the coil.

[qtip]

Originally posted by jslimjeff
What kind of wire are you using that is melted easily. I only use the MA sensor when the fan is off since it is the closest to the coil.

This was a HIGH steam pressure system so the temps where abnormal.

[jslimjeff]

Ohhh. Now it makes sense...

[glennwith2ns]

Originally posted by qtip
The biggest problem I have seen with the valve open and the fan off is melting the wires inside the AHU.

Other problem i've seen in this situation is the grease in the lubrication lines turning into "oil" :>)