Control Valves for Terminal Equipment

[wberner]

Does anyone have a recommendation of whether to place control valves upstream or downstream for terminal heat transfer equipment (I.e. VAV boxes, fin tubes, convectors, unit ventilators, etc.)?

I am speaking of hot and/or chilled water coil scenarios.

I have been told for VAVs that downstream is better to avoid water hammer.

Any help would be greatly appreciated.
Thank you

 

[HVACctrl]

I have seen it placed just downstream of the coil almost 99% of the time- in VAV hot water coils too.

Not 100% sure of the reasons, but I think they could be these:

1)Keeps the coil filled all the time (which avoids air pockets, avoids having to refill the coil, helps keep thermal momentum after closing the valve, etc...)
2)Makes for more stable control

I think there are other reasons I am not thinking of at the moment, but certainly downstream of the coil.

HOWEVER, on occasion, when a 3 way valve is used, particularly at a boiler, it may be in a diverting ionstead of a mixing scenario which will actually be more upstream of the boiler. This is not seen very often, just wanted to throw it out there.

Ed

 

[GMcD]

The control valve should be on the "top" pipe connection to the terminals to avoid dirt collecting in the valve seat. Whether that top pipe is supply or return is immaterial, over time fine dirt in the hydronic system will collect at the lower piping. Some smaller control valves are particularly susceptible to crap in the seat holding it open to partial flow.

 

[walkes]

Try the attached thread

http://www.eng-tips.com/viewthread.cfm?qid=25360

 

[wberner]

Thanks for the help everyone.

One additional question for HVACctrl,

Your reply mentioned, "Makes for more stable control". Could you elaborate on this? Not really sure why?

Thanks