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VAV box flow station not reading any more

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EnergyProfessional

EnergyProfessional

Mechanical
Jan 20, 2010
1,279
We have an office building where many of the VAV box flow stations don't read the flow anymore (just read 0). I also measured the dP manually and didn't really get much of a reading. My suspicion is that over the years the pitot-tubes in the flow rings got dirty.
this seems to be the only building where we observe this problem and the affected VAV boxes basically act as CV.

None of the contractors I talked to ever heard of such a problem. The VAV boxes are not very accessible, so we can't easily cut open the duct to clean the flow station. I believe they have the flow ring like shown here.

Did you ever have such problem and how did it get resolved? the AHU has MERV 13 filter, so there should be little dirt. I wasn't here for construction, so it is possible there was construction dirt.
 
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Just looking into the same thing; in this case the default Metasys program settings did not execute. We test the emergency generators weekly, and at least one or two boxes don't go to local whenever we do it.

The building is 30 years old and has about 500 boxes-never seen dirt loading hit more than a few at a time, and never across every box controlled by the same NCM or NAE. In this case, we get a question mark instead of a zero. Are the failures common to a single NCM/NAE?
 
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In our case the VAV boxes in question just read "0" cfm. this is not a communication issue, the flow station just doesn't seem to read enough pressure. At this point I can just assume they are clogged with dirt. We tried blowing them out with pressurized air.
 
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Did you try blowing the pitot heads out with compressed air?
B.E

You are judged not by what you know, but by what you can do.
 
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If you have cleaned the pitot and tried connecting to an external pressure sensor and still not getting a reading,it could be that the box is oversized.I would like to see at least 500 fpm velocity across the box at the minimum flow setting.Flow control will generate noise and you need to have measures in place to treat that noise.
 
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Based on VAV inlet size and max flowrates it should be in 1400 fpm range.
the problem is as it is we can't physically access the flow-ring to see if it is dirty.
 
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Have you put a draft hood on the register/s to see what is actually coming out of there?
B.E.

You are judged not by what you know, but by what you can do.
 
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the zone is sufficiently (some say too sufficiently) cooled and there is a lot of flow. We didn't measure exact rates, but the VAV boxes were able to heat and cool as designed.
 
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Are you getting any measurement at all, on the air tubes, can you get air to flow through the pitot and the static lines?
Can you get a Manometer connected to the airlines? I am presuming that the existing DP sensor has a static, or does it just use a general duct static from somewhere else in the system.
B.E.


You are judged not by what you know, but by what you can do.
 
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Why can't you access the VAV box, I guess because of tenants, what if this box need to be replaced. contractor can do it after office hours.
 
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The VaV boxes are "hidden" behind wires, pipes etc. the flow station in general is inaccessible with all VAV boxes and accessing it would require cutting into the duct.
 
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Yes, but still in maintenance, you need to access, and your case is a maintenance case.
this may be a good opportunity to provide access panels to help for future maintenance not only for HVAC.
 
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As a start, do you have sufficient access to drill a hole in the duct and take a look with a boroscope?
 
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I'll have our mechanics check that with boroscope or even take duct off. For a current and all future projects I specify accessible flow rings with adding such a drawband so it is less effort to access. the mechanics think if they could wipe it off with alcohol it would help.
 
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Then drop a box of beer in the duct, let tenants enjoy
 
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