What's the pressure needed for VAV box?
What's the pressure needed for VAV box?
(OP)
We used to connect VAV to medium velocity system. Can it be used in normal low speed duct system?
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What's the pressure needed for VAV box?
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RE: What's the pressure needed for VAV box?
I may be able to help further but I'd need more info.
Mark.
RE: What's the pressure needed for VAV box?
RE: What's the pressure needed for VAV box?
RE: What's the pressure needed for VAV box?
RE: What's the pressure needed for VAV box?
RE: What's the pressure needed for VAV box?
RE: What's the pressure needed for VAV box?
I recently ran across a job where duct static was dropping to 0.4" on a design day and we were not getting enough air. We corrected some poor fittings to fix the problem.
So back to your qustion, I think you need a good 1" or better on VAV systems. As I recall on the job I just referenced, the boxes were pressure-independent and each had pressure drops between 0.25" and 0.5". So you have to have enough pressure to get through the box, through the downstream ductwork, and out the diffusers with the corrct velocity.
Unless I'm completely missing something in your question, I would think that there is no way the system will work at low pressure.
RE: What's the pressure needed for VAV box?
OA intake = 0.10"wg
Filter = 0.40" wg clean, 1.0" wg dirty
preheat coil = 0.15" wg
cooling coil = 1.0" wg
fan discharge & system effect = 0.25"wg
supply duct upstream & downstream of VAV boxes= 1.5' wg
VAV box = 0.25" wg
Total is 3.65" wg with clean filter vs. 4.25" wg with dirty filter. If VAV use 4.65" wg. If CV use 4.0" wg
The VAV box is only a small part of the total pressure drop. I recommend using the full dirty pressure drop only if the system is VAV. For CV use an average say 0.75'wg. This is because the system is balanced with clean filter anyway and the airflow in the CV system will just reduce as the filter is loaded. If the full dirt filter allowance was used, the unit being balanced at the clean filter condition will just either throttle the damper or adjust fan speed down. If damper is throttled, the system will just use up energy and still the aioflow will drop when filters are loaded. If the fan speed is adjusted, you might as well just have used the clean filter pressre drop and still the airflow will drop when filters get dirty. Using the average filter condition puts you in between. Hopefully the balancer set air terminals to more towards the +10% region and you had designed for 10% safety such that the airflow drop when filters get dirty is acceptable.
RE: What's the pressure needed for VAV box?
I'm doing a new system this week. Let me see what I can find out through testing and I'll get back to you.
RE: What's the pressure needed for VAV box?
Let me explain. Generally as the designer, you design everything around 1.5" - generally. What we do however is once everything is ready is we open all of the VAV boxes to their full max required CFM. Then we set the static to control the fan at 1.0". We see what happens from here. We then bump it up or down accordingly in order to achieve the desired CFM at all of the boxes (this also assumes all VAV's are to be at MAX on the warmest day of the year). Once we find the correct static, that becomes our setpoint (and it's never 1.5").
The reason we do this is because I noticed on many systems we had the fan blasting 1.5" on the days when we didn't need it and just wasting energy. We've done it like this ever since and have NEVER had a problem.
RE: What's the pressure needed for VAV box?
One note of caution, this job had a host of design issues, including some flaws with the air handler itself. The mixing box section was too short, and that caused the freeze stat to trip out on a regular basis. This unit is in Chicago. To correct the problem, we had to add baffles, perforated plate, etc. to help mix the air. This resulted in a higher pressure drop when running in economizer mode as opposed to a design day where OA would be minimum. This is not a normal situation, but we had to consider it in our static setting.