Pneumatic/DDC VAV Box Minimums
A little background on myself... I'm mechanical engineering student doing my third 6-month coop as a mechanical design engineer. I worked as a tech/engineer for Siemens last year, so I have some field experience with controls, but am a little foggy.
I've been asking around my office about VAV box minimum airflows, but can never get a straight answer. We use a 40% rule of thumb for boxes with reheat, and 25-40%(depending on who you ask) for cooling only boxes. I know the lower limit for a VAV box (from box manf.) is based on the velocity transducer in the controls. I also know that a minimum can go below this and still work fine. It might hunt a little more, and is more likely to have problems, but will work.
What is a realistic minimum for a box? How low can you actually go and have the unit still control for typical pneumatic and DDC setups?
Thanks in advance..
The minimun setpoint of the box is the higher of :
1) The minimum velocity pressure operating point of the controller
2) The minimum ventilation rate as per ASHRAE 62. Your offfice should have a copy, or you can go to the ASHRAE website & preview it.
This is where the question comes from. If I'm doing a tenant improvement, reusing existing boxes, they may be slightly oversized... I'd like to know how low a unit can go so I can reuse as much as possible. I'd rather not have to put in a brand new unit because the minimum for the zone needs to be 10cfm less than what the manufacturer says the box can do... And if I start bumping minimums up to match the manufacturers numbers, I'll risk over-cooling the space...
Originally Posted by flyrfan
I can't speak for other manufactures, but Trane boxes will still control in pressure dependent mode if CFM is too low for the flow ring to read. Whose controls are you using?
Last edited by mallron; 04-30-2008 at 02:58 PM.
The job I'm working on right now has existing Enviro-tec VVF fan powered boxes with (I think) Honeywell pneumatics. The building engineer has the boxes zoned out on each floor with around 5-8 boxes per pneumatic zone for occ/unocc control. I assumed the boxes were pressure independent, but am not certain.
Why not just use the minimum stamped on the vav box? To me that would be the most realistic value. The manufacturer has likely put the boxes through multiple tests to come up with that value.
Of course you can go lower but if the pressure transducer goes out of range and your vav controller starts hunting the actuator then of course will be driving back and forth, and of course the hunting will decrease the life of the actuator as you posted.
Now when you need to go lower what I do is look at the individual boxes' controllers info and find how low I can go before the airflows start bouncing around like 50-100 cfm. I do minor increases in my low flow settings and then once I get a stable reading again I'll use that as the low flow setting. The other thing to consider if you are getting airflow readings bouncing around due to too low of an airflow setting your heat may also be short cycling. The vav controllers I deal with have a built in safety in that the actual airflow must be at least 80% of the minimum flow set in the controller, otherwise the heat will not come on.
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Use the inlet size of the box to determine the capability of your minimum controlability. For pneumatic velocity controllers I would not go below an inlet velocity of 750 FPM.
Originally Posted by enerGwizz
Most of the stuff I wok with the vP sensor will read down to 0.01"wc and work. I like to see them no lower than 0.02"wc.
This assumes that the box is installed correctly. Too often there is a mile of flex or a hard turn right before the inlet, causing a poor reading. If this is the case, it will take a good deal more than 0.01"wc to get good control at the box minimum.
If the inlets are oversized, they won't control worth a darn. This causes the controller to constantly hunt from closed to barely open. It will wear out the actuator, and cause comfort issues.
Do yourself and the TC a favor..
1. Make sure the inlets are correctly sized.
2. Shoot any MC installing miles of flex or hard turns before the inlet of the box.
the boxes will be pressure independent if there is flow sensor and velocity controller,how well the box controls at minimun really depends on type of flow sensor,controller and inlet duct connection.Most mfg's spec state .03 to.04vp, but as stated by orion242 i also have hundreds of boxes that control at .01 to .02 vp.As far as the % of minimuns go i see engineered specs daily with 40 to 50% mins on cooling only boxes.In office spaces this does not work,you will subcool. i typically won't go over 15%. good luck
Originally Posted by enerGwizz
Generally down here rule of thumb is 10% minimum when you're not sure. Never had problems when I go with this. If you leave me some way to talk, I can contact you tomorrow.
Also. 10 CFM won't make any difference at all in the grand scheme of things so don't worry about it.. Make your minimums match the box minimums and you'll be okay.
Just to keep in mind. This is a situation I had not too long ago. We had a new office building we did that was 95% unleased and there was a minimal amount of boxes that were actually installed. I set the boxes with 10% for minimum. The situation was coming up that when the outdoor temperature was mild the building went into a negative. What happened was the building exhaust was overcoming the minimums in the boxes. The RTU would see the building pressure and open the OA to its max position, but with the boxes at minimum position it wouldn't move anymore air through the RTU.
So as just a little food for future thought, make sure that the box minimums are going to total at least your building exhaust.
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Wisconsin Code calculates a zone's cooing minimum airflow to be 1.5 air changes an hour for standard office use.
(Length X Width X Height X 1.5) / 60 = minimum CFM
Your minimum Heating will be calculated to give you your proper temperature rise on heating based upon KW or coil size.
I know the code changes from state to state but this may help.
OA CFM is also calculated on 7.5 CFM per person or => total amount of building exhaust.
We have to follow Title 24, which is California's adaptation of ASHRAE codes. Title 24 is actually more strict than ASHRAE, but at the same time, is generally not enforced at the plan check level.
Originally Posted by jslimjeff
We need .15 cfm/sqft for office space (which is like your 1.5 air changes with a 10' ceiling height) of outside air. Then you need to take into the account how much of your supply air is actually outside air.
15x10 office = 150 sqft
ventilation req = 150 x .15 = 22.5 cfm of outside air
ahu supplies 20% OA -> 22.5 x 5 = 113 cfm
If this is an interior office, maximum load demands around 120 cfm.
Therefore, cfm max = 120, cfm min = 113.. hmmm....
Setting minimums at 10-15% will make the box work well, but won't satisfy OA requirements..
BTW thanks for all the posts... Feel free to keep them coming