Vav Box Trouble Shooting

[allstar08]

I am going to be working on these things a lot, and they are new to me. I have a few questions:

1. What is the best way to verify signal at the box?

2. The most common problem you guys have seen with them?

3. Are most of the actuators 4-20ma or 2-10vac?

4. What is the most common duct static that these things see on the input and out put?

[makinmoney]

I guess the first thing I would say is get to know the difference between a "series" VAV versus a "paralell" VAV. Most of the problems that I encounter with the boxes comes from the BAS system that is controlling them...I.E. Johnson, Staefa....etc. Normally it's a problem that wasn't picked up during the original commisioning of the BAS to the VAV's.

A VAV box is a very simple apperatus that modulates primary airflow from a RTU thru the box into a designated area. The box will have heat capabilities if it is a perimeter box. It could be done with electric heat or hot water (coil). If a call for heat comes from the zone, the box will throttle the damper back and use Plenum air with the fan inside the box engaging. If it's a series box the fan runs constantly. The box will the engage the heat wether electric or hot water. Electric should be staged and hot water should be modulated.

The Acuator will be the primary means of communication to the BAS and will also control the heating capabilities of the box. It will also engage the fan on a call for heat in a paralell box.

[d_lee1]

emoney its according on who makes the box. I work on alot of reddi boxes that are phnematic that gets a air signal to operate..been converting alot of those over to ddc carrier comfert id. which you have to have a computer to work onthem.

[madhat]

I try to get the VAV and Controller OEMs service instructions. Then when I work on a box I write my own service instructions. This gets rid of the generic crap. Also If I have a box I've never seen, that's not working, I take a look at a box that is working and take notes. Right now I'm looking at 14 EAVs that appear to be installed backwards, with no instructions, and nothing on Krueters web site.

[allstar08]

For example if I go to a building and I have a customer complaining about no heat in one area, and that area is controlled by a Titus FCU with electric heat. How would I make sure it was a control issue, and not that of the box? How would I verify that the box was not getting signal. Would I go to the front end and check the CPU, or do I go to the box and see if the controller is getting signal from the comm buss? How do I verify signal from the comm buss, that I am getting signal to turn the electric heat on?

[Dallas Duster]

You need to go to the front-end and checkto see if the "element" is up and communicating and the check what the unit mode so you can go to the "element " comfirm its proper operation

[maxster]

YOU will be an expert after 2 months of doing them .....the main unit should be banging 1"-1.5" static when in occupied at 55F supplied to the main trunks before you do any VAV check the unit for operation and the above values.TAKE NOTE....many boxes call for 4X the diameter on the inlet duct work as a straight run into each box off the main trunk for tighter control on the A/B pick-up rings.2-10V with 2V being minimum and 10V being the maximum setting,to check the minimum air put stat to highest temp and adjust...all the way down to lowest temp and it will give you maximum air.remember the VAV application works on ther being a self generated heat load within each space...lights...computers...people so most of your cold complaints will come in from morning warm-up being satisfied and the work force coming in...always take the temp (or off register)within the duct going to VAV/space.always walk the space to see if others are comfortable before changing any unit temperatures on the main unit.MWUP is the last heating cycle before the unit goes to OCCP fan or cooling.NIGHTSETBACK is typically 50F mounted on a perimeter interior wall.when either happen the VAVs should dump to full open(power taken off boxes) and the fan should be driven to 100% air basically the system turns into a CV unit for the heating cycle...clean filters are a must for true VAV operation if the air changes between the inspections the individual A/B rings will pick up the air changes and balance it out so all the boxes get something.TRICK on setting the Max air....if you have no specs adjust it to a point where the you barely hear the air overhead being discharged...straight SUN LIGHT into a corner office eats up the space and isn't considered part of the load tweek the blinds so the sun is off the sill but the worker can still see out.if a rooftop fails in cooling and is reset to run the space will be warm,and most of the boxes will go to max and you will not see major static until the space cools down,and the VAVs start to work towards min settings.most VAVS require at least .5" of static at the rings to at least move

[jogas]

One of the problems I run into a lot is installer did not follow the manufacturer's requirement of X diameters of straight supply air duct entering the box. This insures even laminar flow across the flow sensor and reduces or eliminates turbulence.
Most, if not all boxes have a CFM / Differential Pressure or CFM / VDC graph to indicate box flow providing you've got minimum inlet supply air static pressure.
But realize most boxes do NOT have the perfect inlet straight duct and still manage to operate. What happens then is the graph values shift. I then use a FlowHood to prove actual CFM to commission the box.
I have had boxes with elbows at the inlet that had to be re-ducted to provide straight pipe for the flow sensor to track properly through it's range of deadband CFM to Max Cool Design CFM.
Get the Manufacturer's commissioning instructions, follow them to the letter as it applies to your job. If any problems arise, call them, they want to see their product work.
jogas

[allstar08]

Awesome info guys thanks!

Are most of these systems CV and the VAV box regulates at the space?

Is the barometric relief fan always on, and what regulates it? I have some of these units on some Trane RTU's at a college I work on, and it seems like they are always on. Do they work off of a different system, and maintain a positve building pressure? I have always been told to keep buildings in a slight positive.

[jogas]

Awesome info guys thanks!

Are most of these systems CV and the VAV box regulates at the space?

Is the barometric relief fan always on, and what regulates it? I have some of these units on some Trane RTU's at a college I work on, and it seems like they are always on. Do they work off of a different system, and maintain a positve building pressure? I have always been told to keep buildings in a slight positive.

VAV is the best, CV with bypass IMHO is a cheap POS.
the Trane RTU's sound like they have the StaticTrak option. Get the
IOM (Installation, Operation, Maintenance) Manual for the unit(s) Model.
Good Luck,
jogas

[duke of earl]

A true VAV box consists of an air valve and maybe a heat source only. This truly varies the volume of air to the space based on the space temp deviation from setpoint. A box with a fan(series or parallel)is more of a mixing box because it mixes plenum air with supply air from the air handler. On a series fan powered box, the fan runs continuous and the air valve modulates between a minimum and maximum position creating a leaving air temperature to balance the load in the space.On a parallel fan powered box, when the space temp falls below 1 degree under setpoint, the fan is energized and is considered the first stage of heat.This is called fan offset. If setpoint is not made then second stage of heat is energized. Never use pulse width modulation for electric heat as you will never make setpoint and your tannants will be cold. This is in the controls setup(BAS).This method pulses the heat contactors instead of leaving them energized until setpoint is made.A properly setup vav box will provide reasonable comfort but usually lacks in the heating mode because of minimum airflow for fresh air blowing down on the occupants after setpoint is made. Resetting the air-handler supply air temperature up a few degrees can help make for a more comfortable system overall as long as the oa enthalpy is not too high.Hope this helps.

[allstar08]

Most of the vav boxes I have seen use a plenum return air and are branched off of a main supply air duct. Would this be a series or parallel application?
Also if I had a hot water coil would PWM work better then constant so I could modulate according to demand?

P.S. what you are saying is helping. Thank You

[maxster]

if your walking past a rooftop and the exhuast fan(dampers closed) is running that is telling you the OA dampers are moving around and your economizing.those dampers on the exhaust are waiting for the space to pressurize like a ballon and they will srat to open to dump some of that return coming up from the space.....that pressure is senced from the space either a 1/4" tube sticking thru a tile in the cieling or behind a stat cover...give us the model/serial on one of those rooftops and i will give you info on that design.this balancing of building pressue is all TRANE designed within the rooftop...VAV is always modulating of the main air supply into the space...CV is only applied in MWUP...NSB in HEATING modes...if you had VAV boxes and a CV unit were would the air go when the boxes satisfy?that is what the vfd/igv are or to back the air down when the staic setpoint is reached.i think on those VAVs your talking FAN-POWERED VAVs mostly used on the perimeter air..if they have a filter and a fan within thats them...the ducted air is for normal VAV cooling as interior has,but when they call for heat the supply air closes he fan comes on and pulls return plenum air in and heats it up to make the setpoint.the reason for these o the perimeter is during the winter you have those exterior walls with cool air in the main ducts being supplied,your better reheating the plenum air then 55F main air...and there is no heat cycle on VAV after the unit comesinto OCCUP MODE...imagine changing the rooftop unit to heat the perimeter air the rest of the space would go nuts with 70F air being suppied..interior space don't have that wall loss the perimeter has.FAN-POWERED VAVsare totally stand alone in operation and the only common ite they have with the rooftop is the ducted supply air.

[duke of earl]

emoney,

Series/Parallel refers to the type of box in itself. If the fan draws air from both the plenum and the main supply duct, it is a series fan powered box. If the fan is mounted so that it does not see supply air and only pulls in plenum air, it is a parallel fan powered box. A series configuration reduces the horsepower requirement from the air handler fan since the vav fans run continuously.

[allstar08]

Duke,

So the one I am working with must be a series box. It has a duct coming from the supply that is high velocity and it is pulling from the ceiling return system. The fan on it does run full time. I know because I had to go turn it down to its slowest setting, because it was too noisy. So we take high velocity air we pinch it with an actuator and push it out with a fan to meet the design requirements. Then a element inside will come on to heat later as needed. Am I getting close?

[makinmoney]

If what you say is accurate then you are working with a series box. If the heating element or coil is after the fan then it is most likely a series box, if it is behind the fan, and the fan comes "on" with a call for heat then it is a paralell box........

Keep going you are getting it!

[Control Man]

You are taking the right approach , if not sure ask or discuss it.

My biggest problem found with VAV boxes is that they get played with by people ( building operators ) who have no idea of what they are doing, they make drastic changes then its the control techs problem.

[duke of earl]

emoney,

I would caution you about changing fan speeds because they are typically set by the TABS people (test and balance). You may be creating a problem by making these changes. I would look at the balance report and see where the speed is set and stick with it for now. It is possible someone changed it before you got to it and maybe you got it right by changing it yourself but who knows. By lowering the fan speed you may have reduced your plenum air intake which is your first stage of heat. When the air valve goes to the heating minimum position, the fan is drawing mostly warmer plenum air. Now bring on the electric heater (usually borderline sized at best)and you only have to heat the warmer air. Also you have to have a certain amount of airflow across an electric heater to keep the manual reset hi-limit from tripping. See where I'm going with this? Some heating minimum airflows are set pretty low so you are dependent upon proper fan cfm to prevent heater trips.

[allstar08]

You are right, and you should always check. I did, this time anyway, check with the balance report, and I was there with the balancer and a hood at the time I changed the fan speed. The box in question was specked out over sized so we had to do a lot of dampening to get it in spec.

[duke of earl]

Yeah the engineers tend to oversize the boxes cause it makes them quieter. Good job on checking the balancing. Too many techs just go in and wreck a vav system because they don't understand how the systems works and they start tweaking stuff thinking theyr'e fixing the problem.You are doing the right thing by asking questions first before doing something that may cause problems down the road. I will check your profile for your email address so I can send you something that will help you learn vav more in depth.

[B_roche]

Duke I hate to be a pain but would you mind forwarding that to mine as well? I really need to learn more about this.

broche404a@yahoo.com

[tech202]

I could also use that link to learn more about VAV's.. apreciate it.. Greenears, just in case Duke don't get back to me soon, you could forward that this way, I would apreciate it.. My email adress is hbtstar@aol.com

thanx guys.

[Shophound]

For example if I go to a building and I have a customer complaining about no heat in one area, and that area is controlled by a Titus FCU with electric heat. How would I make sure it was a control issue, and not that of the box? How would I verify that the box was not getting signal. Would I go to the front end and check the CPU, or do I go to the box and see if the controller is getting signal from the comm buss? How do I verify signal from the comm buss, that I am getting signal to turn the electric heat on?

I always verify what the front end is indicating the box is attempting to do. In my case, the front end does not control the box, each box has an application specific controller (ASC), but the front end will attempt to reflect what the ASC is doing.

Typically I can derive via reading the front end that something may be wrong with the box. For example, the primary air is showing to be wide open but the zone temperature is not anywhere near set point. That means get the ladder and go above the t-bar to see what's going on...probably the actuator slipping on the damper shaft. Zero out the actuator, set the shaft manually closed, tighten down, light the box off again and see how she behaves. Keep an eye on it until it acts like it is supposed to.

In my case, where the ASC's operate stand-alone, the communication to the front end is for monitoring and control purposes. Problems with analog inputs/outputs and digital inputs/outputs will be diagnosed at the ASC vs. the front end. That is when a meter that reads miliamps and DC is handy. Even better, a signal generator that can create both signals, so you can verify if an actuator, for instance, responds to the proper signal when you intentionally create one right at the box with the generator. A good signal generator can save a lot of time and grief when dealing with controls troubleshooting.

[airdata870]

hey shophound brand of signal generator do you use? i am looking for one, any info would be great.

[Kool Beans]

I agree with maxster.

[Shophound]

hey shophound brand of signal generator do you use? i am looking for one, any info would be great.

I am completely having a brain fart right now as to what brand the generator. I want to say Extec...when I get to work on Monday I'll verify it and get back with you. Sorry for the sudden bout of CRS. :o

[jogas]

shophound,
I have looked at signal generators, but never have been able to justify their cost.
Here's why. When I'm checking an actuator problem, I'll set the front end or room set point to call for 100% cooling demand. I'll disconnect the actuator wiring from the output, and sometimes, if I have to, I'll disconnect the actuator from the damper shaft and close the damper to insure a 100% output from the controller. I then use my Fluke to see what Ma or VDC signal is being outputted. This isolates the problem to either the output or actuator.
I'm not saying not to use a signal generator, but I've so far been able to work without one. It's also one less thing to carry/maintain/try not to lose or get stolen.
jogas

[whec720]

Interesting jogas. I'll have to try your tip some time but for those who like toys, (I'm guilty of this, BTW), here you go:

http://www.kele.com/olcat/TE7/ASG-DS.PDF

[Shophound]

shophound,
I have looked at signal generators, but never have been able to justify their cost.
Here's why. When I'm checking an actuator problem, I'll set the front end or room set point to call for 100% cooling demand. I'll disconnect the actuator wiring from the output, and sometimes, if I have to, I'll disconnect the actuator from the damper shaft and close the damper to insure a 100% output from the controller. I then use my Fluke to see what Ma or VDC signal is being outputted. This isolates the problem to either the output or actuator.
I'm not saying not to use a signal generator, but I've so far been able to work without one. It's also one less thing to carry/maintain/try not to lose or get stolen.
jogas

Well, I'm an in-house guy, and tend to frame my responses along those lines at times. The signal generator I use has been handy for setting up actuators after they have been replaced, and verifying actuator operation without running back and forth between the front end. Sure...I could hook up my Fluke and verify DC or milliamps, and sometimes do that. The signal generator I have can also read both, as well as create both. That's handy. When setting the span and range of my Honeywell mod motors, the signal generator is wonderful for that task.