VAV System Control Sequence

[twisted pair]

AutoPilot,

Okay the boxes are all single duct cooling only. How do you provide heat? Does the AHU have a reheat coil or is there any baseboard radiation? Does this AHU have a morning warm-up mode? If zones require heat during the occupied mode it looks like they have to go to minimum position and that’s it.

Typically I see a low supply air temp control scheme with series fan powered boxes. Why is it a problem to use a supply air reset?

What is wrong with using an average temp of some interior and exterior sensors to reset the SAT. If you include the meeting rooms in the average I would think it would be a good start.

What did they require for VAV minimums? 20%, 30% or higher?

Why are they concerned about mold, because you are raising the SAT above 55 deg? Can you trend the space %RH with a return air %RH sensor?

[keepyoucool2003]

You said way back an engineer was involved, he designed it I'm assuming, he should be directing you what to do. Are the meeting rooms Occupied when your checking the temp? If all your controls are functioning properly (going to min) this all says return to engineer, the fact that it made it on here, says it even more, your banging your head against the wall. Just my two cents but I avoid leaving work with a headache.

There are quite a few possible changes to the system and quite a bit of work. With our company that equates to QUITE a bit of money. The problem is usually found by the control guy... and the control guy doesn't know the design conditions. Maybe it's acceptable for the zone temps to drop so many degrees below set point.

Anyway, I see there is more than one issue. You should put your boxes to full cooling, watch all the dampers, increase the static pressure set point until they all make. It doesn't matter what a mag reads compared to the control sensor, the mag isn't controlling the system. You lkily have a mechanical problem if the fan is at 60Hz and you can't make static, if your just controls that needs to be addressed by someone before you try to fine tune this system. Maybe it's a box that's opened wide open or something but short of an air balancer you can't know if that's it or the fan is flaking out. Anyway I have a project leader who asks me what the balancer was reading on his chilled water DP and what ours transmitter was reading. I want to smack him around a few times because he thinks we should control to the measurement off his gauges.

Orion242 you're correct the minimum cut off is .02"wc... typo on my part.
Static pressure is 1"wc new and calibrated in large main duct in good location with magnehelic Dwyer 604d I think (don't like em terminals are to small and I can't see in the dark). TAB reported all rooms between 68° and 72° with fan at 48hz. 1"wc at SAT 55.2°f while unoccupied
during TAB with fan scheduled on at 7am and off at 5pm. They set all box Kfactors, min, max airflows etc. Only reported one room with noise problems.

TP.. All box's are cooling only single damper with zone sensor. floor layout is elevator lobby, two large waiting room type areas at each end of elev lobby.
No interior/ exterior.. All have windows with view.

Some good sized meeting rooms and the rest are office areas. Elevator lobby and large waiting areas have good control. meeting rooms and offices get damn cold if SAT is at set-point 55°f.

I noticed on a Saturday with a few people at the site, things look good.
RAT @ 71 (warm for this system)
SAT @ 62 Fan 60hz static @ .68"
warm zone @ 74.6 (only zone with damper at 100&#37
cool zone @ 66.2

4 or 5 zones satisfied at around 50% cooling. all the rest are at minimum flow
with temperatures well below set-point. (2°f or more below). Tenants control
room set-point and most have them set at max 78°.

[twisted pair]

It doesn't matter what a mag reads compared to the control sensor, the mag isn't controlling the system. You lkily have a mechanical problem if the fan is at 60Hz and you can't make static, if your .

If the sensor was out of calibration, how would you know if you did not verify the reading. This would eliminate the static pressure sensor as the problem.

[AutoPilot]

Guys, thanks for all your help.
The Owner and Engineer have admitted (reluctantly, thanks guys) the space is NOT being used as originally designed. The system is less than a year old. After looking at some trends, one thing they changed was the time schedules for all zones, (was 6am to 7pm) now system is on 24/7.

It appears all over cooling problems started when they switched to 24 hour operation. Half the floor operates 24/7 while the other half is 9-5 6 days a week and all the vav schedules are set for 24/7. The vavs will goto 0 CFM if scheduled off, so they want release the vav schedules back to automatic for the after hours unoccupied offices.

Thanks Again
All your input is appreciated! Great Job.
Seems they always blame the control guys when the tenants complain

[klrogers]

Anyway, I see there is more than one issue. You should put your boxes to full cooling, watch all the dampers, increase the static pressure set point until they all make.

Just like Orion242 said earlier

"Diversity is my guess, ie the AHU total CFM does not equal to the total of the VAVs max CFM. Nothing strange there, at least in our area it’s pretty common."

You will not make static pressure set point on MOST systems if all the VAV boxes are in full cooling.

Kevin

[keepyoucool2003]

Since were going to beat the dead horse here, I was going to let it go, but no. I had to read this three times to follow because the there wasn't much logic to the madness. Relating SAT to static readings, then talking about TAB with fan schd on and in unoccupied. Well that should have been a clue there to him. I missed it, but I'm not on the job or being paid to KNOW the sequence, and typical operation. Sounds like there isn't much working relationship between control guy and the AB. We usaully talk to each other so we understand what conditions the system must be in for the balance to take place, and why. In this case, low box CFM flow so everything is controlling and the system can maintain a setpoint. At some point the balancer must have asked "can you put everything to unoccupied" I'd wonder why, MAYBE that's just me. End of me reading this tread since the subject was closed by the guy asking the questions.

[crab master]

First of all I just briefed this, but if you are over-cooling the building with a chilled water system I would think the fix here would be "relatively" simple.
1. You do not have to maintain a certain airflow to protect the equipment as you would in a DX system.
2. If all the zones are satisfied why continue to supply cool air to the zones? If you only have 15 zones then why not monitor the zone temps vs. the setpoints and allow cooling only when a zone needs cooling? Granted you will have to make this somewhat "sensitive" so that when a conference room gets loaded up in a hurry the system can handle it. Maybe you don't have all that data available? Granted this design goes away from a VAV design but why waste the extra energy when the cooling is not needed?
3. I don't know the type of controllers you are using but also why not pass the the supply air temp out from the air handler to each VAV box so if it has the capability it could change it's own airflow settings based on whether or not the VAV controller thinks the plant is in warmup, central plant heat, or cooling mode?
4. I would also look into adjusting your minimum flows on all the boxes to a lower value.

What type of controllers are you using? Maybe someone here is more of an expert on the Brand and type and can give you more insight.