HVAC Control problem!

waterlucky,

What kind of "lab" are you dealing with?

My first guess as to the lab having thermal sensors in the return is that the lab has a significant amount of exhausted (non-recirculated) air, presumably from "fume exhaust hoods", and sensing the supply temperature might not adequately take the hood exhaust into account.

Having sensors in the return air would accurately determine the temperature in the occupied space, regardless of whether the hoods were operating or not.

Your ramp-up/down should be longer than AHU 1 & 2, because the effect of the damper setting will take far longer to be detectable at the stat. Also, your control linkage could be 1,) too far out on the motor actuator, or 2.) too close (to the pivot) of your damper linkage quadrant, resulting in over-response of the dampers.

You could set a duct stat in the supply that would limit the operable range of the dampers based on a closer range than the return stat, say 18 to 20, possibly resettable based on demand/cycle depending on the weather extremes.

Pneumatic? electronic? How large a space, unit, airflow?

A little more info might help.

Rich

Also, heating, cooling, or both? Day/night? Preheat/cool? Freeze stat, on heat, cool, or both? Occupied/unoccupied function?

Rich

It is possible that the PID loop is causing the "hunting" in the system. If the RESET RATE is too fast, it could cause hunting. I don't know if the VFD ramping rate would cause hunting, but could cause other control problems or unacceptable environmental conditions. R.A. control should only be used when there is no acceptable location for a Room sensor. One solution you might consider would be to incorporate an AHU supply air controller and reset its control point with the R.A. control. You are aware that rapid swings in temperature and or O.A. & R.A. quantities can keep the system out of balance, and create undiserable humidty problems.

Dear Waterlucky,

As a general rule, particularly where large quantities of freah or exhaust air are concerned, you should NEVER contro; from either return and certainly not from supply air. Space temperature at occupant level is the only way! Particularly if you have large heat generating loads in a zone such as solar or machinery.

Hope this helps

Phil L

Good Morning Waterlucky,

Quik question, is the system direct expansion or chilled water?

I work with a large chill water system, chillers, and over 50 AHU's. Most, but not all of the time, it is a matter of getting the PID loop set correctly. Don't go into it messing with settings if you're not sure of what you're doing, it will only make it worse.

A thermostat is essentially an on/off switch, with two set points, depending on how you define your comfort zone. Putting the sensors in either the inlet or outlet air flow will not accurately control the conditioned zone. Put the sensors in the occupied areas, and then adjust the set points wide enough to eliminate hunting.

Here is some food for thought. A constant volume (CV) air system supplies constant air volume at variable air temperature to a space. The sensor is typically located in a space, or in the return air duct to sense an average of multiple spaces. A variable air volume (VAV) system supplies air at variable volume, but constant temperature. The sensor is typically in the supply air. VAV systems then have sensor and modulating dampers for individual zones You may have a combination of CV and VAV systems at your site.

Large fluctuations imply "Hunting" which means you system is modulating either too fast, or too slow, or too greatly. Monitor the different modulating devices, to see which is changing too quickly. If possible, put some of the devices in a fixed position while you check the modulation of others. Good Luck!

Thank you very much all for your generous comments! :-) Hope to share..

currently we are having greatest temp control problem ["hunting"] in the lab; some lab rooms are using CAV while lab offuce use VAV system; all these ducting comes from the same AHU. we have tried to swap the temp control instead from the return duct side into supply duct side. Inaddition we have increase the supply stat. pressure. we are currently monitoring the "hunting" situation for a few days to see how...

However, we did not change any VFD setting as yet... Btw, has anyone any experience of Danfoss VFD? any troubleshooting guide to recommend? Thanks..

go to the dan foss website, you can download any info you need and they have a free on line training website

To troubleshoot the systems:

1. you have to draw all components and their auto-control trade-off connection. I mean units with dampers, sensors, valves, etc.

2. To build the process in the Psychometric chart.

3. Analyzing these trade-off you can find the solution.

I can help you if you send to me the data < apcco@verizon.net >

RE: My previous note...........You cannot just swap controls. Controlling the discharge air by resetting it through the Return Air Sensor will stabilize the discharge air temperature, preventing those large swings in temperature. The R.A. tempeature control measures the average room temperature which generally changes rather slowly. If there are large and / or rapid changes in room air temperature, the air circulation rate could be the cause. You have not stated how the VFD is controlled. If through static pressure, then the reset is much too large. During normal operation of the VAV system, the duct static pressure will change as the VAV units reposition themslves. When the VAV units begin to close on the supply air side, the system static pressure will increase, causing an increase in air flow in the CAV system, etc. If the VFD is not properly adjusted, it will allow the static pressure to vary, possibly causing the hunting condition.

If you are having humidity problems, it could be caused by the amount of outside air being introduced in the system. You can either pre-condition the O.A.(energy recovery coil or wheel) or add re-heat humidity control to the Lab CAV system. In either case, make sure the O.A. and R.A. are properly mixed in the air handling unit prior to entering the coil section.

g scott

can you send more info, like what typ of controls do you have and how old are they? also are the labs clean rooms if so what class are they? is the system built up dx or is it chilled water? what typ of reheat do you have if any ? hot water steam or elect. how much osa are you useing

whether unit no 3 $ 4 are isolated. or the return air for both r same.