Trane Summit DX VAV setup Question

[Ancoma Tom]

I am involved with a Trane Summit system and three of the AC systems on the network are DX VAV systems. The VAV AHU systems are set up to control to a supply air temperature and the VAV's control to their zone temperatures. The problem is overcooling of spaces. As the VAV boxes satisfy and close to their minimums the AHU continues to pump out its 55 degree or 60 degree air (depending on outside air temperature) and overcools the spaces. I thought the VAS settings were to prevent this by shutting off the air handler but this doesn't seem to work as I thought it should. Anyone with experience in this? Thank you.

[toocoolforschool]

What happened to the reheat?Doyou have hot water reheat or do you have electric heat strips.

[Ancoma Tom]

All the zones are satisfied but the AHU continues to cool. Really don't want to have the reheat come on when its 90 degrees outside, so it is locked out.

[toocoolforschool]

Your system is designed for reheat. You could always overide the damper and close it 100%. Or you could change the supply temp. setpoint. Set it a couple of degrees higher seee if that helps tempur the space better.

[Ancoma Tom]

When you say override the damper, I assume you mean the VAV box dampers. Since it is a DX system I can only go so low on the minimum air flows without worrying about low airflow/flood back to the condensing unit. The discharge air temperature from the AHU is setup with a reset program that keeps the SAT setpoint at 55 until it gets down to 80 degree outside, then it resets up to 60 degree.

[toocoolforschool]

Time to fire the boilers

[maxster]

what is the static set point there whe IGV/VFD should be backing down as the boxes start to move from max to minimum air then the DX will react to the lower air an make set point and unload or cycle off.

[Ancoma Tom]

I have dropped the static from 1.5" to 1.3" and it is controlling fine. Just leary of dropping the minimum airflows on the boxes too low since it is a DX system. You are right that the lower air flow will cause the condensing unit to cycle on it's low pressure switch. The problem with the spaces overcooling may only be remedied by lowering VAV box minimums even more.

[maxster]

is the fan VFD or IGVs might try 1" if the air handlers are on the floor being conditioned 1" might help that minimum over cooling.two location questions...where is the discharge air sensor for th DX,and then where is the static sensor.outdoor air reset is to high keeping the DX banging away not letting it react to the drop in static when you go to minimum.if the reset wasn't in the picture the static air would control the cooling with the air changes going thru the AHU.

[LU91Tech]

Lowering the static shouldn't help. The vav will only compensate to maintain cfm min/max. Sounds like you have too much airflow/cooling for the heat load. Is the space occupied or vacant? Was an engineer involved and a test and balance done? I always prefer to use space temp reset rather than outside air temp...seems to work better to me.

[Ancoma Tom]

Maxster,
Presently IGV, soon to be converted to VFD. AHU's on the same level as the boxes and only 6-8 boxes per system. Not sure what minimum inlet static pressure the Trane boxes need to operate correctly. I am not exactly sure where the discharge and static sensors are located but I don't think they are the cause of any issues either. I don't follow what you mean by "outdoor air reset is to high", if anything it being down at 55 most of the time is causing the overcooling.

[Ancoma Tom]

I agree that it seems like too much capacity for the load. It is an occupied space, in operation for 10 years but not very efficiently. Just recently got involved with the account and trying to save energy and make the occupants comfortable. Space temp reset may work better than OA as you say. Trane used an archaic programming method to do the reset control, almost looks like DOS. I'm not sure how easy it would be to change the reference from OA to space but I could look into that.

[19characterusername]

Running VAV w/o reheat is doable but it requires return air reset, average or critical space reset (easy with DDC boxes). For return air reset make sure that sensor doesn't see mixed air or you under cool in moderate condition and overcool in hot conditions. Return air reset works well when the boxes aren't already networked with the ahu. Most VAV ahu controllers should have some programming for resetting rthe supply air temp based on....

The lead comp on a DX VAV system needs hot gas bypass, full stop. Check if you've got this. It may be worth retrofitting and setting the bypass to maintain a minimum suction temp.


Another cool thing is static reset. The fan runs a starve box program that gets boxes to 95% open but maintaining their CFM set point. Huge fan savings. On your system you might be able to get away with as low as .3-.5 entering a box (you need to work out what that means at the ahu).

[maxster]

i took it OA was resetting the discharge air setting?the static sensor and discharge air locations are critical to the AHU reacting to the swing from anything but max air.when that VFD is installed the ramping signal is going to depend on it being in a general location to track the air might want to look into its location.8 boxes on the same level as the system 1.5" is to high for such a medium application.one thing i've seen with VFDs when the space is comfortable and the boxes are floating to maintain area set points..the VFD screen should be at 25Hz to 30Hz

[Ancoma Tom]

maxter,
Yes the discharge air is reset from OA. Above 80 OA it maintains 55 DA, below 75 OA it resets to 60 DA. Hasn't been too much time below 75 yet here in Florida so most of my experience on this account so far has been with 55 DA. I did drop the static to 1.3 but will try going down to 1" to see what effect that has.

[maxster]

tweeking that static by 10ths is a good way to go trying to get the minimums not to cool the spaces.anoter question,how are the compressors being staged thru either a UCM/TRANE style digital controller or....VAV rooftop Trane staging controller(W7100 type item).the maximum air setting for the spaces on the boxes are you hearing major air out of the registers during cooling....any self powered VAV reheat units around the perimeters...

[Ancoma Tom]

Being staged by the Summit outputs directly to LSV's and compressor contactors (2 stages). Haven't noticed any objectionable air noise, but haven't really listened while they were calling for maximums either. When you say self powered, if you mean fan powered boxes, no none of them on this job, just standard VariTrane dampers with electric reheat..

[alcomech]

AREA: This is where you will set up temperatures and individual set points related to individual areas. You can set AREAS to control any group independently from other AREAS such as office space, floors, common areas, warehouse, etc. etc. The area itself is unique as your space can be subdivided for individual temperature control, and also has the capability to have different start and stop times. This is helpful when as an example you have subdivided areas and or have different tenants on individual floors or areas which require different schedules such as holidays or if necessary exclusion dates can be created. This information is available for your use at the bottom of the screen located in the help menu. You should familiarize your self with the help screens which are available at all windows. It is very helpful to have more than one AREA so that individual times can be established.

VAS: Variable air volume air systems (VAS) is made up of VAV boxes, air handling equipment which supplies hot or cold air to boxes.
In addition ventilation equipment such as Outside air VAV boxes, make up air fans and building exhaust fans. VAS works with AREA CONTROL where it receives start and stop status through the TIME of DAY SCHEDULING. Remember it is the AREA which helps manage the VAS to control operation of all the VAV boxes and air moving equipment.

VAV boxes are listed as MEMBERS which are related to the VAS and controlled by the AREA which it serves.
This is where you set up the votes for your VAS and when changes are made you will need to reset the votes. This needs to be checked and if not maintained will give you an erratic control which you are experiencing. Such as in the case where a remodel has occurred and originally you had 10 VAV boxes but you now actually have only 8. This will alter your votes.

CPL: Custom Program Language allows for custom programs and calculation which cannot be done in other applications in Tracer summit. This is somewhat difficult to do and changes to CPL are not recommended at first. In order to familiarize your self you may want to open CPL and print out its program. When exiting do not save changes. You should be able to review the list of VAV's and UCM's which will be responsible for the temperature reference. This is accessible and listed in the CPL editor. You will need to locate the VAV's in CPL and verify they actually exist as well as positioned where there temperatures are accurate.

As far as the set up of the DX system you should set the static pressure to 1.5" W.C. which is the design of your VAV box and will give you the best accuracy and a good starting point for the commissioning of your system. This VAV box is capable of working with less static and much more up to 3" but should not be necessary for any setting other than 1.5" especially when balancing.
You will need to verify actual static pressure 2 thirds downstream of the air handler with a Magellanic gauge to confirm accuracy of your set point which controls inlet guide vanes. It is required to have a higher static than some have suggested so that when under a full load you will have an average of 1.5". Unless you want to stand around and regulate this control as needed...Let the equipment do the work. Lowering your static will do nothing whatsoever. As a matter of fact, the higher air static will identify problem areas such as dumping from a VAV box which has a bad motor or air valve. This can be checked by overriding control and closing or opening the air valve and checking air flow at the display.

Your balancing will be set up as two positions- Minimum airflow and maximum airflow. Minimum is based independently for cooling and more important for heating which will require airflow to meet its requirements. You will need to command the VAV's to self calibrate which is done in the set up. This is done by selecting calibration and choosing a Binary output for its operation. You will need to save it and wait a minute. Then choose its reference which is indicated by the threes (...) as its reference.
In addition the calibration should be scheduled to perform by itself which is done through scheduling just like you would start and stop. This may need to be set up in CPL if its not already set up.

It is not recommended to alter the set up for the VAV's other than set them up as their actual size. If you have a 12" round inlet it should be set up to it's corresponding CFM output which is around 2000 CFM. The air flow ring is designed to individually fit each different size box and accurately measure CFM.

I hope this helps.

Trouble shooting is not part of the repair............understand the symptoms and you will find a solution.

[gwoner]

Why not just raise the cooling setpoint on the individual vav box to please each zone thats an issue. This can be done in Tracer.

[joey791]

Tom,

While you have been receiving great information in this thread(minus a rebuttal against one of maxsters post), one other thing I have come across with the same issue you are having at more than one location. When you are checking your static pressure are you doing it from the front end or have you actually took your magnehelic and checked it. I've seen jobs designed to run 1 to 1.2" that with the IGVs completely shut the system would maintain 2" because of the air bypassing through the center part of the IGV where the shaft passes.

Can you get a set of plans? If so have you checked some of your vav settings versus what the plans say? Has this been an ongoing issue on a new building or old building? Have you also checked to see if the static pressure sensor is set correctly or the right range was used for the job? For static I usually use a 0-5" sensor.

Again everything that has been posted has been right on, just something else for you to look at.