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.

[alcomech]

Have you looked at the "AREA" set points and set the heat/cool set points there. How are you reading the space temperature? Is it set up as an average through a few stats or do you have a single zone sensor.
What is controlling the DX units? Are they controlling themselves or is Tracer Summit controlling.
These are just basic questions and with this info it can be determined where the control is getting its reference from.


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

[alcomech]

As far as the VAV's are concerned they they are controlling based on supply air temperature when in either heating or cooling mode. What is the size of one VAV as an example? What is the minimum and maximum air flow set point? Do most of the VAV's have reheat and if so what type? Do the VAV's have a discharge air flow temperature sensors?

[Ancoma Tom]

Each VAV box/zone of course has its own sensor.

I believe the reset was set up through analog outputs, there is a discharge air setpoint and if you go under setup, it looks at a property reference of a CPL program (OA reset). I've tried to not use the property reference and use a default value of 60 degrees to see if this might work better.

The Summit is controlling the DX.

[Ancoma Tom]

VAV's are controlling by their own wall sensors.
Biggest box is 1500 cfm, smallest is 90. Average box size around 500 cfm.
I tried setting minimums to about 50% but still overcooling some of the spaces.
Electric reheat but when it's 90 outside I really don't want to even think about running that! Only one box has a discharge air sensor.

[alcomech]

Each VAV box/zone of course has its own sensor.

I believe the reset was set up through analog outputs, there is a discharge air setpoint and if you go under setup, it looks at a property reference of a CPL program (OA reset). I've tried to not use the property reference and use a default value of 60 degrees to see if this might work better.

The Summit is controlling the DX.

As you know the VAS is controlled by the schedule of the AREA which has its own individual set up and temperature input which will dictate the MODE of the VAS either in heating or cooling mode. As an example and depending on the season you can set up the AREA for 69 degrees heat and 72 degrees cooling. If the AREA is is below 69 it will be in the heating mode, and above 72 it will be in cooling mode. As you know the VAS space temperature is done in your case through CPL which will be based on an average of say 4 stats.

For a reference only and (not for actual control) to verify space temp with CPL, you can also set up the DX unit to indicate space temperature by a zone sensor in addition to referencing space temperature through CPL. If the DX does not have a space temp indication you can simply at it by modifying your graphics and reference a zone sensor. This is sometimes helpful in the event of com failure for say a package unit it will default to minimum set points.

Again the AREA and VAS are two separate configurations and should not be confused.

Reset can be done at the individual DX unit based on OSA, Return air

[alcomech]

VAV's are controlling by their own wall sensors.
Biggest box is 1500 cfm, smallest is 90. Average box size around 500 cfm.
I tried setting minimums to about 50% but still overcooling some of the spaces.
Electric reheat but when it's 90 outside I really don't want to even think about running that! Only one box has a discharge air sensor.

Unless you have a minimum requirement of air flow so as to maintain positive pressure or minimum OSA, then you should simply be able to minimize air flow to a much lower setting when cooling has satisfied and the VAV box is at its minimum position. You may want to consider setting minimum air flow for the biggest VAV to no more than 200 CFM and as low as 150. As far as the reheat is concerned if you are over cooling and dumping air you may want to consider raising your DX discharge set point but not to the point where humidity is an issue.

Again if you can not lesson the amount of air, you don't have much choice but to use re-heat.
The VAV will not accept a value less than 10 percent as a minimum CFM air flow setting, but it will accept a 0 CFM value if needed for say as an example a box that does not have reheat.

It is important to check the setting of the AREA it self which controls the VAS and will have a great effect on the MODE of the DX unit. The VAV will know through software what mode it is in an is also displayed in VAV - MODE. Remember if your average space temperature is cooled to set point it will not be necessary to run cooling and if staging is available it will unload. (unless you have a humidity issue)

Hope this helps

[Ancoma Tom]

alcomech,
Thank you for the thorough replies. As I understand from your post, AREA controls MODE, so in your example as the temp drops from 73 to 71 will the system stay in the cool mode until it gets down to 69?

As for VAV box minimums it appears that if the box is a Trane 1700 CFM box but is is set up for 600 CFM maximum, the lowest minimum that can be programmed would be 170 CFM. That actually ends up being closer to 30% minimum. The only way to get a 10% minimum would be to tell the system that it is a Trane 600 CFM box, right?

Regarding your statement - "Remember if your average space temperature is cooled to set point it will not be necessary to run cooling and if staging is available it will unload", i have observed that if my average zone temp is less than the VAS cool setpoint, the cooling continues to run to maintain DA temp, is this how it should operate?

In setup, if I go to VAV, then VAS, Supply Air Setpoints, I can input my cool and heat temps that are for MODE, correct?

Then if I go to Inputs/Outputs, Analog outputs, AHU-x Discharge Air Setpoint, Setup is where I can select between a Property Reference, (CPL Program, Supply Air Reset) or no reference if I wanted a constant discharge air temp, correct? If I chose property reference, CPL Supply Air Reset then the discharge would control from the CPL, correct?

It appears if I go into Setup, Unit Control, PCM, AHU-x, Setup A, Setpoint value for DATMPSTP, Select Property reference, I can also select between the AO or CPL. Do these do the same thing?

[JNXMN]

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.

You'l need to a relatively constant flow across coil for a DX system. You'll need to have a by-pass with motorised damper and pressure sensor from S/A to R/A so when the VAV close up the damper the by-pass terminal can relief excess air and maintain pressure in the supply terminal.