VVT Help Please
One of our buildings has a 30 ton Carrier (48AKE030) with gas heat that was originally setup with a VVT system with 17 zones. The building has ceiling return. This unit replaced the old unit 5 years ago. The old system used a constant volume blower and relief dampers that dumped supply air directly into the unit return. The new unit came with a VFD to control motor speed.
We have up graded the controls because the Carrier parts are becoming obsolete.
The question I have is on a VVT system do I want to use the VFD to control static duct pressure, or relief dampers.
I have called our Carrier distributor and received two different answers. It depends on who I talk to.
Also if the relief dampers are used, shouldn't the supply air be discharged away from the unit return and mix with the building return before it enters the unit?
If more information is needed let me know. This in not the best system setup, but I have to live with it for a while.
You use the VFD to control the fan, Than fan should be controlled from static pressure. As you boxes all close off your fan should cycle down as static pressure increases, If the pressure gets to great the relief dumps the air, i have seen most reliefs on the retun at the RTU.
Originally Posted by RedRunner
Do you have relief dampers on the RTU that dump the air outside or are you refering to a bypass damper???
Definately use the vfd is really serves 2 purposes. It does the same job as the relief/bypass damper but also is a energy saving feature. You might have to play with the static pressure setpoint on the comfortlink , because it has been my experience that vvt systems run a lower static pressure setpoint than the 1.5 " defaulted on the RTU
One thing you will want to check is at the lowest speed on the VFD the blower fan will be delivering enough CFM for heating so you have enough air flow as to not trip the hi limit or in cooling mode in summer freeze up the evaporator coil. This is very critical up here in denver as our air is less dence and we have very little humidity.
If you find it bad 3 different ways- ~its bad~
yes remember that a vvt system votes.
if one vvt is voting for cooling and it is a priority vvt it
will still run the unit in cooling until the one vvt is satisfied.
I would go to the biggest zones and give them more votes than the small zones.
otherwise you will freeze up and go out on high limit
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I use carrier 3v with vfd's all the time. Make sure your rtu will monitor supply air temp. The lennox L series maintains 55F supply air in cooling and cycles stages as req'd, Also monitors discharge air in heating mode and cycles heat. Bypass dampers are the old ineffecient way to do zoning. If your ductwork split's after you go through the roof you might need more than one static pressure sensor. I did a retrofit on a hot deck cold deck unit and req'd a static pressure sensor in each duct. Then fed each sensor into math modules to average the readings. Worked like a charm. "Knock on wood" It's been a year without one call
refrigeration workers of Ontario
OK I am going to seal off the original bypass dampers and use the VFD. Carrier does not want to get involved since it is not their controls. They did say either way would work.
I just had concerns about the air flow over the heat exchanger and cooling coils. There will be no bypass.
There are 18 zones, and sometimes all but 3 may shut down. These three zones may have 200 cfm each. Thats a total of 600 cfm.(maybe a little more with air leakage) The static pressure sensor does lower the blower speed.
I was just concerned about not getting the proper air flow across the unit, even if it does stage down.
You are right about the minimum airflow. The old bypass served two functions, it maintained static pressure on the supply duct and maintained a minimum air flow to prevent compressor failure due to floodback. The bypass dumped into the return far enough back to allow mixing of the return and the supply air to prevent shortcycling. The old VVT worked pretty good when set up properly. It's a nightmare if everyone tinkers with it. The Carrier rep should answer your question, no matter what controls you are using. There was a formula in the old VVT books to set up the bypass. Use the formula to obtain your minimum airflow and set the VFD to that minimum. Use this link for factory info: http://www.commercial.carrier.com/co...cal_literature
Originally Posted by RedRunner
Last edited by ACCMan; 02-14-2008 at 05:19 PM.
Another vote for VFD, but definitely factor in minimum airflow. The controls may be able to do this if the have pickup tubes on each box so you then could add up all the airflows at the boxes and then "verify" you have the minimum cfm for heating/cooling before you allow heating/cooling. Also setting your vfd to a minimum speed for airflow will not matter as your zones are actually controlling your airflow.*
ACCMAN - your link is/was expired when I looked at it.
*If you set your vfd at a minimum speed to maintain proper airflow then you must also set your zones at a minimum flow to insure that you actually will have minimum flow. I would likely set the dampers mechanically to a minimum position make sure that the controls cannot close down the dampers any further than minimum - just an added safety factor. Get a new programmer/different maintenance personnel and they set the minimums below what they should be could be a real problem if you don't.
Last edited by crab master; 02-15-2008 at 05:17 PM.
Reason: Better explaination
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I didn't suggest removing the bypass. As I stated, a properly set-up VVT system works fine. The full link address was listed in the edit area.
Originally Posted by crab master