VAV vs VVT

[dneil75]

Can anyone tell me what is the difference between a VAV and VVT systems. I would like to know the difference as it relates to their operations. Cant seem to find a definitive difference between the two systems?

[Paul Pippin]

VAV = variable air volume this unit usually maintains a 55 degree discharge temperature and maintains a static pressure in the duct usually done with a variable frequency drive. These systems usually have some sort of reheat on each vav box or at least the exterior zones. Usually this system will only go into heat mode during the morning warm up period.

VVT = variable volume and temperature this unit will maintain the static pressure in the ductwork either with a variable frequency drive or bypass damper. The VVT system doesn't maintain a 55 degree discharge it can be in either heat mode or cool mode which ever it needs to maintain the temperature in the space.

Sorry these are short answers to the question but cover the basic points.

[triton99]

Paul Pippin is correct. Just wanted to add one thing to it. The manufacturers refer to their VAV boxes as pressure independent boxes and the VVT boxes are pressure dependent thus requiring the bypass dampers. You cannot guarantee your airflow with VVT systems. Most VVT systems are found on unitary type equipment such as rooftop units. The bypass is needed because in the case of a rooftop unit, most of their fans are forward curved fans which are overloading type fans so the bypasses are used to "relieve" the pressure from the supply duct to the return.

[sysint]

I have to disagree with your assessment on why VVT has bypass.

On units 20T and under, constant flow must be maintained across the heat-x so you have bypass. (instead of VFD or other) Above 20T manufacturers make VFD units instead of bypass that are VVT.

In our area VVT is a no-no. Got to verify proper OA.

[triton99]

You may be correct. This is the answer that Trane gave me a couple of weeks ago. Wouldn't you still have constant airflow across the coil since the bypass is between the supply and return duct? As far as outside air is concerned, that would be set at your outside air damper whether manual or automatic. According to Trane, the bypass damper is controlled based on pressure. They call their system Varitrac. We primarily use VAV because most of our buildings are chilled water. This question came up because I have a building with packaged units and I wanted some type of zone control. My larges unit is 15 tons down to 4 tons. This was Trane's recommendation. What is your take on that? Maybe I got bad information.

[triton99]

sysint: We may be talking about the same thing. Take a look at http://www.trane.com/Commercial/Equi...g%20060804.pdf
They talk about the bypass and how it is controlled in the Application Considerations and Features and Benefits section. Again, I prefer pressure independent VAV but why would they not allow you to use VVT? I do not see how that would affect yout Outside air if you are maintaining a constant static pressure which is what the bypass is controlling off of. Just asking for my own benefit of learning.

[jvrba]

triton, what is the maximum bypass percentage? If your floor is satisified what percentage will bypass?
Here's another one: What is the difference CAV VVT?

[fat eddy]

VAV is variable air Volume

VVT is variable Volume & temperature,

The VVT system relies heavily upon all the distributed zones having similar conditions.It is a cheap setup IMO to provice any heat to an area it must stop cooling the others . We are not talking about dual duct roght ?

VAV controls cooling by controling the volume of air,this also has problems. VAV terms are usually set to about half their cfm in the heating mode , heat is usually electric reheat or hw reheat.


Series fan, constant volume boxes with hot water reheat is the best situation. JMHO, with these types of boxes you can usually eliminate a need for additional perimeter heat. JMHO

[Edited by fat eddy on 03-04-2005 at 07:13 AM]

[sysint]

Triton, VVT is space temperature driven.

If you don't know how much CFM you are delivering to a space you don't know how much air you are delivering to each space. Since the VAV's has set minimums you are good.

You will note that Carrier's VVT in later versions came out with an absolute minimum later to combat this issue.

Again, bypass is used on units 20T and under to keep constant flow across that heat exchanger. Now, maybe with these rumored new units that will change. Now, if somebody spent the money on AHU's with HW, then you could go back to VFD control on smaller tonnage.

[seaboard]

sysint & Triton, you are both correct. Many older rooftops under 20 tons would be set up like a VAV system with nothing to vary the air flow. When all the downstream terminals shut the curve of the fan wasn't capable of producing great static, the air dead headed, supply air temp dropped and satisfied the controller (usually a W973). Today's unit's have more efficient fans and can produce much more static. Manufacturer's did this because much of the market is replacement driven and crappy ductwork has to be allowed for. This is the explanation I got from Carrier and Trane both. Although the bypass keeps airflow across the coil from dropping below nominal the air entering the coil will be pretty cold or hot depending on the mode. Putting VVT in a building that has a conference room and several offices on the same zone makes sense. What I don't like to see is a building with 10 VVT systems. Should have "biggie sized" the units and gone to VAV.

[triton99]

Thanks for the information. Good discussion.

[edbarco]

Can I use vav boxes to control a specific zone(private room) from an AHU system that is feeding the dining room (outside), and a private room (inside) of a restaurant?
If so, what is the best way to handle it?

[orion242]

>The VVT system relies heavily upon all the distributed zones having similar conditions.It is a cheap setup IMO to provice any heat to an area it must stop cooling the others . We are not talking about dual duct roght ?

Cheap is right! In my experience these systems are very touchy. Properly designed they can work fairly well. That said there next to never properly designed. A gas fired / DX RTU with a bypass damper has a major problem in my opinion. Just because you have the proper airflow to the heat exchanger doesn’t mean you’re in the clear. If you discharging 120F air and most of it is flying though the bypass (typically within feet of the unit) your putting it right back into the heat exchanger or DX coil. Can you say limit trip. You almost have to control these units off discharge air temperature to prevent this, which then cycles the hell out of the equipment. That coupled with the one room that always need heat or cool and the rest of the zones need the opposite, leads to nothing but complaints.

They work, but everything needs to be designed and tuned for proper operation. If you see exterior rooms and interior rooms all on the same unit, it’s bound to have comfort issues. Or better yet, a communications/equipment closet fed by VVT along with a ton of office zones. The spaces have to be roughly the same heating/cooling load, and preferably hot/chilled water coils in the unit.