Changeover-Bypass Variable Air Volume Systems
Recently, I've been dealing with a significant number of issues with changeover-bypass VAV systems. This is what Carrier made popular with the term VVT for Variable Volume and Temperature. Trane also has a solution referred to as VariTrac. The problems that I've seen are typically based on unmatched expectations for the system... Years ago on a trip with Trane, when I asked one of the Representatives about Trane's solution, they responded, "What, the poor man's VAV system?"
Why even bother?
Back in the days before digital scroll compressors and cost-effective variable frequency drives, smaller tonnage (les than 20 tons) packaged unitary HVAC equipment was constant volume airflow. Constant volume airflow solves a bunch of issues and none more important than when you have the compressor going 100% (which is when it's on) you need to add heat to the refrigerant coil sufficiently to prevent the condensate coming out of the airstream from freezing. Once your coil freezes bad things happen, usually including water in places you don't want it...
So, you would buy a constant volume unit, because that's how they made them. The problem with a constant volume unit is that you get one thermostat to tell the unit to provide heating or cooling and you put that thermostat in one place. No matter what you do, someone will think that the placement is wrong... In a commercial environment, to meet the ventilation requirements you have the fan run continuously (or "Fan On" on the thermostat). The unit brings in some outside air, mixes with return air and delivers to the space. In the winter, the mixed air temperature is usually around 55ºF. In the summer, the mixed air temperature is usually around 80ºF dry bulb, 67ºF wet bulb. So, when the thermostat says it wants heating, the 55ºF air gets heated (usually up to 90-100ºF) and delivered to the space until the thermostat is happy. The heat shuts down and then the supply air drops back to the mixed air temperature. When the thermostat wants cooling, thw mixed air gets cooled down to about 55ºF and delivered to the space unti the thermostat is happy. Then the air temperature will go back up to 80ºF.
This sounds like a mess, but it is inexpensive and if the Owner knows what to expect it can work. However, if there is a need to break up the one unit to serve multiple thermal zones with multiple thermostats there is only one way to do that and that is with a changeover-bypass VAV system.
System setup and problems
What happens now is that you install a motor-actuated damper (bypass damper) connecting the supply and return duct.
Then you add a bunch of motor-actuated dampers (zone dampers) out in the ductwork to serve different zones in the supply duct.
Traditional VVT uses pressure dependent zone dampers, which means you don't know how much air is going through the dampers. They open and close as required based on the thermostat requirements. This doesn't sound too bad, but what happens is the system might reduce the airflow to a value too low to provide proper ventilation or the system could deliver an airflow that is too high and cause air noise. There is a way to try to accomodate this-
The bypass damper generally monitors the supply duct static pressure. As the zone dampers close, the bypass opens to prevent the static pressure from increasing. Therefore, in theory, if the system is balanced properly, you shouldn't get more air than designed through the zone damper.
Usually, you have a controller that looks at all of the zone damper sensors and asks whether they want heating or cooling. Then the controller takes the vote and tells the rooftop unit what to do. When a zone damper wants heat, but the unit is in cooling, the zone damper senses the incoming temperature and goes to a minimum position to prevent a lot of heat from going through. Then, when the voting of all the zone dampers says the majority are happy, the controller switches back to the other mode and the zone damper that wants heat gets heat and opens (and all the other zone dampers close).
However, another problem arises. The packaged units do not usually have big fans on them. They are designed to provide constant volume at a relatively low pressure. The worst thing that you can do to a system that modulates airflow is have low pressure. The control is just hard to achieve at low pressures. So, all of your zone dampers are bigger than they should be to control properly. This could lead to having the zone dampers all but shut most of the time trying to meet the space temperature requirements in some areas. This reduces the air to the space, but it also creates noise...
Sometimes, the bypass damper is not connected directly to the return air duct. Perhaps it is a plenum return and there is little ductwork to connect to, or perhaps you read the VVT manual that says to avoid a bypass too close to the return air inlet. What can happen to the equipment is that the 55ºF air being bypassed now mixes with the outside air and you get a mixed air temperature in the 60sºF rather than in the 80sºF and once again you freeze your coil- not because you don't have enough airflow, but because your air is too cold... However, when the bypass goes into the plenum and there are return air grilles near the bypass opening, the 55ºF air bypassed to the plenum likes to fall out (well it gets pushed out because now you are pressurizing the negative pressure plenum- ugh!) of the ceiling through the grilles on top of people- they get cold...
Another potential problem with these systems is ensuring that ventilation air is provided to the spaces in a building. This setup can lead to having a whole lot of unused outside air in the ductwork system, but not enough ventilation air in the space. There are ways around that, but they adversly affect the relatively poor temperature control you already improved on...
Attempted solutions
We recently designed a project that had a VariTrac setup from 1988 and the rooftop equipment needed to be salvaged. We knew there were going to be issues, so we told everyone and indicated that we would try to resolve them as best as possible.
We added pressure independent VAVs (over-sized). We added fan-powered VAVs to help mix plenum air and help trim the supply air temperatures as the units switched from heating to cooling. We upgraded the bypass dampers and designed a complete Direct Digital Control system.
Before the job was complete, the rooftop units were replaced with brand new equipment that had VAV capabilities. The biggest problem is that the rooftop units have pre-canned control algorithms (and none of them include VariTrac or VVT) and the units won't operate to meet the sequence of operation outlined. Why? Well, because the newer equipment is not designed to operate like this because the new components do not need the constant volume setup that the old equipment needed. Slowly, with hard work between the Contractor and the equipment Vendor they are getting the system operating to a point where the Owner will be happy.
So, what should I do?
1. Any new systems should avoid changeover bypass VAV. There is no need for these anymore, real VAV systems are inexpensive enough today that even the Poor Men should be able to afford them. Ventilation can be controlled properly in a realy VAV system and temperatures will be more uniform.
2. If you have an existing system or existing CV system that you want more control, you can add some dampers to the system. I would first recommend looking at thermafusers and seeing if they can solve your problem. If you don't think they will help, then you need to make sure the rooftop units are zoned properly and you will need to ensure that the control system installed can provide the sequence of operation to acheive the VVT solution. Finally, ensure that the people in the space know what to expect- there will be hot air in the summer, cold air in the winter, probably some air noise.
This type of system doesn't make everyone comfortable, but ensures that everyone is equally uncomfortable.
