The VAV terminals will adjust from Max to minimum depending on space demand. Your system may have reheat or not or perimeter radiation for heating. In either case your computer room may have a separate cooling system to avoid high cooling demand on the system. VVT is a piece of garbage and suffer occupant complaints therefrom.
On heating, you're stuck heating rooms requiring cooling. Properley balanced, I have consistant zone temperature variations of less than one degree total on an eight zone system. The system allows remote or local programing of setpoint limits, airflow etc.
Very dependable. I do not work for Trane. Red Flag This Post Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework. Learn methods and guidelines for using stereolithography SLA 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts.
Download Now. This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. 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 W 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. Thanks for the information. Good discussion. If so, what is the best way to handle it? 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. If you discharging F 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.
Re: vav boxes. Actually, Carrier's new boxes are pressure independent and traditional VAV systems will be a thing of the past. They are energy hogs.
VVT Systems need to have additonal controls installed in the return air to limit the operation of the burner and the mechanical cooling to prevent failures. Where do I subscribe? The VVT is called variable volume because it delivers a variable volume to each zone, as load dictates. It is called variable temperature because the temperature of the air supplied by the central unit varies with time. The VVT is an economical all air zone system that is ideal for many commercial jobs.
It works well for systems up to 25 tons of total cooling load. The system is provided with a complete factory packaged control system designed to provide multiple zones of temperature control.
Each box modulates its volume control damper in response to the zone thermostat. If in one zone the damper is starting to close, then the extra air is bypassed into a ceiling return air plenum or a ducted return.
Indeed, the zone airflow is variable but the rooftop unit airflow is constant. Each box has a minimum cfm setting to ensure adequate room air circulation and outdoor air ventilation regardless of zone load reduction.
VVT System pictured above. When all the zones require some cooling, the unit stays in cooling mode. The same thing happens in heating mode.
However, when both heating and cooling loads occur at the same time, it becomes a time-share system. The definition of time-sharing is: the sharing of a computing resource among many users by means of multitasking at the same time.
In our case, the electronic controls determine the greatest need heating or cooling and they first satisfy that mode centrally. Then, once satisfied, it switches over to the opposite mode. The system can continue switching over from cooling to heating, back and forth, to satisfy all zones. In the mid-season, the result is inevitable dissatisfaction while the unit is in the wrong mode and the damper to that zone is closed.
For a better comfort, the perimeter zone damper units can be equipped with hot water supplementary heaters. Electric heaters can also be used instead. It will vary the airflow at a constant temperature. There are two kinds of variable air volume units, the pressure dependent and independent.
The first one represents the basic control of a variable ATU. It will modulate the damper actuator from the zone thermostat regardless of system conditions. However, the pressure independent unit will deliver the required air volume to the room, even if the supply static pressure increases.
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