VAV system for Winter Heating & Demand Ventilation

[biswaji]

I have been involved in designing a small one storey building having multizone areas. Can I use a roof top package unit having gas fired heat exchanger and cooling coil and terminal VAVs (without reheat) to control the individual zones during winter by modulating the airflow (similar during cooling period) to maintain the zone temp. The supply air temp shall be fixed (or resettable). Heating boiler is not an economical solution for such small application and I do not want to use terminal electric heating (Baseboard or VAV reheat) either. Besides if I intend to utilize demand ventilation concept to modulate the economizer damper based on CO2 sensor input how can I integrate this with the design mentioined earlier?
Apprciate your thoughts.
Thanks

 

[DrRTU]

Generally, your design of zero heat at the terminals will under heat the perimeters and over heat the interiors. Your design will only work with high winter design temps. Run your loads by changing the LAT. The Carrier comfort link controller actually does an excellent job at this in the RAT mode.

 

[biswaji]

I agree with your comments on under/over heating but again think that these problems are mostly felt in larger perimeter rooms having substantial glass walls. The perimeter rooms here are small (250 sq ft) and so I think that there will not be much temp stratification between perimeter and interior within the same room that might cause discomfort provided diffuser is strategically located. Core area has its own VAV and the loads patterns are uniform. Thanks.

 

[NCDesign]

A VVT system will not work if need to be able to heat and cool a space at the same time. If you are doing a small zoning system be sure that your system covers an area that will for the most part all need cooling or heating at the same time. These small zoning systems are all designed to allow you to differentiate temperature from zone to zone, ie person A 70F, person B 72F, person C 68F as long as their loads are such that they all will need heating or cooling at the same time to maintain their temperatures. You can get around part of that by supplying reheat to a zone that needs heating when the rest of the building needs cooling, but you're pretty much dead in the water if you have a bunch of zones that need heating and one that needs cooling (Think several exterior zones with glass on the same system as a conference room in the interior). The system will index to heat and that zone that needs cooling will just get hotter. Keep in mind that this isn't determined just by load conditions but also by the temperature requested by the zone sensor.

 

[DrRTU]

I agree - VVT will not work. VAV has a chance with a high winter deign conditions i.e. 45 degrees F. The key would be the amount of glass and internal loads. You need all spaces to have the same use and thermal envelope. I believe you have one big design liability. Design a system with heat in the terminals, use parallel boxes to keep power down. A single duct re-heat design will also work. I am not taking the lazy design, just keeping your E&O rate low. Power wiring is expensive to run after the space is built out. Good VAV application is all about zoning, if not, use one CV system per exposure and call it done.