Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Multiple VAV Zones, sizing of the main heating coil

Multiple VAV Zones, sizing of the main heating coil

Multiple VAV Zones, sizing of the main heating coil


I was wondering if someone could help me a bit regarding sizing of the main heating coil inside AHU in VAV system, single duct, multiple zones...etc. So, I am aware of principle how VAVs with re-heating coils are operating (first, airflow is reduced until setpoint in the room is reached, and if temp is still dropping heater will start).

However, in this system (which is 100% fresh air), there are many VAV rooms and in addition some of them are CAV. In the main AHU there is a heater and humidifier. Re-heating coils (which are just at the room, after VAV) are sized based on temperature before it and after. This temperature before the re-heating coil is what is delivered by AHU.

Can someone please explain, if heating coil inside the AHU should be sized for minimium airflow in the system or for maximum? I size for example re-heaters based on the fact that temperature upstream re-heater is 15deg. But, main heater in central AHU how should get these 15deg, based on minimum airflow of the whole system or max?

If for example is considered minimum airflow, then since there are many zones and many CAV rooms it can happen that during winter if some VAVs are opened system will not be in minimum airflow and temperature after AHU will not be 15deg but it will be lower and then re-heater as well is going to be undersized.

Same question is applicable for humidifier which is also inside AHU.

Appreciate any feedback and reference to any literature.

Thank you

RE: Multiple VAV Zones, sizing of the main heating coil

In a classical VAV system, central AHU heating requirement is calculated from the greater of 1. Supply air tempering requirement or 2. Morning warm-up heating requirements. These values are found in your load calculations.

RE: Multiple VAV Zones, sizing of the main heating coil

How could you size on any other basis but on basis of maximum possible air flow? What the heating coil would serve in any other scenario?

The purpose of you heating coil is to get planned temperature upstream of vavs in any regime. As simple as that.

what you don't mention, though is heat recovery. i doubt your 100% fresh air system can be planned without heat recovery. so it has to be be calculated for all scenarios: minimum outdoor temperature, maximum flow.

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.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Research Report: Augmented Reality for Maintenance, Repair and Overhaul (MRO)
The term Industry 4.0 denotes a cluster of technologies that’s poised to fundamentally reshape manufacturing and bring about a new industrial revolution. These include 3D printing (AM), the Industrial Internet of Things (IIoT), artificial intelligence (AI) and mixed reality technologies, more commonly known as virtual reality (VR) and augmented reality (AR). Download Now
Research Report: The Next Product You Design Might Be a Service Thanks to the IoT
For this report, the editors of engineering.com interviewed companies on the leading edge of implementing products as a service with a view to describing and categorizing the types of products that can be delivered as a service and the technologies that make it possible—one company in high-tech manufacturing, another who helps ensure access to clean water in developing nations, and a third in the mass transportation industry. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close