VVT System

[317069]

Let us say:we have a VVT system that serves an area which consist of three zones A, B, and C
this system has No reheat coils and the control system works as follows:
The set point is 21 C for all zones
At 22.5 C and over, the zone damper will be fully opened and calls for the 1st and 2nd stage of cooling.
At 19.5 C and below, the zone damper will be fully opened and calls for the 1st and 2nd stage of heating
Between 19.5 C and 22.5 C there is no call either for heating or cooling, it is just air circulation.


situation 1
The initial parameters were as follows:
Set point for all zones is 21 C
Zone A temperature is 20C
Zone B temperature is 18.5C
Zone C temperature is 22 C
The system is on the heating mode as zone B needs.

situation 2
Now, let us say that the zone C temperature became 23C while the zone B temperature became 19 C and zone A temperature stays the same.

How will the system respond to this situation?

- Does it stay on the heating mode to satisfy zone B first then switch to cooling mode?
In this case does “satisfy zone B” mean that its temperature is 21 C as the set point or just when it goes above 19.5 C?

Or

- Will it switch to cooling mode, and in this case will zone B damper be fully opened or will it be closed to the minimally opened receiving cold air in both situations (fully opened and minimally opened)?

Thank you

 

[ChasBean1]

This sounds like a pretty cheap (but not atypical) system and there is no proportional control, so prepare for adverse occupant feedback. You can sometimes get away with this type of system if it’s zoned right (no mixed sun/shade zones, no interior/exterior zones).

For your question it depends on how it is programmed and if a heating vote trumps a cooling vote. There’s probably a setup guide and some programming at the ‘brain’ thermostat that will allow you to determine and set what will result from the voting scheme, or if there is just a master thermostat and no voting scheme.

 

[317069]

Thank you for reply.
I undestood from you that we can program the system to respond the way we need.
we can tell system: close the zone damper up to 100%or leav it fully open. is that right?

 

[MrFreeze42]

Great question. I have the same question, but I've come to the same conclusion.

http://www.hvacknoxville.org

Repair Services.

 

[walkes]

Some VVT systems can be programmed to adjust the zone voting. If A is okay, B needs heat, C needs cooling: could be an issue of poor zoning and stretching the limitations of a VVT system. If the unit goes to Cooling, C will be fully open, B should close and A should modulate, then the unit will switch to heating to satisfy B, and so on. If you have too much diversity in your zones the system will be constantly flipping from heat to cool.
You are better off using a VVT with very similar zones such that only minor adjustments need be made to the controls.