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Single Zone VAV serving multiple zones

asked 2017-03-23 21:11:09 -0500

updated 2017-04-16 11:09:43 -0500

I haven't seen a definitive guide on this, but enough info to make me believe this is possible.

I have a single zone VAV RTU (single T-stat) serving multiple spaces/zones. The spaces were modeled with a thermal zone per space, because they have different internal loads; ex: retail sales floor and a stock room. I can configure the demand side of the zone splitters & supply path to feed both zones, but where/how do I have just the sales floor zone's t-stat control the unit? From the BigLadder documentation for Group – Design Objects, I need a t-stat in all zones to account for the load/airflow of all zones. What isn't clear is how to designate/configure the master & slave zones; and make the master t-stat the only one that matters.... Is one of the example files configured or this? Any help appreciated.

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Would a SetpointManager:SingleZone:Reheat be what you're looking for? It has a field for setting a control zone that determines supply air temp, which is the master zone you were after.

pflaumingo gravatar image pflaumingo  ( 2017-03-23 21:42:32 -0500 )edit

The larger issue is how to vary the air volume to all the spaces in the same way. I think you need to write an EMS program to set the flow at all the 'other' terminals to the same percentage as the control vav terminal is requesting.

Adam Hilton gravatar image Adam Hilton  ( 2017-03-24 08:13:11 -0500 )edit

You could try using a VAV terminal in the master zone and AirTerminal:SingleDuct:Uncontrolled in the slave zone (doesn't require a Tstat), however, I am not sure how the uncontrolled TU will respond to variation in main air loop flow rate.

rraustad gravatar image rraustad  ( 2017-03-24 08:36:15 -0500 )edit

Ya, that's actually something I've tested and it doesn't respond like that. The terminal devices just like to live in their own little world. That would be an incredibly useful terminal to add in the future though!

Adam Hilton gravatar image Adam Hilton  ( 2017-03-24 08:57:14 -0500 )edit

Yeah, I'm a bit surprised as this is a very common setup in design/real world. I figured/hoped there was some AirLoop object I just overlooked that had a 'control zone' input (ala eQuest single zone system)

Yes, the 'easy' work around is to just make it all a single thermal zone, and average out the internal loads...but that seems kinda wonky as well. The two spaces are one 'thermal zone' for control, but they aren't in any sense of load or ventilation...arg.

since OS has 'spaces' and multiple spaces can be assigned to a thermal zone, this may be one thing OS has on E+.

dradair gravatar image dradair  ( 2017-03-24 11:08:12 -0500 )edit

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answered 2017-03-24 08:10:15 -0500

For VAV systems, the zone Tstat controls the zone air flow (and reheat coil if used) and the system (air loop) coil is controlled by a set point manager.

Any of these terminal units should work.


If there is just a heating coil in the main air loop then using SetpointManager:SingleZone:Reheat or SetpointManager:SingleZone:Heating to control the coil will provide some temperature at the heating coil outlet node and also at the terminal units. The Tstat in each zone will look at this temperature and calculate the air flow rate needed to meet the zone load.

  MasterZone,  !- Control Zone Name
  HeatingCoilOutletNode; !- Setpoint Node or NodeList Name

If there is also a cooling coil in the main air loop then another set point manager should be used to control that coil.

You can see what that air temperature is by reporting node temperature and heating coil outlet node set point temperature) Output:Variable,*,System Node Temperature,Hourly; (where * = node name of zone inlet node) Output:Variable,HeatingCoilOutletNode,System Node Setpoint Temperature,Hourly;

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I think I'll start with this approach. Though, I don't have any VAV terminals in reality, just diffusers. So, I may adopt this with the use of VAV:NoReheat and no turndown allowed. And have the setpoint manager(s) reference the Master Zone.

dradair gravatar image dradair  ( 2017-03-24 11:04:22 -0500 )edit

alright, have been testing this with the 5ZoneAutoDX example file. I have modified the file to have all 5 zones with a AirTerminal:SingleDuct:VAV:NoReheat. Have also created & assigned the Setpoint mangers to the main heating &cooling coils. It runs. But - I have a bunch of unmet hours... in the controlling zone?? The quantity depend on how I configure the heating or cooling Setpoint mangers & their nodes. With one controlling the coil outlet, and the other the AirLoop outlet & vice-versa. In both cases the hating coil runs during almost all occupied hours and ....

dradair gravatar image dradair  ( 2017-03-27 16:48:29 -0500 )edit

....and the cooling coil runs like mad to try to overcome the heating coil in the summer. So. Its almost as if the heating coil doesn't know to shut off. At this point I am purely concerned about the control zone unmet hours; b/c in the 'real' model they will be the far bigger/needier zone than the attached 'slave' zones associated with them....So, any ideas how to dynamically shut down a heating cool? or a different setpointmanager to use?

dradair gravatar image dradair  ( 2017-03-27 16:51:34 -0500 )edit

Try the SetpointManager:Coldest for the heating coil and SetpointManager:Warmest for the cooling coil. Report the node set points to be sure they don't fight each other.

rraustad gravatar image rraustad  ( 2017-03-27 20:28:14 -0500 )edit

SetpointManager:Warmest with a Setpoint Node on cooling coil outlet is happy. SetpointManager:Coldest assigned to either the heating coil outlet node or the system outlet node yields a severe/fatal error of:

* Severe * SetpointManager:Coldest="SPMNGR_COLDEST-HEATING", no zones with heating found: * ~~~ * Air Loop provides no heating, Air Loop="VAV SYS 1".

ergo....either it thinks none of my zones need heat (false) or it thinks I need reheat coils (doubtful though)....I'll keep digging and see.

dradair gravatar image dradair  ( 2017-03-28 13:28:46 -0500 )edit

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Asked: 2017-03-23 21:11:09 -0500

Seen: 723 times

Last updated: Mar 24 '17