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asked 2015-10-06 17:24:24 -0500

mleach's avatar

updated 2015-11-07 13:49:12 -0500

I'm trying to create a modified version of the E+ example file (5ZoneSupRetPlenRAB.idf) that contains the SetpointManager:ReturnAirBypass object. In the example file, the cooling coil and heating coil are on the main HVAC branch. There is a separate bypass branch that routes return air to a point beyond the main HVAC branch. SetpointManager:ReturnAirBypass is assigned to the airloop and controls the flow of bypass air to hit a scheduled setpoint temperature. A mixer connects the two parallel branches and an outlet branch containing just a fan carries the mixture of bypass and conditioned air to the demand side of the airloop. Everything seems to work fine in this case, as would be expected since it is an example file.

However, if I move the heating coil to the outlet branch (after the mixer but before the supply fan) and add a SetpointManager:Scheduled to either the fan outlet node (and place a corresponding SetpointManager:MixedAir object on the outlet of the heating coil) or to the heating coil outlet node, the mixing behavior changes. Bypass flow is decreased compared to the base example file case, resulting in many hours in which the heating coil needs to run to hit the desired loop outlet temperature.

Is SetpointManager:ReturnAirBypass designed only to work in the configuration represented by the example file? Is there some reason why my configuration doesn't work? Am I missing something?

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answered 2015-10-07 00:06:52 -0500

updated 2015-10-07 00:08:14 -0500

For SetpointManager:ReturnAirBypass, the setpoint temperature is the supply air temperature at outlet node of the airloop outlet branch.

The setpoint temperature is modified to account for any potential fan heat gain.

Tset,mod = Tset - (Tloop,out -Tmixer,out )

The calculation of the two branch:

m,bypass = (m,totalTset,mod - m,nonbypass T,nonbypass )/T,bypass

So you may need to put all your equipment in the nonbypass branch.

At least, you should not put the SetpointManager:MixedAir after the heating coil.

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I tried two different configurations. In one, I had a SetpointManager:Scheduled on the fan outlet node and a SetpointManager:MixedAir on the heating coil outlet node (which you are suggesting I NOT do). In the other, I just put a SetpointManager:Scheduled on the heating coil. However, I saw the same problem for both cases, and I can't not have a setpoint manager on the heating coil, so those seem to be the only two options. Can someone state definitively that you cannot put conditioning equipment on the outlet branch? I couldn't find any such statement in the documentation.

mleach's avatar mleach  ( 2015-10-07 11:07:27 -0500 )edit

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Asked: 2015-10-06 17:24:24 -0500

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Last updated: Oct 07 '15