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Choosing a Setpoint manager?

asked 2017-12-07 08:45:58 -0500

hoffmann1320 gravatar image

updated 2017-12-07 16:04:13 -0500

I am currently trying to model a 10,000 sf community center in climate zone 4a that uses a packaged rooftop system with 2 stage dx cooling with an electric heating coil, and VAV with electric reheat terminals in zone. The template from Openstudio for this type of air loop wants to use a scheduled set point manager. The deck temperature on this setpoint was set at 55 all year, and running this model results no unmet hours,but a significantly high heating energy load for the year (about 500,000 kBtu per year heating and 160,000 kBtu cooling). I tried using the Setpoint manager: Warmest, and had success in lowering the heating energy load by about half. What is the correct set point manager to use in this situation?

I am new to using openstudio/ energyplus, and haven't fully grasped how to pick set point managers for each situation. Besides the energyplus setpoint manager engineering reference, could someone explain how they choose their setpoint manager in their systems.

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answered 2017-12-07 18:17:15 -0500

updated 2017-12-11 19:49:57 -0500

A common control strategy for the cooling supply air temperature in VAV reheat systems is warmest zone reset.

SetpointManager:Warmest resets the cooling SAT based on the cooling demand of the warmest zone, at the maximum zone supply air flow rate.

Assuming that you are in a climate that requires heating at some point in the year, it makes sense to me that if you are supplying 55F air all year round you will see very high heating energy from reheat. Applying a reset to the SAT will absolutely yield significant heating savings.

This strategy is used to minimize the amount of reheat that will occur at the zones. VAV reheat systems typically supply air to the zone at 55F to 60F, then will reheat the air up to 95 F at the zone level for zones that require heating. However, if most zones are in heating mode, than it is beneficial to reset the supply air temperature to a higher value so that it minimizes the amount of reheat and resulting heating energy.

The setpoint manager is placed on the supply side, at the outlet node of the supply fan:

image description

For more information on setpoint managers, look at this post: https://unmethours.com/question/26270...

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Where would you put the reset setpoint manager within the air loop? Would it be next to the VAV with reheat terminals?

hoffmann1320 gravatar imagehoffmann1320 ( 2017-12-11 10:49:08 -0500 )edit

In OpenStudio it is placed on the supply side, just after the fan (at the supply fan outlet node). I've edited my answer above to include an image of where this is located.

Anna Osborne Brannon gravatar imageAnna Osborne Brannon ( 2017-12-11 19:48:15 -0500 )edit

Thank you for your help, greatly appreciated. By doing, my heating load was brought way down, but still seems a little high. With the warmest setpoint manager in that location with 55-60 as the parameters, I am still seeing heating loads in the 200k kbtu and cooling in the 60K range. Is there a way to reset the SAT in the heating months to have the air loop electric heat coil to do the main part of the SAT conditioning, so the system isn't relying on the VAV reheat coil to do all the conditioning this time of the year.

hoffmann1320 gravatar imagehoffmann1320 ( 2017-12-12 10:29:53 -0500 )edit

You might want to look at your zoning strategy to circumvent this. If you have zones on an AHU that require cooling, in a VAV reheat system strategy, there isn't a straightforward fix, because if you don't send cold enough air to those zones you will get high unmet load hours. Try zoning your system so there is one AHU for core zones, and different AHUs for each orientation (or combination of orientations, whatever makes the most sense).

Anna Osborne Brannon gravatar imageAnna Osborne Brannon ( 2017-12-12 15:41:21 -0500 )edit

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Asked: 2017-12-07 08:45:58 -0500

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Last updated: Dec 11 '17