Unmet Hours

Two heat sources in the same room - both sized for full load?

asked 2020-06-02 13:55:45 -0500

TheEnergyTutor
848 ●5 ●12

I noticed in my model it has a VAV and baseboard heat in the same thermal zone, intended to work together, but OS sizes both for full load. What i mean is if I remove one of the heat sources, the sizing doesn't change. I noticed in the baseboard config I can set the "Fraction of Autosized Heating Design Capacity" but in the VAV there is no way to reduce the size of the heating coil to account for the supplemental baseboard heat. Does anyone have a suggestion? The system is presently solving for an oversized VAV heating coil and is this is not correct.

answered 2020-06-02 17:29:02 -0500

Aaron Boranian

13830 ●76 ●24 http://bigladdersoftware.com/

Yes, by default EnergyPlus will try to autosize each component to meet the full load calculated from the design day. Some objects have input fields to change this, like the baseboard heater, but most do not (terminals, window air conditioners, etc.). In the case of terminals, EnergyPlus needs the DesignSpecification:AirTerminal:Sizing object added to the input file so that you can set adjusted capacities, supply temperatures, or flow rates from the design day. There is a similar DesignSpecification:ZoneHVAC:Sizing object that makes similar adjustments to zone HVAC components (window air conditioners, etc.).

Currently, both of these options are not available in either the OpenStudio Application or the OpenStudio SDK. If you want to incorporate these objects into your OpenStudio workflow, you will need to do so through an EnergyPlus measure that adds EnergyPlus objects to the input file created by OpenStudio -- something like Inject IDF Objects could work.

An alternative would be to avoid EnergyPlus measures and to manually hard-size the terminal once you know the final baseboard heater capacity.