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split system with air loop with Fan:OnOff

asked 2015-09-02 07:21:58 -0500

MattStewart's avatar

updated 2015-09-02 09:43:17 -0500


For smaller buildings with no OA requirements we normally see HVAC equipment in which the fan cycles with the heating/cooling load. From previous posts it seems that Air Loops are not compatible with Cycling (Fan:OnOff). Is it proper to model such equipment as Zone Equipment even though there is an air loop?

As a second question. Is it possible to determine the savings associated with moving the equipment out of a hot attic and into a conditioned space or specifying the amount of ductwork that is in conditioned space versus that which is not?

Thank you.

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answered 2015-09-02 08:01:06 -0500

Previous posts have questioned the ability of VAV or OnOff fans to cycle the HVAC system. It is not the fan that varies the air flow, but the parent, or the system that houses the coils and fan. Terminal boxes control air flow for VAV systems.

  • Air loops are compatible with Fan:OnOff, to the extent that the parent object controls operation via the fan operating mode schedule (and the parent must allow Fan:OnOff)
  • Air loop equipment can also be modeled as zone equipment given the correct parent object choice (e.g., Furnace as air loop or PTAC as zone equipment). The results should be similar if no outdoor air is specified.

Modeling attic vs conditioned space could be accomplished using a duct loss/leakage model. AirflowNetwork has this ability.

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Asked: 2015-09-02 07:21:58 -0500

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Last updated: Sep 02 '15