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VAV Damper Heating Action

asked 2026-03-24 20:30:33 -0500

Keigo's avatar

updated 2026-04-01 10:26:24 -0500

EnergyPlus I/O Reference, AirTerminal:SingleDuct:ConstantVolume:Reheat, Damper Heating Action states:

With Reverse and ReverseWithLimits (the default) action, as the heating load increases, the unit starts at minimum air flow and minimum hot water flow. The hot water flow is increased until it reaches maximum flow or the user-specified maximum reheat air temperature is reached, then the air damper starts to open to meet the load. For Reverse the damper can open all the way. For ReverseWithLimitss the damper can only partially open to a maximum flow rate given by the following two fields. These options are used if the minimum air flow rate is not adequate to serve the peak heating load. This is sometimes called the dual maximum control logic as illustrated in following figure. For heating coil types other than the hot-water coil, e.g. electric, steam, and gas, the reverse action works the same as the normal action – always keeping the air flow at the minimum during heating.

Is the last sentence correct? I tested electric coils. Reverse worked as the reverse action. The air flow increased beyond the minimum flow to meet the heating load.

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@Keigo the object type you reference is ConstantVolume, but you linked to documentation for VariableVolume. Which air flow control are you asking about?

Aaron Boranian's avatar Aaron Boranian  ( 2026-03-30 09:57:15 -0500 )edit

Sorry if the link is wrong. I meant to ask about Airterminal:SingleDuct:VAV:Reheat.

Keigo's avatar Keigo  ( 2026-03-30 10:18:08 -0500 )edit

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answered 2026-03-25 07:30:17 -0500

Well, that's true that the damper will open when using reverse action. But for those heating coil types there will NOT be an increase in heating rate unless the maximum reheat temperature was tripped and then there can be an increase in heating rate since at higher flow rates the supply air temperature will decrease. For the HW coil the heating rate will actually increase because the coil leaving water temperature will decrease thereby allowing more heating capacity. So using reverse action with these non-water heating coil types does not always help to meet the zone load and will just waste fan heat. The documentation should be updated to discuss these impacts.

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Thank you for the answer. I explored a bit. For heating sizing runs, the air terminal air flow rate did not increase with reverse action, but the electric coil heating capacity was autosized based on the zone heating load regardless of the air flow rate. So, the electric coil should have sufficient heating capacity when autosized. For annual run, the air terminal air flow rate increased with reverse action when the min air flow at the max reheat air temp couldn't handle all the load. As the autosized electric coil has sufficient capacity, the heating rate increased without supply air temp drop

Keigo's avatar Keigo  ( 2026-03-27 02:28:52 -0500 )edit

The only case that didn't work well was setting the air terminal min air flow fraction to 0 (although that's unlikely in reality). The electric coil was autosized correclty in sizing runs, but the air terminal air flow rate remains 0 with reverse action in the annual run.

Keigo's avatar Keigo  ( 2026-03-27 02:40:19 -0500 )edit

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Asked: 2026-03-24 20:30:33 -0500

Seen: 112 times

Last updated: Mar 25