Add support for powered attic ventilation fans in HPXML/OpenStudio-HPXML

[danielas-r96]

Feature request

Please add support for modeling powered attic ventilation fans in OpenStudio-HPXML, either through a supported HPXML extension or through a first-class HPXML schema addition if the HPXML Working Group agrees this belongs in the standard.

The current attic ventilation support appears to allow passive ventilation rate for vented attics, but not a fan-driven attic ventilation device with its own airflow, power, controls, and operating logic.

Why this matters

This is a significant model-form gap for hot-humid and cooling-dominated homes, especially homes with ducts and/or air handlers in the attic.

In a recent residential calibration case study for an all-electric single-family home in Edinburg, Texas, the model had a persistent summer cooling residual even after diagnosis-gated calibration. The home had a severe attic-and-duct cooling-load condition: low ceiling insulation, no radiant barrier, leaky/weakly insulated ducts in the attic, and a severely overheated attic. The calibration improved the annual electricity fit substantially, but the remaining summer deficit was tied to attic physics that the current workflow could not represent well, including the absence of a powered attic ventilator input.

This is not the same as:

• passive attic ventilation rate
• whole-house fan operation
• local exhaust ventilation
• whole-building mechanical ventilation
• generic miscellaneous electric load

A powered attic ventilation fan affects the attic thermal zone and the heat transfer environment around ducts, ceilings, and roof assemblies. A whole-house fan affects the conditioned living space and is a different device.

Requested capability

Please consider supporting a powered attic fan attached to an attic/roof assembly, with inputs such as:

• associated attic ID / attic zone

• fan type: roof-mounted, gable-mounted, solar-powered, grid-powered, or unknown

• rated airflow, preferably cfm

• fan power, preferably W or W/cfm

• control type:

  • thermostat
  • humidistat
  • thermostat + humidistat
  • schedule
  • always on
  • solar availability / solar-powered operation

• thermostat setpoint, e.g. fan turns on above 90°F, 100°F, 110°F, etc.

• optional humidity setpoint

• operating schedule or seasonal availability

• whether sufficient passive intake/free area exists, if represented

• reporting of fan electricity use as a separate or traceable end use

Suggested modeling behavior

At minimum, the fan should be able to:

1. remove air from the attic zone to outdoors,
2. introduce makeup air through passive attic intake paths or an equivalent simplified assumption,
3. add fan electricity use when grid-powered,
4. affect attic temperature,
5. affect ducts located in the attic,
6. affect ceiling/roof heat transfer into the conditioned space,
7. expose annual and timeseries outputs sufficient to inspect the effect.

A simplified first implementation would still be useful if it clearly documents assumptions and limitations.

Possible implementation path

A reasonable first implementation might translate the fan to an EnergyPlus attic-zone ventilation object with a design airflow rate, fan power, and availability/control schedule. A more detailed future implementation could use AirflowNetwork or a more explicit attic airflow model if needed.

The key need is not perfect attic CFD. The key need is a supported, documented, inspectable way for OpenStudio-HPXML users to represent a common residential device that materially affects cooling-dominated homes.

Example use case

Single-family detached home in IECC 2A / hot-humid climate:

• Location: Edinburg, Texas
• All-electric home
• Central air conditioner
• Ducts located in attic
• Weak attic/duct thermal conditions
• No radiant barrier
• Severe summer model-to-bill underprediction
• Existing physical issue: attic heat and attic duct losses dominate the residual
• Needed representation: powered attic ventilator with airflow, power, and temperature-based controls

Questions for maintainers

1. Is there already a recommended OpenStudio-HPXML modeling workaround for powered attic ventilation fans?
2. Should this be implemented first as an OpenStudio-HPXML extension input, or should it start as an HPXML schema issue?
3. If implemented as an extension, where would maintainers prefer the input to live: under Attic, VentilationFan, or another structure?
4. Would a simplified attic-zone ventilation implementation be acceptable as a first step?
5. What tests/examples would be most useful for a future PR?

[shorowit]

I think the current workaround would be to model this in two pieces:

1. Specify a vented attic ventilation rate that is greater than the default of SLA=1/300. If you are targeting a certain CFM, you could look at hourly attic ventilation rates to calibrate the input.
2. Add an exhaust only mechanical ventilation system to capture the power usage and approximate the airflow rate of the air being pulled from the conditioned space due to attic depressurization. (This air will be exhausted to outside, not to the attic, so it's obviously not perfect.)