Outdoor Air & Ventilation Strategies with John Semmelhack, Ty Branaman & Tim De Stasio

In this live-stream episode, Bryan is joined by Tim DeStasio, John Semmelhack, and Ty Branaman for a deep dive into residential ventilation — the often-overlooked “V” in HVAC. The group opens with quick life updates before launching into a wide-ranging conversation covering the three core ventilation strategies (circulation, extraction, and dilution), how tight home construction changes the ventilation equation, and why there is no single “magic number” that works for every house.

A major theme is the physics of pressurization and depressurization. Tim walks through balanced strategies (ERVs and HRVs), positive-pressure approaches using ventilating dehumidifiers, and exhaust-only setups — explaining why exhaust-only, while still code-compliant, is his least favorite option due to the risk of pulling air from crawl spaces, attics, or combustion appliances. Using REDCalc, Tim demonstrates how even modest airflow can swing a tight house’s pressure by ten or more pascals, and the group discusses how wind, stack effect, and seasonal conditions constantly change a home’s natural air exchange rate.

Ty brings a service-focused perspective, emphasizing measurement before intervention — checking CO2, carbon monoxide, and other indicators before jumping to solutions like bigger fans or added dehumidification. He shares real-world stories, including a rental home with elevated CO from an unvented gas range, and stresses that building science problems (duct leakage, poorly placed bath fans, depressurized combustion appliances) often hide behind symptoms that look like simple ventilation shortfalls. Large kitchen range hoods get particular attention: hoods moving 1,200 CFM or more can seriously depressurize a house, and the group discusses code thresholds for interlocked and powered makeup air, along with practical products like the Fantech MUAS.

John rounds out the discussion with a walkthrough of a real high-performance home design, showing how an ERV can serve both whole-house outside air and continuous bathroom exhaust, how MERV 16 pre-filtration has become a common upgrade since the 2021—2022 wildfire smoke events, and how an ERV can be configured to boost into a makeup-air role when a range hood kicks on. The episode closes with a look at ventilating dehumidifiers, demand-controlled ventilation, the tradeoffs of leaving blower fans in the “on” position, and recommended resources for anyone wanting to go deeper into building science.

Topics Covered

• The three ventilation strategies: circulation, extraction, and dilution
• How tight, high-performance construction drives the need for mechanical ventilation
• Calculating and understanding natural air exchange rates
• Balanced (ERV/HRV), positive-pressure, and exhaust-only ventilation strategies
• Using REDCalc to model pressurization and depressurization
• Why exhaust-only ventilation can create combustion safety risks
• Designing ventilation with flexibility beyond code-minimum rates
• Measuring first: CO2, CO, and pollutant levels before choosing a solution
• Large range hoods, makeup air requirements, and the Fantech MUAS system
• Passive capture and range hood geometry (corner hoods vs. center islands)
• A real-world high-performance home ERV design walkthrough
• MERV 16 pre-filtration for outside air following regional wildfire smoke events
• Using an ERV in a makeup-air configuration for range hood operation
• Ventilating dehumidifier strategy and sizing considerations
• Demand-controlled ventilation and the new ASHRAE 62.2-2025 guidance
• Risks of leaving blower fans in the “on” position, including duct leakage effects
• Recommended building science resources: Joseph Lstiburek, Allison Bailes, and TEC classes

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