HVAC School - For Techs, By Techs
Outdoor Air & Ventilation Strategies with John Semmelhack, Ty Branaman & Tim De Stasio
· Summarized
Key takeaways
- Ventilation is a critical but often overlooked component of HVAC that directly impacts indoor air quality and safety.
- There are multiple strategies for ventilation—circulation, extraction, and dilution—each suitable for different home conditions and needs.
- High-performance, airtight homes require mechanical ventilation systems like ERVs or HRVs to maintain healthy indoor environments.
- Accurate measurement of pollutants such as CO2 and COantes enables targeted solutions and prevents unnecessary interventions.
- Large kitchen range hoods can cause significant depressurization; using powered makeup air systems like Fantech MUAS helps mitigate this problem.
- Balancing pressurization and depressurization is complex and influenced by house tightness, wind, stack effect, and external conditions—modeling tools like REDCalc can help evaluate these effects.
- Demand-controlled ventilation and automation can optimize energy use and indoor air quality, especially in variable outdoor conditions.
- Proper duct design and insulation, along with careful control of outside air intake, are essential for safe and effective ventilation systems.
- Regular testing, measurement, and adjustment of ventilation systems ensure they perform as intended and maintain safety and comfort.
- Building science resources and continuous education are vital to understanding and implementing effective ventilation strategies.
Summary
The Importance of Ventilation in HVAC
This section emphasizes the often-overlooked 'V' in HVAC, which stands for ventilation. The speakers discuss why proper ventilation is essential, especially as homes become more airtight and energy-efficient. They highlight the differences between various ventilation strategies, the need for understanding house-specific requirements, and the role of the physics of pressurization and depressurization. Proper ventilation improves indoor air quality and prevents issues like humidity buildup, pollutants, and safety hazards related to exhaust strategies.
Types of Ventilation Strategies and Their Practicalities
Different ventilation methods are explored, including circulation, extraction, and dilution. The discussion underscores that in high-performance homes, natural air exchange tends to be minimal, necessitating mechanical solutions. Balanced systems like ERVs and HRVs are favored because they control moisture and energy exchange. Positive-pressure systems with ventilating dehumidifiers and exhaust-only systems are also discussed, with an emphasis on the potential risks of exhaust-only strategies, such as drawing in contaminated air from attics or crawl spaces. The conversation highlights modeling tools like REDCalc to evaluate house pressurization effects.
Measurement and Tailored Solutions in Ventilation
The speakers stress the importance of measuring existing conditions, such as CO2 and carbon monoxide levels, to identify actual problems before applying solutions. They share real-world examples like high CO from gas ranges and discuss how measuring enables more precise and effective interventions. The value of testing and adjusting systems, understanding airflow dynamics, and avoiding one-size-fits-all solutions are emphasized to ensure health, safety, and efficiency.
Designing Balanced and Effective Systems with Advanced Technologies
This section covers high-performance home design approaches, including how ERVs can serve multiple roles like whole-house ventilation and continuous bathroom exhaust. The use of MERV 16 filters and configurations for boost mode during activities like cooking are explained. The integration of demand-controlled ventilation and automation through sensors is discussed as a way to optimize energy use and indoor air quality. Properly designed makeup air solutions, like the Fantech MUAS, are highlighted to prevent depressurization caused by large kitchen range hoods.
Considerations for Dehumidifiers and External Air Intake
Ventilating dehumidifiers are presented as a strategic solution in humid climates, capable of controlling moisture and providing outdoor air filtration. Proper sizing, integration with HVAC, and automation are key points. The speakers also discuss the importance of duct insulation, controlling outside air intake during cold weather, and avoiding excessive energy costs or safety risks from poorly designed systems.
Handling Fan Operation and Building Pressure Dynamics
The discussion touches on the implications of leaving blower fans in continuous operation versus cycling, particularly regarding duct leakage and humidity control. The importance of measuring house pressure, understanding natural air exchange influences like wind and stack effect, and using appropriate tools and calculations (like REDCalc) to model pressurization effects are highlighted. The goal is to achieve effective ventilation without creating negative or excessively pressurized conditions that could compromise safety or efficiency.
Resource Recommendations and Final Tips for Learning
The speakers recommend resources for further learning, including articles by Joseph Lstiburek, building science courses, and educators like Allison Bailes. They emphasize the importance of testing installed systems, understanding psychrometrics, and observing building science principles to find natural balance in homes. Practical tips include measuring before and after implementation, automating when possible, and continuously learning about the intricacies of ventilation.
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Show notes
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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