2025-12-22 – Weekly HVAC News : Origin of the 400 cfm/ton rule

Last week’s HVAC forum was buzzing with discussions on optimizing airflow and efficiency. Members delved into the intricacies of static pressure management and explored advanced tools for airflow calibration. Conversations also touched on historical aspects of HVAC technology, with intriguing insights into the earliest heat pumps. The debate on the financial impacts of SEER ratings versus duct leakage also drew significant attention.


This Week’s Hot Topics

Static pressure log sheet with auto-flags
A practical tool for technicians—this discussion focuses on enhancing efficiency by automating flags in static pressure logs.
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ESP-to-CFM quick check for MERV 13
Members are exploring how to quickly verify ESP-to-CFM relationships when using MERV 13 filters, crucial for maintaining system performance.
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Balancing speed and quality on no-heat calls
An important conversation about maintaining high service quality without compromising speed during no-heat emergencies.
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Earliest heat pump used for salt works
A dive into history, this thread uncovers the pioneering use of heat pumps in salt production processes.
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Origin of the 400 cfm/ton rule
Curious about where this rule of thumb originated? This discussion digs into its historical roots and relevance today.
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Quick tools for dialing in airflow
Efficient tools and techniques for precise airflow adjustments are the focus here, offering actionable advice for fieldwork.
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Why we drill 1/4-inch test ports
This topic clarifies the reasoning behind using 1/4-inch test ports, emphasizing accuracy in measurements.
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SEER or duct leakage — what hits bills harder
An intriguing debate on which factor has a greater impact on energy bills, providing practical insights for cost-conscious homeowners.
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Best quick-read manometer for comfort calls
Members share recommendations for the most reliable manometers to use during comfort-related service calls.
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Stay engaged and keep sharing your experiences. It’s your contributions that enrich our community and drive the conversation forward.

Switched a ranch last week from about 420 to about 360 cfm/ton after checking TESP with a Testo 510i; latent improved and blower noise dropped. > Best quick-read manometer for comfort calls — I grab the 510i because I can zero it outside and read TESP in under 2 minutes, but 400 cfm/ton is only a starting point, so I confirm with the blower table and the unit’s ESP rating.

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