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ROT Can Cause Duct Issues – Short #160

In this short podcast, Bryan explains how rules of thumb (ROT) can cause duct issues. He talks about the role of friction rate in duct design as well as its intent and limitations.

Friction rate is a value located on duct calculation tools, including Ductulators. We use friction rate to predict the operating static pressure of the system, but it is often misapplied when people design their ductwork around rules of thumb. The friction rate is expressed in inches of water column (“WC), which we also use to measure static pressure. However, the operational static pressure and friction rate are NOT the same things.

Ductulators provide information about friction rate based on 100 feet of straight ductwork in the size selected, which we almost never see in the field; fittings and turns add effective length (EL), so the total effective length (TEL) is often more than 100 feet. When duct designers apply rules of thumb, like a 0.1″ friction rate, and apply it to the CFM, they don't consider the actual length of the duct. So, the ducts are often undersized and don't properly account for the actual resistance to airflow.

If you want to stop using rules of thumb, ACCA Manual D and related software can help you get more precise design parameters and account for other restrictions.

The following tech tips contain more information and specific equations to help you find the total effective length: The Friction Rate Chart (and What it Means), What the Heck is a Friction Rate? (Eric Kaiser), How to Determine the Friction Rate for Residential Duct Design (Neil Comparetto).

Learn more about the HVACR Training Symposium or buy a virtual ticket today at https://hvacrschool.com/symposium.

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HVAC School – For Techs, By Techs

Real training for HVAC ( Heating, Ventilation, Air Conditioning and Refrigeration) Technicians. Including recorded tech training, interviews, diagnostics and general conversations about the trade.

Matthew Taylor delivers an expert-level presentation on EPRs, building on his previous work on parallel rack systems. While his earlier content focused on the similarities between air conditioning and refrigeration, this session explores what makes commercial refrigeration unique—particularly the critical role of EPRs in maintaining optimal operating conditions across multiple evaporators running at different temperatures. This presentation was shared at the 6th Annual HVACR Training Symposium.

The discussion begins with a fundamental review of the refrigeration cycle in a typical supermarket setting, where 30 to 80 evaporators may share a common suction line. Matthew explains why EPRs are essential: when multiple cases need to operate at different temperatures (ranging from -13°F for frozen foods to 24°F for fresh products) but all connect to the same compressor rack, EPRs become the solution that makes this possible. Without them, cases would cycle on and off constantly, creating efficiency nightmares, oil management problems, and potential food safety issues.

Matthew walks through the mechanical principles of various EPR types, from the high-efficiency Sporlan SORIT valve with its pilot-operated design to the Parker A8 valve that can be installed directly in the store. He also addresses the industry’s shift toward electronic EPRs, particularly the CDS modules that offer temperature-based control rather than just pressure regulation. Throughout the presentation, Matthew emphasizes practical considerations: how EPRs affect compressor staging, oil system pressure, defrost cycles, and ultimately, the core product temperatures that determine food safety. The session includes real-world troubleshooting insights and addresses common misconceptions about setting superheat on systems with EPRs.

This technical presentation provides HVAC professionals with the knowledge needed to understand, diagnose, and service EPR-equipped refrigeration systems confidently. Matthew’s approach demystifies a component that many technicians find intimidating, breaking it down into understandable principles while highlighting the critical role EPRs play in modern commercial refrigeration efficiency and reliability.

Topics Covered

Basic Refrigeration Cycle in Supermarket Applications – Understanding parallel rack systems with 30-80 evaporators sharing common suction and liquid lines
Oil Management Systems – Oil separators, oil reservoirs, oil regulators, and the critical pressure differential required for proper oil flow
Compressor Staging and Capacity Control – How parallel rack compressors operate as multi-stage units to match system load efficiently
Saturated Suction Temperature (SST) – Why racks are designated by temperature (e.g., “13-degree rack” or “-13 degree rack”) and how this relates to the coldest evaporator requirement
Temperature Difference (TD) Engineering – The relationship between evaporator temperature and case leaving air temperature, typically 10 degrees in traditional systems
EPR Fundamentals – Why EPRs are necessary to maintain different evaporator pressures on cases operating at various temperatures while connected to a single rack
Mechanical EPR Types – Comparison of Sporlan SORIT valves (pilot-operated, low pressure drop) versus Parker A8 valves (self-contained, higher pressure drop)
Electronic EPR Systems – Modern CDS modules and other electronic controls offering pressure control, temperature control, or hybrid approaches
System Stability and Load Management – How proper EPR settings prevent compressor hunting, reduce energy consumption, and protect oil management systems
Subcooling Requirements – Why liquid receivers eliminate natural subcooling and how mechanical subcoolers restore it before expansion devices
Core Product Temperature – The critical relationship between runtime, EPR settings, and food safety in refrigerated cases
Dual-Temperature Applications – Converting medium-temp cases to low-temp operation (like holiday turkey displays) using EPR pilot solenoids
Superheat Setting Procedures – Why EPRs must be overridden to 50-100% open position when setting TXV superheat
High Glide Refrigerants – Special considerations for setting EPRs with refrigerants that have significant difference between dew point and bubble point temperatures
Troubleshooting Philosophy – Understanding EPRs and TXVs as independent systems that don’t directly affect each other due to non-critically charged liquid receiver systems
Pressure Drop Considerations – How EPR pressure losses (0.5-2 psi depending on type) affect compressor suction setpoints and energy efficiency
Electronic Control Integration – Various controller brands and approaches to managing electronic EPRs, from pressure transducers to temperature sensors and PID algorithms

Have a question that you want us to answer on the podcast? Submit your questions at https://www.speakpipe.com/hvacschool.

Purchase your tickets or learn more about the 7th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.

Subscribe to our podcast on your iPhone or Android.

Subscribe to our YouTube channel.

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Search Episodes

EPRs and Rack Refrigeration w/ Matthew Taylor

01/22/2026

Bryan Orr

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