Short #64 - Diagnostic Duct Design

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Short #64 – Diagnostic Duct Design

July 23, 2019

For a technician who comes across a customer balancing complaint, How do you think about and help to solve the problem?

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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.

Glycol secondary loop systems are showing up in more and more grocery stores, and in this episode of the HVAC School Podcast, Bryan Orr sits down with Max, one of the company’s lead startup techs and its in-house subject matter expert on glycol skids. Max explains what these systems are, why clients are choosing them for retrofits, and what it takes to commission one without taking a live store down. The big picture: a traditional DX rack chills a water and propylene glycol mix in a heat exchanger, and that glycol is pumped around the store to cool the medium-temperature cases.

The conversation starts with the “why.” Reduced refrigerant charge inside the building, keeping flammable or higher-GWP refrigerants out of the sales floor, regulatory pressure, and zero-emissions initiatives all push clients this way. Max is upfront that efficiency isn’t one of the reasons at this scale, since the extra heat exchange step, lower suction temperatures, and pump energy all cost something. He also covers the trade-off in reliability: case-level controls are simple, but a fault in one of the skid’s main valves can take out every case at once, so techs have to move fast.

From there, Max walks through the skid itself, from the brazed plate heat exchanger, VFD-controlled pumps, and air separator to the expansion tank, strainers, suction guide, controller, and EEV drivers. He then covers flow control with balance valves, defrost strategy, and the importance of proper vents and drains in the piping. The two get into the details of startup and commissioning, and some key takeaways include pressure testing instead of pulling a vacuum, filling and purging air, and verifying that the refrigerant-side valves are wired and driven correctly so a swapped valve doesn’t flood the rack.

The back half of the episode is practical field advice. Max shares how to recognize air in the loop, why store humidity and dew point matter so much for case performance, which specialty tools and fittings he carries, and how to handle glycol mixing and top-offs.

Topics Covered

What a glycol secondary loop system is, and how it differs from direct expansion
Why retrofits are happening: refrigerant charge reduction, safety, EPA regulation, and zero-emissions goals
Efficiency penalties at small scale, and why efficiency isn’t the driver
How glycol behaves in the coil: no latent exchange, a temperature gradient, and frost patterns
Reliability trade-offs: simple case controls versus centralized, mission-critical valves
Staging a retrofit so the store stays open, including pre-piping, pre-testing, and pipe cleaning
Brazing versus soft solder, and whether nitrogen flow matters with glycol
Skid components: heat exchanger, VFD pumps, air separator, feeder tank, expansion tank, strainers, and suction guide
Controller inputs, EEV drivers, and emergency mobile chiller connections
Balance valves (automatic versus manual) and defrost methods, including off-time and electric
Fluid distribution: pitching pipe for air and drainage, plus vents and drains
Propylene versus ethylene glycol, the 35% mix, freeze point, and why more glycol hurts heat transfer
CO2 as a secondary fluid for low-temperature applications
Commissioning steps: electrical checks, pressure testing, filling, purging, and testing valve operation
Proving performance: flow, glycol concentration, temperature, and superheat on the rack side
Troubleshooting air in the system and pressure changes as glycol chills
Store humidity, dew point targets, and case performance
Heat reclaim using glycol loops
Tools: PT ports, manometers, female-to-female hoses, refractometer, and transfer pumps
Service tips: distilled water, matching glycol brands, and glycol safety