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A Steam Heat Primer by Dan Holohan (Podcast)

By Bryan Orr

In today's podcast episode, Bryan gives an audio recording of the legendary Dan Holohan's article, “A Steam Heat Primer.”

Read that article HERE.

Check out more about Dan's work at heatinghelp.com.

If you have an iPhone, subscribe to the podcast HERE, and if you have an Android phone, subscribe HERE.

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Author:

Bryan Orr

Company: HVAC School

Position: Founder

Check out our most recent video

How to Evacuate an AC system, Full Vacuum Procedure

Our latest Podcast

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.

In this action-packed live stream episode of HVAC School, host Bryan is joined by Eric Kaiser, Ty Branaman, and Roman Baugh for a continuation of a deep-dive conversation on vacuum practices — picking up where a previous episode left off with Andrew Greaves and Jim Bergmann. The team sets out to both reinforce the foundational best practices every HVAC technician should follow and to explore some genuinely uncharted scientific territory around how vacuum gauges actually work, how refrigerant contaminates micron gauge readings, and what really happens to moisture inside a system when temperatures drop below freezing.

A central revelation of the episode is Eric’s explanation that modern electronic vacuum (micron) gauges do not actually measure pressure directly — they measure heat transfer and translate it into a pressure reading. Because these gauges are calibrated to nitrogen or air, the presence of refrigerant vapor in a system (which has roughly three times the heat conductivity of nitrogen) can cause the gauge to display a falsely high reading. This means a technician could believe the system still has poor vacuum when it may actually be further along than indicated — or, more concerning, that a system appears to have passed vacuum when contamination is still present. The team acknowledges that controlled experiments are needed to quantify exactly how much refrigerant affects the reading, and they commit to designing those tests.

The conversation then pivots into the physics of water at the triple point — the precise pressure (4,580 microns) and temperature (32°F) at which water can exist simultaneously as solid, liquid, and vapor. Eric walks the audience through a phase diagram built from International Association for the Properties of Water and Steam data, explaining that once pressure drops below the triple point, liquid water no longer exists. Any moisture in the system either sublimes directly from solid ice to vapor or remains frozen. This has major practical implications: a system with ice inside can still pull down to a very deep vacuum, but will not pass a decay test until that ice is fully sublimated — which requires both sufficient vacuum depth and available heat energy. The colder the ambient environment, the deeper the vacuum must go to create the temperature differential needed to drive sublimation.

The episode wraps with an illuminating discussion on refrigerant oils — specifically the differences between POE (polyolester) and PVE (polyvinyl ether) oils and how each interacts with moisture in fundamentally different ways. POE chemically bonds with water through hydrolysis, breaking down into acid and alcohol and permanently degrading the oil. PVE, on the other hand, physically traps moisture through surface tension and can hold up to twice as much water as POE, but remains chemically stable. This distinction affects vacuum strategy, dryer sizing, and long-term system reliability — particularly in VRF and cold-climate heat pump systems where compressor oil management is far more complex.

Topics Covered

Core vacuum best practices refresher: large hoses, removing valve cores, skipping the manifold, using clean pump oil, micron gauge placement, and decay testing
Why micron gauges measure heat transfer — not pressure — and how refrigerant vapor causes false-high readings on the gauge
The impact of refrigerant retained in compressor oil on vacuum accuracy and the potential role of nitrogen sweeps in displacing refrigerant molecules
Triple point science: what happens to moisture when pressure drops below 4,580 microns and why liquid water no longer exists below that threshold
How ice inside a system can allow a deep vacuum pull-down while still failing a decay test, and what that means for cold-climate HVAC work
The role of heat during evacuation: why adding heat accelerates moisture removal and how deep vacuum increases temperature differential to drive sublimation
Cold-climate challenges: vacuum pump limitations, micron gauge accuracy at low temperatures, and the physics of dry air in freezing environments
Triple evacuation and nitrogen purging: whether nitrogen disrupts oil pockets, displaces refrigerant, or both — and why the team wants to test it
Nitrogen tank quality concerns: the possibility that low-grade nitrogen could introduce moisture and whether an inline dryer would help
Using system flush chemicals: why Ty cautions against flushing agents and the risks of adding additional chemicals that break down oil
POE vs. PVE oil chemistry: how POE undergoes hydrolysis when exposed to moisture (creating acid) versus how PVE physically traps water without chemical breakdown
Dryer strategy for large commercial systems, VRF, and VRV: filter dryer sizing, core pulls, oil sampling, and why an “oil dialysis machine” would be a game-changer
Plans for future controlled experiments: testing refrigerant effects on micron gauges, ice behavior at various temperatures, and vacuum performance in cold climates
Industry influence over time: how community-driven knowledge sharing has already shifted vacuum and refrigerant practices over the past decade

Whether you’re a residential technician looking to sharpen the fundamentals or a commercial refrigeration specialist wrestling with VRF oil contamination, this episode delivers both practical takeaways and a front-row seat to the scientific inquiry that drives best practices forward. As Bryan puts it: “Don’t wait for us — if you want to do the experiment, be part of the conversation.”

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.

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