Month: June 2019

As an A/C tech I can sometimes get the terms “defrost termination” and “defrost fail safe” mixed up because they sound pretty similar. Before we cover these terms lets set the basic defrost groundwork for refrigeration (coolers and freezers)

Defrost is accomplished in one of a few ways, these first two only apply to “coolers” where the box, air and product temperatures are above freezing but the coil temperature drops below freezing –

Off Cycle Defrost – In Medium temperature applications where the box air temperature is above freezing there is often no set defrost and instead the coil defrosts when the system naturally cycles off. This relies on appropriate over-sizing and can lead to issues when the heat or moisture load is high, especially when the door are opened a lot for loading and unloading.

Timed Defrost – In medium temperature you can use a defrost timer to simply pump down or cycle off the compressor at particular times while keeping the evaporator fan running to force a defrost  few times per day.

Next we have the methods used for defrosting low temperature applications which are below freezing and generally 0ºF to -10ºF depending on what is being stored.

Electric Heat Defrost – On a set schedule (time) the compressor is pumped down or cycled, the evaporator fan is (generally) shut off and the electric heaters are turned on inside the evaporator coil.

Hot Gas Defrost – On a set schedule the evaporator fan is shut off and hot discharge gas is pumped through the evaporator coil.

In both of these situations the goal is to get the ice off the coils as quickly as possible but to stop the defrost cycle as soon as the coil is ice free but no sooner. We don’t want to terminate or stop the defrost too early and leave ice but we also don’t want to keep adding heat to the coil for no reason.

Defrost Termination

This is where defrost termination and fail safe comes in. The evaporator coil cannot go above 32ºF so long as there is still ice in that area, so it stands to reason that if heaters are running on the coil and the coil is still at 32ºF or lower then there is still ice. A defrost termination thermostat is mounted onto the coil to detect when the coil is free of ice and will often be set to “terminate” or stop the defrost heat when the coil reaches around 55ºF – 60ºF to ensure the entire coil is ice free.

So the defrost starts on a scheduled time of 2 – 6 times per 24 hour period and terminates once the coil defrost termination thermostat ends the defrost.

It is also common for defrost cycles to have a “drip” time once defrost end to allow water to drip off the coil after defrost and then a fan delay once the refrigeration begins again to prevent the fan from blowing water off of the coil into the box. This is often set to 30º or lower before the fan can come back on.


Fail Safe Time 

There needs a to be a time limit to how long a defrost can go before it goes back into refrigeration to prevent catastrophic product loss in the case of defrost termination failure. This is part of the defrost clock and is often called the fail safe or fail safe time.

The fail safe time can be a wide range of times depending on the application and frequency of defrost but 20 to 40 minutes is common. If your fail safe time is 30 minutes this means that once a defrost cycle begins the LONGEST it will remain in defrost in 30 minutes regardless of the defrost termination thermostat.

Demand Defrost

This strategy is of using time and temperature for defrost is still the most common found in the trade. There is a more advanced strategy called demand defrost that only initiates defrost when sensors predict that defrost  is required. This is often done via trend analysis between sensors to “learn” when ice is present and when it is fully defrosted and will require some manufacturer specific understanding of the particular controls scheme.


Regardless of the strategy the goal is the same

  1. Defrost when needed to keep heavy ice buildup off the coil
  2. Stop the defrost cycle as soon as the ice is gone
  3. Don’t blow water off the coil into the box / case by starting the fans too soon
  4. Use strategies that don’t cause catastrophic product loss if a sensor fails

From a technicians standpoint its important that you fully understand the defrost strategy being used and that you fully test the defrost cycle after you make any changes.

— Bryan



Low pressures are often measured in inches of water column or “WC. Like most units of measure, it has a very simple origin, in a water manometer 1″ of water column is literally the amount of force it takes to raise the column of water by 1”. While some water manometers (water tube) are still in use the vast majority are either dial or digital gauges that still use the same scale.

1 psi is equal to 27.71 inches of water column; this is why water column is most often used to measure pressures under 1 psi. These low pressures are most often read using a manometer or a magnahelic gauge.

When we measure water column with our tools it is calibrated at atmospheric pressure or the gauge scale instead of the absolute scale. This means that for a manometer or magnahelic to be properly used they  MUST be recalibrated before each (many auto calibrate to zero) to compensate for changes in elevation and barometric pressure. At altitudes over 2000′ above sea you will also need to follow manufacturer recommendations to adjust the gas valve and even change orifice sizes in some cases due to the effect the lower atmospheric pressure has on the gas.

Gas pressure is usually measured in “wc, most commonly we set single stage appliances to 3.5” wc on natural gas and 11″wc on propane. This varies based on manufacture specs, combustion analysis and meter clocking tests. Always read the manufacturer specs.

We also use “WC to check air static pressure on systems. Static pressure is pressure that is exerted in all directions in a contained space, it is not the directional force of the air.

We use a manometer or a magnahelic and measure the negative air pressure in the system return side before the blower (and after the filter whenever possible) and the positive pressure supply air side directly after the blower. By calculating the differential you come up with the total external static in water column. For example, if the return static is -0.3″wc  and the supply static is +0.2″wc the total static is 0.5” wc.

Many manometers and all magnahelic gauges (to my knowledge) have two ports so you can read the differential pressure all at once. This also comes in handy when reading/testing differential pressure on many furnace air pressure switches to ensure they make and break at the proper pressure.

— Bryan Orr

I’ve always liked old books.

Think about an old printing press somewhere in Chicago or Boston or Scranton, Pennsylvania.

Imagine workers with their hands covered in ink up to their elbows, setting type while giant machines of iron, steel, and brass stamped out a book page by page. Then those pages went on to be bound, crafted in a way that few things are nowadays.

At that time the pages were new and crisp, fresh ink and fresh paper giving off a distinctive odor.

But that’s not the part I like the most.

The part that piques my imagination is the people who wrote it and the world they lived in. In most of my imaginations, the past is all in black and white, full of dull people, living dull lives.

But that’s just wrong.

When I open one of these old books they talk about problems we still face today, with information that still applies

Pretty quickly you begin to see the genius of these writers. You start to understand that their lives and work were often very similar to our own with many of the conveniences stripped away.

These people had to be resilient and resourceful. They had to memorize more and read more because access to information was rare and precious. They were more reliant on experimentation and discovery because much of what they knew they had to find out for themselves and pass on person to person.

Many of these books for the trades are written between the industrial revolution and the Second World War. A time in the world when anything seemed possible both good and evil.

Great leaps in technology and progress on the positive side. Abuse of workers at home and a looming enemy abroad seeking to tear the fabric of civil society apart on the other.

In reading the Building Trades Handbook from 1899 I learned that there was a booming correspondence school in Scranton Pennsylvania that educated thousands by mail correspondence in the trades and engineering one page at a time.

The books start with very simple skills like working with fractions that can be so daunting for Tradesmen Even today.

I learned in the American Electricians Handbook from 1921 that we knew so much about electrical motors and electrical engineering at that time. So much of it is well explained in that text using explanations that would make sense to the average Workman.

On the other hand, electricians from that era were not nearly as concerned with preventing electrical shock. The practices used to diagnose electrical circuits are laughable and frightening by modern standards. It does show that the Tradesmen that came before us were tough… even to the point of being a bit crazy.

While all of this is very interesting I’ve noticed something else. Most of the really great educators in our trade have gone back to old books to find answers.

Jim Bergmann told me that he went to old books to find answers about carbon luminous flame in old furnaces and boilers.

Text From The Philosophical society of Glasgow 1881

Dan Holohan always speaks about going back to books by “dead men” to learn about steam heating.

Joe Lstiburek (Building Science) talks about going back to very old construction books to learn about capillary action and capillary breaks to prevent moisture intrusion.

Why is that? Why do old books contain information that some of the new ones don’t?

Remember when you played that game of telephone as a kid where you say a phrase to one person and it’s repeated around the circle. By the time it gets back it’s either nothing like the original or a good portion of the information is missing.

That’s what often happens in education.

Those who make significant discoveries, invent practical machines and applications and work out the math are the first educators in a particular field.

Not only do they write about it, but they also LIVED IT.

The generations after that tend to get split, with the educators focusing mostly on the teaching and the field workers focusing mostly on the doing. They both have a piece of the puzzle, but over time the message gets diluted and breaks down until nobody REALLY understands the whole anymore.

We see this in our field today all the time –

Engineers know lots of theory and math but not what commonly goes wrong or the practical elements of the field.

Manufacturers understand their products but not necessarily the application.

Installers know how to assemble systems but not why or how to properly design.

Techs know how to fix “most” problems but really understand the why of a design? Forget about it!

For those of us who really want to understand the work we do we are left with going back to those people long dead who made the discoveries themselves.

The ones who worked in unsafe buildings and grabbed hot wires, and worked in sweltering labs before A/C existed and also the ones who wrote old books.

I just got in a book like that…

Never stop learning, never stop reading old books. Take a look at Ebay and Amazon and let me know the treasures you find.

— Bryan



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