Month: December 2017


Dehumidification features are common on residential systems ever since the introduction of variable speed blower motors. The system is set up so that the blower can produce less CFM per ton when latent (humidity) load in the space is higher than the setpoint relative humidity. Slowing the blower increases moisture removal by reducing the sensible load on the evaporator coil and therefore dropping the coil temperature and surface dewpoint.

Most variable speed fan coils and furnaces have a terminal designated for Dehumidification and it can be called D, dehum, DH or something else depending on the manufacturer.  In all cases I am aware of, this dehumidification terminal must be energized for the blower to go to full speed and when that terminal is de-energized the blower speed (usually) drops to 80% of full speed.

For years we have seen thermostats with designated dehumidification terminals to match up with the fan coil/furnace terminal, so it was just a matter of disconnecting a jumper from the dehumidification terminal to the R terminal in the unit and connecting a wire from the designated thermostat terminal to that dehumidification terminal in the unit. The diagram below is an example of this on an old Carrier Thermidistat with a variable speed Carrier fan coil.

We now have 24v control smart thermostats like Ecobee, Cor, Nest and Lyric with a lot more flexibility in how they can be set up rather than having a single, designated dehumidification terminal.

I am a big fan of EcoBee for many reasons including their Alexa integration, remote wireless sensors and application flexibility… but you need to be really careful with how you set them up, ESPECIALLY when setting up dehumidification.

The image above is a GIF and should show you the first part of the dehumidification setup. I am setting it up for a single speed compressor heat pump with a variable speed fan coil. EcoBee has contacts labeled acc+ and acc- that can be set up to do a wide variety of functions. For this typical dehumidification function using the system you would select Menu>Installation Settings>Dehumidifer >1 Wire ACC+>Open contact state to activate dehumidifier.

This setup uses 24V power from the R terminal to energize the acc+ terminal and therefore the dehumidification terminal in the fan coil/furnace when there is NO call for dehumidification.

Now for a controversial part. Go to the equipment menu and select Dehumidifier to “dehumidify with fan”= no. We have seen several occasions where the blower continues running with no cooling call if this setting is set to yes when there is a dehumidification demand and no cooling demand.. According to the EcoBee website HERE it appears to say the opposite, but we have confirmed on a few occasions that this occurs and there appears to be no adverse effects from setting it to off becasue the blower is still controlled by the thermostat for cooling operation and dehumidification without cooling is not possible without an external dehumidifier.

In order for the system to over cool below the temperature setpoint to dehumidify you need to go into the thresholds menu and set up AC over cool Max to the maximum temperature below setpoint that would be allowed during dehumidification by the equipment.

— Bryan



Does setting a thermostat too low directly cause an air conditioning system set in cool mode to freeze?

The answer is, no, at least not directly.

However, low evaporator load (low return temperature or low airflow) and low outdoor ambient temperature can both lead to evaporator coil freezing. Low indoor set-point can lead to low return air temperature which is a form of low load condition that can lead to coil frost accumulation on air conditioning systems… but it is actually pretty rare.

It isn’t the act of setting the thermostat too low that causes the freezing, it is only when the return temperature drops below the acceptable limit that the freezing can occur.

There is an important distinction we need to make before you go any further. Just because low indoor temperature or outdoor temperature CAN cause freezing, that doesn’t mean that it is the actual cause.

Many units are misdiagnosed as freezing because of these two causes when the actual cause (low airflow, metering device issues, drier restriction etc..) are left undiagnosed.

When a system is found frozen, it must be fully defrosted and tested for other issues before a conclusion is made that low set-point or low outdoor ambient were the root causes.

Most air conditioning systems are set up for around 400 CFM per ton of Cooling.

This will generally result in a 32° to 38° DTD (design temperature difference) on the evaporator coil with 35° being the established standard. This means that the evaporator coil will be 38° to 32° colder than the return air that passes over it and ice will begin to form at 32°.

Based on this, return temperatures of below 70° begin to enter a zone where freezing becomes possible (on systems set up for 350 CFM per ton for example). At a return temperature of 64° frost formation on the evaporator coil becomes likely.

Again, this is due to the return air temperature not just the thermostat setting.

Just because the thermostat is set to 67° doesn’t mean the return temperature will achieve 67°. In the case of a supply to return bypass damper as is common in some residential zoning systems, you may see a low return air temperature even when the thermostat is set normally.

Systems that have long run times and high humidity return air are the most likely to freeze the coil due to low return temperature cause by low thermostat set point. For example in Florida, I have seen vacation homes on a rainy Spring day where a vacationer sets the thermostat to 50° in cool and leaves for the day… that makes for a nice frozen coil when they return because the combination of low sensible load (outdoor temp), lots of moisture to freeze and an all day continuous run time.

A typical residential system with no special low ambient controls or freeze protection shouldn’t be set below 70° indoors or run for a significant amount of time when it’s below 65° outdoor.

This isn’t to say that they WILL freeze if you set it colder or run when it’s colder than that, but given enough run time it is possible.

It isn’t a system “running too hard” that causes it. It isn’t the set-point itself, it is the heat load on the evaporator coil that can cause the suction saturation to drop below 32°.

Once again, as a technician, never blame set-point until you have exhausted all other possibilities.

— Bryan

All fuel-burning appliances require oxygen to burn and sufficient oxygen to burn clean and safe, without soot and CO (Carbon Monoxide).

I live and work in Florida where most of our fuel-burning appliances are 80% efficient with open combustion that utilizes air and oxygen from the space for combustion.

With these low-efficiency appliances whether the appliance is forced vented or natural draft that combustion air is leaving the space, and exiting the flue.

This causes negative pressure that must be allowed to equalize as well as consumes oxygen from the space. It is because of this that these open combustion appliances must either be in a sufficiently large space or communicate with (be open to) a larger space or outdoors.

When you consider that other gas appliances also need to use oxygen and need to vent to outside you can see that without sufficient communication to outdoors that negative drafts can occur on natural draft appliances like water heaters.

This is why all open combustion appliances that utilize combustion air from inside the space must be in an “unconfined space” or connected to an unconfined space or the outdoors using an approved method.

I see many furnaces jammed into tight closets and mechanical rooms with little thought or planning regarding combustion air.

According to NFPA 31, 54 & 58 an unconfined space is a space that has at least 50 cubic feet of open area for every 1,000 Btu of input. This means that a 100,000 Btu furnace must be in a 5,000 cubic ft space to be considered unconfined.

If the appliance is not unconfined then additional combustion air must be made available to the space with one opening at the ceiling level and one near the floor.

If the air is coming from another unconfined space then the openings should be at least 1 square inch per 1,000 BTU and 1 square inch per 5,000 BTU if it is connnected to the outdoors.

While these openings and are needed in many cases to allow for proper combustion and venting it helps illustrate why modern sealed combustion “direct vent” appliances that take all of their combustion air from outdoors make so much sense.

Not only are direct vent appliances more efficient on the fuel utilization side, they also prevent the negative home pressures and/or thermal losses associated with having vents in walls and ceilings.

So either make sure you have an unconfined space, you are bringing air in from an unconfined space or outdoors or you have a direct vented appliance.

— Bryan

Every year…. We watch it EVERY YEAR and it still gets to me again and again. Call me a sucker, but especially living a blue-collar life working in the trades It’s a wonderful life describes the way many of us live, and even more so the way we can start to feel about our lives and our life’s work. Now, if you HAVE NEVER watched “It’s a Wonderful Life” don’t read this until you do… it won’t make sense.

We work day in and day out, for people and with people who don’t always notice what we do or see the value in it (or so we think). The truth is (like the movie reveals) we all matter more than we think. What we do, and to even a greater extent WHO WE ARE has an enormous impact on others and the world around us.

But this story isn’t about you or me, this story is about compression refrigeration and what it did to transform the world

Remember Mr. Potter from the movie? Take a good look at the man above, those fluffy white mutton chops belong to Frederic Tudor, and this grouchy looking man is the “Mr. Potter” of our tale…

but let’s not get ahead of ourselves

The year was 1806 and Frederic Tudor returned from a trip to the Carribean with the idea that exporting ice from the lakes and ponds of Massachuchets would be his path to fortune, and he was right, but not right away.

The Ice failure

Now, Frederic Tudor (from here on referred to as Fred) DID NOT invent the idea of using ice to cool food and drinks… as long as humans have roamed the earth they have been using ice for one thing or another. What Fred did was to create a commercial ice TRADE, and at first, he was REALLY bad at it. Fred’s first few shipments of ice from Massachusetts to the Carribean island of Martinique ended in failure, not only because much of the 80 tons of ice had melted away but also because the locals saw no need for Fred’s frozen water. Most of the ice, like Fred’s dreams of profit, just melted away.

In fact… between the years of 1809 and 1813 Fred racked up some major debts he couldn’t pay, debts that landed him in prison THREE TIMES, this was Massachuchets after all and they didn’t shy away from burning witches or throwing tea into Boston harbor or sending debtors to prison. Fred was no exception to this rule, and not unlike Bill Belichick in his days with Jets… he may have been cast off for a time, but Boston loves a winner… and Fred was a winner.

The Ice King Cometh 

After the war of 1812 had settled down…..

you know the war of 1812 right? the one that was just a big misunderstanding where the British burned the Whitehouse to the ground? Well, that war had an impact on the newly created international trade for ice. By 1815 everything had calmed a bit and Fred learned that in order to sell ice he had to –

  1. Become a pitchman for the product. Sure, prevention of meat spoilage was the most important market for his product but serving ice cold drinks in the Summer… that was the sizzle that sold the frozen steaks.
  2. Cut the ice more efficiently. Cutting by hand was labor intensive, he figures out how to cut and move the ice with horse-drawn sleds and it saved a TON of time. Now…. the horses added some FIBER to the ice occasionally, but it was just a small consequence of progress.
  3. Insulate Better. After losing so much ice shipping it great distances, Fred learned how to pack the ice better and insulate more effectively.
  4. Diversify. Shipping ice to a tropical location, saving some of the ice and the shipping back fruit preserved in ice…. that was a genius move.

All of these gains added up and by 1833 Fred was known as the “Ice King” when he shipped 180 tons of ice from Boston to Calcutta, India. The British elites in that far off place were able cool their drinks and their tempers.. we didn’t want another accidental Whitehouse incident after all.

But Fred was worried…. sure he dominated the natural ice trade, but there was a new technology that kept him up at night

The Machines of Cold

All the way back to the 1750’s (before Fred was born) inventors had been experimenting with the effects of evaporation of certain substances on the removal of heat. I can only imagine one of these men,  sitting in Philadelphia in the hot Summer of 1776… thinking about how his earlier experiments might cool Liberty hall, or at least their drinks as they hammered out a new country. Benjamin Franklin was one of the first to consider and experiment with changing the state of liquid to vapor to produce ice… but he certainly wouldn’t be the last.

The was inventor Oliver Evans in 1805 who talked about a method of continuous vapor compression to make ice produced by machines a reality… but he didn’t actually make it a reality himself. After all, making a few cubes of ice with a machine would be a novelty, a parlor trick in those days. Ice needed a market, a distribution system, market DEMAND. Fred was the one who created those things more than two decades after Oliver’s bright idea.

The Idealist

John Gorrie was not an inventor, it’s unlikely that he studied the papers of Benjamin Franklin or Oliver Evans. John was a doctor, born in the West Indies, raised in South Carolina and educated in a prestigious New York medical school in a time when medicine was a respected profession filled with misinformation and HORRIBLE practices. This is a time when writing a constipated patient a prescription for toxic mercury or soothing a colicky infant with narcotic “soothing syrup” was commonplace. So when John moved to steamy Apalachicola, Florida in 1833 t the age of 30, heaven only knows what sort of crazy nonsense he was doing to his patients unintentionally.


John Gorrie was a good man by all accounts who really cared about helping people get well. He had left a comfortable life in to deal with tropical diseases like malaria and yellow fever, brought into the Florida port town by sailors and spread by mosquitoes. John must have had to adjust to the oppressive heat and humidity of a Florida Summer and it was this HEAT that caught John’s attention.

By this time John had had some of Fred’s incredible ice in a drink and it sure made him feel better on a hot day. What if some of these illnesses could be helped… even cured if he could control the temperature the rooms of his patients with ICE.

His first experiments with using Fred’s expensive imported ice and it sure appeared to help lift the spirits of those in his care, but he just couldn’t get enough of the stuff to make a difference.  John’s theory was that diseases like malaria were a “vapor” carried in from the swamps and that by draining swamps and cooling, dehumidifying and ventilating patients rooms you could reduce or eliminate the disease.

So like most people who have a problem, they want to solve he began reading and experimenting and in 1844 John wrote about a machine he wanted to create that consisted of

“two double-acting force pumps, one for condensing, and the other for rarifying air and an air magazine or receptacle for condensed air. It may be placed in any part of a house or ship..”

Just to save you from looking it up, rarify means to “make less dense” or more simply to expand. In essence, John described a very simple vapor compression system that uses air as the refrigerant.

An Icy Reception 

It was the Summer of 1847 as the story goes, and a French nobleman Monsieur Rosan was celebrating Bastille Day in steamy Appilachicola. As the guests were commenting on the oppressive heat the nobleman rose and announced to his guests that they would be able to enjoy chilled wine, thanks to ice mechanically produced by doctor Gorrie and his prototype machine.

In 1851 John was granted a patent for his ice making machine, and as the story goes he also found a deep-pocketed investor willing to put up the money to make this prototype a commercial success. A Boston investor lost to history, a Boston investor who died before the deal was final.

It was then that the wheels of John’s ideas began to come off. In publications across the country, the articles would write of Dr. Gorrie and his folly, of how inefficient his machines were and how foolish the invention was… after all, the “Ice King” had already solved the problem by making ice available all across the globe.

Did Fred cause this trouble to fall upon John? Probably not. Did he help the process along the way? I would be willing to bet he did.

In the end, John and his friends would speak of how he was undermined by the ice lobby, by Fred and his ice gathering and shipping machine. In 1855 at the age of 51 Dr. Gorrie died a broken man, a man who in his own words had “had been found in advance of the wants of the country”, in other words, he accepted that he was a man before his time.

This story has no happy ending. There was no Clarence the wingless angel to show John the present that would have been without him or more appropriately the future that was. partially because of John and his contributions, a future all of us who are reading this work in every day.


Real change within industries and societies requires someone dogged and determined to take up the cause and push the idea to its destination. Inventors like Edison and Bell, industrialists like Rockefeller and Ford and even military leaders throughout history from Alexander the Great to Napoleon.

These people are applauded in retrospect for their brilliance and foresight, but to a man, their greatest asset was their determination. For every one of these “greats” whose names you know, there are thousands… Millions more, who have lived simple lives, working hard towards ideas they care about. People of determination who like John Gorrie and George Baily do the thing they were set on earth to do.

In the story of Mr. Potter and George Bailey or Frederic Tudor and John Gorrie, we clearly paint a hero and a villain. One man motivated by ego and greed, the other by altruism and vision. The fact is that every person has a bit of both in us. We all want to make a living, provide for our family and maybe buy a boat or a new truck along the way. It’s a noble thing to live a simple life and put some money away to enjoy, but don’t miss the opportunity to do the big thing that gets put in your path once or twice in a lifetime as well.

The fact remains that like John Gorrie we all play a part in the long, winding and noble story of our trade. You and I are a link in a chain back to Gorrie and Tudor and Franklin and to the people and systems we serve, we are more important than those men, dead and gone.

I leave you with a fitting (if a bit dramatic) poem, written for Gorrie 50 years after his death.

Give him a niche in the temple of Fame
Give him his place and hallow his name!
He, who in love for his suffering kind,
Lent them the use of his wonderful mind:
Pointed the way by unheard of device
To make in the Tropics the purest of Ice.

Give him a niche! May his name never die!
Build him a monument stately and high;
Who, in the ages, has equaled his thought?
Who for his fellows such solace has brought?
Think of the troubles his skill has allayed!
Think of the inroads on pain he has made.

Give him a niche and enshrine it with flowers!
Honor the man with divinity’s powers!
He who, no matter how sultry the day,
Drove from damp foreheads the fever away:
Pay quick a tribute that nobody shuns,
To GORRIE — greatest of Florida’s sons

Now go move some heat, and Merry Christmas.

— Bryan



I need to warn you…

This is the actual process we use at the company I own for our typical “silver” residential maintenance. I’m sure you will find some things you do differently. Take it for what it is and I’m happy to get any feedback you may have.

  • Read the call notes, property notes and customer notes. Check the last service call and last notes and readings so you are aware of the service history.
  • Check the filter size and ensure you have the PROPER filter if possible
  • Wear shoe covers in the home, ask the customer if they have noticed anything unusual with the system.
  • Inspect and set the set the thermostat to run, shine a light in the return to check for filters, blockages, debris or damage.
  • Visually inspect the system operation to make sure all of the components appear operational before beginning the maintenance. Note anything out of ordinary you observe to the owner and address / diagnose before proceeding.
  • Remove disconnects / shut off breakers. Check for proper breaker sizes and inspect disconnect wires, lugs and pulls.  
  • Remove the condenser top and panels and place them carefully in the grass away from damage.
  • Remove any debris from the bottom of the unit. Inspect wires and compressor terminals while doing this. Use a vacuum to remove dirt and leaves as required. If grading is poor, use a shovel to scrape dirt/leaves away from the base of the unit.
  • Wash the coil well, starting from inside out and from top to bottom. Only use coil cleaner according to the labeled dilution and only use it when the coil requires it.
  • Check the inside of the condenser one final time for any potential wiring or copper rubouts and repair / isolate as required. Note any rust on the compressor, roto locks or accumulator.
  • Inspect the crankcase heater if the system has one. Confirm operation by amperage or ohming out.
  • Look for any signs of refrigerant oil (potential leak points)  
  • Replace the top carefully, ensuring that you don’t pinch any wires. Rewire the fan.
  • Inspect all wiring connection in the condenser control box for tightness and damage. Check contactor points and note condition.
  • Move inside, if the air handler has a finished floor around it, lay down a drop cloth. Always keep the work area inside clean during and after work.
  • Check the evaporator coil condition and cleanliness top and bottom. If the coil is a slant coil orientation and has any dirt on it clean the surface with evaporator coil cleaner, pump sprayer, rag and a soft brush.  If the coil is dirty and an A or another coil type that cannot be easily cleaned in place you may quote a removal and clean if the coil has no known leaks and is less than 7 years old.
  • Connect a wet vac to the drain outside
  • Remove the panels from the air handler and begin pouring water into the drain pan all the while helping any sludge in the pan get removed by moving it toward the pan outlet. Use duct tension straps or zip tie ends / small bottle brushes to help pull sludge out from the sides of the pan under the coil and clean all channels.
  • Run a minimum of 2 gallons of water through the pan and then empty the vacuum. Run another gallon through and repeat the process until the pan is visibly clean and the water in the vacuum is clean.
  • Run 1 more gallon through the drain one done and ensure it runs out.
  • Spray and wipe down the inside and outside of the air handler, including wires with a safe anti-microbial solution.
  • Remove any dust from the blower motor body and end bell with a vacuum, rag or soft brush, being careful not to force dust further into the motor. When the dust buildup is severe, use compressed air or nitrogen and a vacuum to remove it.  
  • Test the blower capacitor by removing the leads, testing with a capacitor tester and reconnecting the leads.
  • Inspect the blower wheel for cleanliness. If it is dirty check the particular maintenance type to see if removing and cleaning the blower is extra or included.
  • Check the blower motor bearings for play.
  • Inspect all wires for rub out inside and outside the air handler. Inspect and disconnects and check any lugs for tightness.
  • Check low voltage wiring and dip switch/pin settings. If the system has an advanced interface you will need to check for proper settings at the controller.
  • Check coil feeder tube location and condition. If tubes begin to rub out, isolate them and strap them together using foam tape and zip ties.
  • Inspect the float switch for proper installation and wiring. Test the float switch to ensure it breaks the circuit when the float rises. If there is no float switch, quote to install one at the end of service. 
  • Check air handler panel insulation, glue or retape as required.
  • Spray down the coil surface lightly with antimicrobial and add 3 pan tabs to the front of the drain pan away from the outlet.
  • Install a new air filter with date and your name.
  • Replace the bottom air handler panels and turn the air handler breaker on.
  • Run the heat and test the heat strips on and off using an amp meter. Note heater and blower amps.
  • Shut off the air handler, replace all panels and double check the drain cleanout cap is in place and the float switch is in the correct position.    
  • Put the system into cool mode and turn the air handler back on (condenser disconnect still off / out)
  • If the system has been checked during previous calls and has no history of leaks then use the historic data to perform a Non-invasive refrigerant test. If this is the first maintenance or if the system has a history of issues then connect gauges.
  • Connect an amp clamp to compressor common and observe as you turn on the condenser breaker/disconnect.
  • Test the L1 & L2 voltage and ensure it is in the acceptable range
  • Allow the system to run for at least 10 minutes. During this time you can Begin Cleaning up
    1. Start filling out the service call/collecting model and serial numbers if required
    2. Inspect suction insulation and thermostat wire outside for damage or poor splices
    3. Perform the “under load” capacitor test
    4. Read across the contactor points to see if there is any voltage drop in the contact points
  • Once the system has run for 10 – 15 minutes perform either the non-invasive test protocol or check and note suction, head, superheat, subcool and delta T according to the “5 pillars” tests and any manufacturer guidelines.
  • If the system is a heat pump, test the opposite mode (usually heat) to ensure the reversing valve shifts properly and the system runs.
  • Note and suggested repairs or improvements to the customer and get their response.
  • Clean up fully and double check drains refrigerant caps and disconnects. Call in standby.
  • Wrap up paperwork with the customer.
  • Ask the customer if there is anything else you can do to make their experience better and if they can think of any way you or the company could improve.
  • Complete all call notes, finish timesheet entry, neaten van in preparation for the next call.

   — Bryan

P.S. – If you have a good maintenance procedure for gas, oil or other types of systems would you be willing to share it? you can email it to me at / I can give you credit or make it anonymous, whichever you prefer.

P.S.S – I just put out my first attempt at a tool list. It is sortable by sector and my thoughts on which tools are most critical but I would like your feedback HERE IT IS

P.S.S.S. – We have a cool giveaway going on right now



Let’s take a deeper dive into the magic that is gas defrost..

Most techs who are familiar with heat pumps understand the basics of a gas defrost but when we apply this strategy to a larger system where we’re only reversing a small part of the system while we need to add some controls and valves to get the job done optimally.

Since we’re already familiar with the basics of defrost systems and controls, I’m not going to dwell on things like frequency or duration of defrost but we will get into some unique terminations methods and defrost efficacy testing that only work with reverse cycle defrosts.

There are 2 basic types of gas defrosts.   Hot gas defrost where superheated discharge gas is directed into the evaporator and “Kool gas” a trademarked name for a defrost that directs saturated vapor from the top of the receiver unto the evaporator.    Each have advantages and disadvantages but both work essentially the same way.

So, defrost starts and a whole lot starts happening at once.   3 electrically actuated valves all have to work together to make this happen.

First, we need to create a pressure differential between the gas we’re sending into the evaporator and the liquid line.   This is to allow that gas to flow through the evaporator and back into the liquid line.  There are many different valves that are applied to do this and an in depth treatment of each valve isn’t really possible here, so we’ll just look at the 2 most common places they’re applied.

Discharge line

This is more common on hot gas defrost system as opposed to Kool gas systems.  A valve is installed in the discharge line that, when activated, creates a pressure differential.

Liquid line 

Same thing, really.   This valve will work for either but is really necessary for a Kool gas system.   A discharge differential won’t work for Kool gas.


Regardless of the location in the system, the valve is typically adjusted for an 18-20 PSI (1.24 bar – 1.47 bar) differential setting.   If your equipment is significantly higher than your evaporator this may need to be set even higher.  We’ll get into a method to test this and ensure that the defrost is working properly towards the end of the article.

Differential created, we now need to direct defrost gas to the evaporator.   To do this, we have 2 valves.   One that stops flow from the suction line into the compressors and one that directs gas into the suction line and back towards the evaporator.   At the same time the differential valve activates, both of these valves activate and start the defrost process.


Photo caption:  the grey bodied valve, installed in the vertical line stops refrigerant flow to the compressors.   The brass valve installed on the horizontal line opens to admit hot gas to the evaporator.

Out in the evaporator, we’ve got a check valve piped to bypass the TEV


 not visible in this photo is the actual check valve.   The line leaving the distributor allows condensed liquid to leave the coil, bypass the metering device and re-enter the liquid line through a check valve.


Last thing is that, with all this heat being forced into the evaporator we normally want to turn the evaporator fans off and sometimes turn on small heaters to prevent water running off the coil from freezing on a cold drain pan.   Using either a pressure switch that cycles

Let’s “follow the gas” and try to visualize what’s happening during this defrost.   So, we’re sending high pressure, superheated vapor into a cold suction line.   That gas immediately starts rejecting heat into the surrounding pipe and any frost or ice that’s in contact with it.  Remember, we’re going backward, so we hit the outlet of the evaporator and we’re heating it up, melting that frost away and rejecting heat from the gas all the way.   As we continue to pass through the evaporator, we’re going to reach a point where we’ve rejected enough heat to condense and possibly to even subcooling as a liquid.  Eventually, we reach the metering device and are routed through a check valve that bypasses that and winds up in the liquid line.  With a Kool gas defrost, we aren’t starting with superheated vapor, but the concept remains the same.  Warm, saturated vapor is sent to the evaporator where it condenses and is subcooled and forced back into the liquid line.

As liquid is condensed and pushed through the check valve, more and more hot gas is allowed into the evaporator to provide more heat to completely defrost the coil.  Without the pressure differential, we wouldn’t be able to push the liquid out of the coil because a pressure differential is required for anything to flow.

Is one ‘better’ than the other?

One drawback to hot gas defrost is the expansion and contraction of refrigerant lines due to temperature swings can be extreme if the lines run far enough.  Remember that copper can expand over an inch per 100’ of pipe with a 100°F(55°K) change in temperature, so we have to consider the expansion and movement of the piping.

Using a Kool gas defrost helps with the pipe expansion problems but tends to have less heat available for defrost and, combined with a modern push to lower compression ratios for efficiencies sake, can have problems clearing the whole coil during colder weather.

So, what can go wrong??

Sounds like a great system.  We’re reusing heat that would ultimately be wasted to melt frost from a coil.   Economically and ecologically awesome, right?

As with any complex system, there are multiple points of failure.  If any of the 3 electrically activated valves fail to operate either because of a control system fault or a mechanical problem with the valve itself, we set ourselves up for trouble.

If the differential valve fails, we won’t have an adequate flow of refrigerant to get enough heat for a complete defrost.  Similarly, if the solenoid valve that opens to allow defrost gas into the suction fails to completely open, we won’t have enough flow.

If the suction stop solenoid fails to close, we’ll can see a range of problems from inadequate defrost from the amount of bleed through to a complete failure to close that allows all of the defrost gas to flow straight into the compressors.   You can see this same problem if the hot gas solenoid fails to close properly after a defrost.


Testing defrost


I promised earlier that I’d give a method to test gas defrosts to ensure that they’re working properly.

For this test to work properly, we need a coil that is free of large ice buildup but that has a ‘normal’ frost on it.   If I’m troubleshooting a particularly difficult system, I’ll first clear all ice from the coil, then disable defrost overnight and return in the morning to ensure that I have the right conditions to test the defrost.

Now, I’ll connect a thermometer to the line that bypasses the TEV at the evaporator and allow that to stabilize.  I really like to use a thermometer that record Min/Max readings for this job. You can also take the temperature on the line leaving the evaporator or really anywhere along the liquid line that is dedicated 100% to that circuit.   It that line runs all the way back to the compressor unit, you can test it there although the further from the evaporator you measure the temperature, the less accurate the test becomes.

Make a note of the temperature in your notebook and go start a defrost.   Monitor this temperature and a distinct pattern should emerge if defrost is functioning properly.   The temperature will hold stable for a couple minutes.  Typically this is already pretty cold because we’re in a refrigerated space, then it will start to drop.   I will normally see a start temperature in the low ‘teens’ here and expect within 2-4 minutes to see it dropping and it will hit a low of -2°F to -6°F(-18.8°C to -21.1°C) ).  This is a rush of liquid that has condensed in the evaporator and has rejected so much heat that it is very subcooled.

This temperature will then start to rise as there is less and less frost to absorb heat from the gas.  Once all the frost is gone, this will start rising pretty rapidly.   Once it hits 65°F (18.33°C) on newer equipment and 75°F (23.88°C) or so on older equipment, you can be sure that there is no frost left on the equipment and that any further defrost is just wasting time and is detrimental to equipment operation and possibly to product shelf life.

Much of the timing depends on the length of the suction line and the amount of frost buildup on the coil.   A shorter suction line will result in a faster temperature drop while more frost on the coil will result in a slower but deeper dip in temperature before it starts back up.

This is also probably the best method to use to terminate this type of defrost.   Monitor that temperature using whatever means available to you and, once the liquid temperature rises above either a manufacturer’s predetermined setting or one that you’ve field determined through testing, you can end defrost.

–Jeremy Smith CMS




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