Month: March 2018

This topic came up because I was testing out the new MR45 digital recovery machine and that machine goes off by itself when it hits a 20″ Hg vacuum. This is a cool feature but it is good to know when that level of vacuum is overkill and when it’s not enough according to EPA requirements.

Why would you need to recover into a vacuum you might ask? Well, so long as you are above a PERFECT VACUUM (and you always do) there are still molecules of refrigerant in a system even at 0 pisg (14.7 PSIA at sea level). In low pressure systems like centrifugal  chillers the entire system charge can often be in a vacuum when the system is off, this means that recovery on these systems means you START below 0 PSIG and go down from there. 

First off let’s pretty much assume that none of you are using recovery machines OLDER than 1993 so really only look at the right side of the chart above.

If you are working on an air conditioning system with UNDER 200 lbs you are safe taking your recovery to 0 or atmospheric pressure. If the system you are working on has OVER 200 lbs of refrigerant or if you are working on a medium pressure or low pressure system you will need to pull the system into a vacuum.

The EPA does make an exception if the system has a know leak and pulling into a vacuum will result in contamination of the recovered refrigerant. Here is an excert from the EPA final rule summary from 1995 (still in force)

Also let me clarify that 25mm hg absolute is another way of saying 25 torr or 25,000 microns, it’s just a finer scale and it goes from 760 torr (760,000 microns) down as the vacuum gets deeper whereas inches of mercury (“hg) goes up as the vacuum gets deeper.

— Bryan


In Florida, there are not many gas furnaces, At least not as many as up North. Sometimes we can look like real dummies compared to techs who work on them everyday.

One thing to know about 80% gas furnaces with cased evaporator coils is that you can often check the evaporator coil by removing the high limit and running an inspection camera up through the opening.

You may also be able to use a mirror and flashlight but you usually won’t see much due to the heat exchanger being in the way. Otherwise, you are stuck removing the entire blower assembly… and that’s no fun at all.

Another practice is benchmarking the static pressure drop across a new coil when it is dry and wet when installed or during the first service call. You can then easily watch coil loading over time without the need to look at the coil visually.

— Bryan

The receiver is also often called a “liquid receiver” and you will see it on everything from small self-contained refrigeration units to very large commercial and industrial systems.

Many new techs who are used to residential air conditioning confuse receivers with accumulators. While an accumulator is located in the suction line before the compressor and prevents liquid from entering the compressor a receiver is located in the liquid line after the condenser and stores liquid refrigerant.

The liquid receiver stores refrigerant when the system is operating at less than maximum heat load and is generally designed so that receiver can hold all of the system charges and still be no more than 80% full. This allows you to pump down the entire system charge into the receiver without danger of creating hydrostatic pressure (very high pressures resulting from full liquid expansion) in the receiver.

The multi-position service valve at the outlet of the receiver is called a “king valve” and can be used for refrigerant circuit access as well as fully front seated (turned clockwise) for pump down.

Because a receiver has both liquid and vapor present inside many techs argue that the refrigerant cannot be “subcooled” in the receiver. The truth is that while the refrigerant that interacts between the liquid and vapor at the top of the receiver is at saturation the refrigerant below the liquid line can be and usually will / should be subcooled.

— Bryan

This article was written by Christopher Stephens, he is a commercial HVAC/R service manager in the greater Los Angeles area. Thanks Chris.


Let’s start by covering the three elements that are needed for a Fire-  Fuel, Air, and Heat (spark). With those three elements in the right condition you can start a fire. So, with if we reduce the oxygen to an existing fire we can slow it down until the Fire Department can arrive to extinguish the fire.

In a restaurant, we use exhaust fans to remove unwanted smoke and heat produced by cooking appliances from the kitchen, however we also need to add makeup air to the restaurant to compensate for the exhausted air. If we didn’t than the exhaust fans would create a negative air pressure that would make it very difficult to open the doors to the restaurant. (I always like to use the paper bag analogy here if you put a paper bag over your mouth and suck the bag will collapse, but if you cut a hole in the bag the air will pass right thru the bag) This makeup air can be delivered back into the building by a dedicated supply fan unit or by using fresh air dampers on the air conditioning units. Now let’s move to some safety devices.

Air conditioning units move large amounts of air, so for safety reasons we install duct smoke detectors in the ductwork to “detect” smoke. if they sense smoke they shut down that air conditioning unit to eliminate the oxygen source and to stop the spread of the fire. At the same time they also send a signal to the fire alarm company that there is a fire. Most duct smoke detectors have two alarm functions. They can detect a Fire condition and they can detect a trouble condition. A fire condition is obvious the detectors sensor is sensing smoke so it triggers a direct short (closed) in a relay and sends a signal to the fire alarm company. Now a trouble condition can be several issues all depending on how the detector is configured. We can configure a Duct smoke detector to signal a trouble condition if someone has taken the cover off the duct detector, if the detector has lost power, or if there is a malfunction in the smoke sensor.

It’s important to note that most fire alarm companies monitor a duct detector with an 18/2 thermostat wire, a fire condition is sensed when there is a direct short between those two wires, and a trouble condition is sensed when a resistor that is placed between those two wires disappears from the system by means of a normally open contact closing. So, on a properly operating duct smoke detector without a fire condition if we place an ohm meter across the alarm terminal we should read the resistance of the resistor (the resistance value is determined by the fire alarm panel manufacturer) If we have a trouble condition we will read open line across the alarm terminal and if we have a Fire condition we will have a direct short across the alarm terminal. This is designed so the smoke alarm system can ensure that the circuit is intact at all times, otherwise the circuit could be broken (open) and when a fire occurs the alarm wouldn’t trigger. The resistor helps the fire system prove that the circuit is intact and ready to rock.

Now let’s talk about exhaust fans as stated above they remove smoke and heat from the building during normal operation. But if there is a fire in the restaurant they can aid in slowing down the fire until the fire department arrives to extinguish the fire. We can do this by turning on the exhaust fans and turning off all supply air to the building (makeup air and/ or air conditioners) and by doing this we will be reducing the oxygen in the building to suffocate the fire. We accomplish this task by using relays built into the exhaust control system. These relays open and shut off all of the A/C systems and makeup air while closing to turn on exhaust. This reduces the exposure of the flame to oxygen and therefore reduces the spread and intensity of the fire.

— Christopher

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