Dehumidifier Facts & Troubleshooting

This article is written by tech and business owner Genry Garcia from South Florida. I met Genry at a Solderweld demonstration and he later offered to write this excellent article. Thanks Genry!

Though dehumidifiers have increased in popularity thanks in part to the implementing of new building codes, at the same time they have become a kind of red-haired stepchild…the runt of the litter if you will. We all know is there but nobody gives it too much attention. I have seen a few that a couple of years later still have the original filter in and likely haven’t worked right in a while. For the purpose of this article, I am only gonna refer to ducted, vapor compression refrigeration ‘whole house dehumidifiers’ like the ones in the picture above.


What is a dehumidifier?

A vapor compression refrigeration dehumidifier works exactly like any other refrigeration system in the sense that has a compressor, a condenser coil, a method of refrigerant metering and an evaporator. The main difference is that the condenser coil is placed immediately downstream from the evaporator and as such the air that has been cooled and dehumidified through the evaporator it’s then reheated before being discharged. Most dehumidifiers are rated at 80 ºF. and 60% RH entering air conditions and their capacity is expressed in PPD (pints per day of condensate removal). Most of these units have all their components arranged inside one single cabinet but, there is at least one manufacturer that offers a split system option where they claim there is no sensible heat load added to the space.

I personally like this hot gas reheat strategy. It removes water vapor from the air in the space we’re trying to condition while adding sensible heat back as to not overcool it. The discharge air can be 20 to 30 degrees higher than the return. This is by design, a sound tactic since the warmer and drier discharge air has a larger specific volume which results in a lower percentage of RH when this air mixes with the rooms. However, special consideration should be given to where and how this warm and dry air is going to be introduced into the controlled space as not create unwanted warm spots.


Why a dehumidifier?

The two main reasons in my experience are to pre-condition ventilation air that is needed and/or required by building codes in humid climates and my favorite, which is to control the inherent water vapor that can accumulate in attics, when the line of the building envelope is moved to the roofline by insulating it with spray foam. What? Inherent? Why is there humidity accumulating in the attic if is not ventilated? Great questions, Dr. Joe Lstiburek explains it in this article. There is also the occasional retrofit job where a dehumidifier gets added (hopefully the discharge air does not get connected to the return side of the system) in an effort to alleviate high humidity issues in a space. A dehumidifier’s application and its connection/integration method it’s a controversial enough subject, here is great new research on what is the best way to connect a dehumidifier  by the Florida Solar Energy Center. Special care should be taken when ducting these, whatever the configuration might be. They don’t move a lot of air to begin with and any scenario that results in mild to high static pressures will seriously tax their capacity.



So, let’s say you get a call where you eventually arrived at the suspicion that the dehumidifier might not be doing what is supposed to. This type of call is usually tied to a consumer complaint for lack of comfort in areas where “it used to feel fine but it hasn’t in the last few weeks” or the most common one in my experience; sudden occurrences of condensation on supply vents and on ductwork surfaces.

First things first, you want to make sure that there is a demand for the dehumidifier to be operating and that both, the fan motor and the compressor are working. If they are not, those issues are normally simple to address by following the wiring diagram. Once we’ve established that all the components are operational and its capacity performance it’s what’s left to check then like almost all things HVAC, the manufacturer’s specs rule. In this article, we are going to be following this one from Honeywell.

There are 3 methods to check the performance of a dehumidifier in the field:

  1. We can measure the volume of condensate that is generated over a pre-determined period of time.
  2. We can measure the inlet and outlet air temperature and humidity.
  3. We can measure its power consumption.

Condensate Volume

One pint is 16 ounces of volume and one pound is 16 ounces of weight and like the saying goes “A pint’s a pound, the world around” so for the purpose of this test we’re going to use them interchangeably. Let’s say that we are working on a DR65 (65PPD nominal capacity) from the specification data document referenced above. As plotted in the chart below, if our entering air condition is 80 ºF and 60% RH then that would mean that the unit should be removing about 68 pounds of condensate per day.

The nominal capacity of 65 pounds of condensate in 24 hours equals to approximately 2.71 pounds per hour, so if we wanted to check this unit’s performance using this method, we would cut the drain line and collect the condensate in a measuring cup. It should produce about 14 ounces in 20 minutes.

Easy right? Not so fast, here is my issue with this method. This would work fine if we knew for a fact that the unit has been working enough time to have produced enough condensate to wet the whole evaporator coil, to have enough collected at the drain pan so it flows out the line and to top it all off, from this manufacturer at least, the drain connection port is under negative pressure so it will need a P trap, which would also have to have enough water in it for it to flow out into our measuring cup.

This is not a bad way to check a dehumidifier’s performance but it has a few variables, it can be deceiving and it certainly takes more than 20 minutes. How much more? Who knows, depends on how much water its already inside the unit and the drain line upstream of where you are collecting the condensate.

Measuring the Inlet and Outlet Air Conditions

Using the same scenario from the first method, we are gonna try it a different way now. At our known entering air conditions of 80 ºF and 60% RH the first step is to remove the duct connections if any (more on that in a minute). After the unit has been running for a few minutes we then take our leaving air conditions which I would expect them to be at 100 to 110 ºF dry bulb and 15 to 20% RH. If instead, our entering conditions were at 75 ºF and 50 %RH, then our leaving air conditions would be closer to 90 ºF and around 22 to 24% RH. On the opposite extreme, if our entering conditions were 100 ºF and 35% RH (as the case may be in a “ventilated” attic) one can expect our leaving conditions to be closer to 130 ºF and 12 to 14% RH.

Disclaimer: These readings are based solely on my personal experience. As a matter of fact, I have in many occasions reached out to technical support reps of at least two different brands and asked them point blank why is it that the math doesn’t add up when I use the latent heat formula (QL = 0.68 x cfm x ΔW) and the response has always been something along the lines of “It doesn’t work that way” or a plain “I don’t know”.

I have consistently logged this numbers on properly functioning dehumidifiers for the very reason of being able to cross-reference them when working on one which performance is questionable.

About removing the duct connections: As you can see below, a dehumidifiers’ airflow does not fare well at even mild static pressures. For this reason, to perform this test it is best to disconnect the ductwork if it’s a ducted application. Consequently, an additional valid test is to, once we have recorded our measurements without ductwork, we can then re-connect it and perform the same tests for the purpose of comparing readings. This can and will reveal that perhaps the unit is not the problem but the duct configuration is.


Caveat: These units tend to have the compressor located right upstream of the discharge air collar, maybe on purpose. For this reason, when measuring the leaving air conditions, the mean radiant temperature of the compressor can affect our reading when taken too close to the discharge air connection, despite the lower emissivity of its black painted shell…in other words move the probe out 6 to 10 inches so you don’t pick up radiant from the compressor body.


Power Consumption

This is probably the simplest and easiest one of the three methods as long as we have access to the manufacturer’s literature. Let’s look at the chart below.

At the established entering air conditions of 80 ºF and 60% RH, the dehumidifier should be consuming around 600 Watts. 597 to be more exact.

We can obtain the Watts reading in one of two ways…or both but one is easier I promise:

  1. Using a multimeter capable of Bluetooth connectivity, we can open the dehumidifier, place the clamp over one of the line voltage wires, close it back up and record the amperage reading in a phone or tablet while is running. We will then multiply this current value by the line voltage to get the power consumption. Depending on which meter you are using, a direct Watt readout might be possible but getting the leads to safely stay on the points where the incoming line voltage can be measured will be tricky.
  2. Get yourself a Kill A Watt and get a faster and safer Watt reading which we can then cross-reference in our chart to confirm the dehumidifier’s performance.


In conclusion, all 3 methods have their merits and disadvantages. As it is the case with many of the issues that we solve in our industry taking the time to analyze and look at all the aspects of a problem and/or consumer complaint will offer a broader view of what the possible solutions can be. Focusing on any single reading or test method and offering a diagnostic based on that alone will invariably lead to mistakes that could’ve been avoided. There is no replacement for good ole’ common sense and thorough research.

— Genry Garcia

Heat Doesn’t Rise

We've all heard some version of the phrase “heat rises” but is that really true? First, we need to remember that heat is energy not matter. Heat is a force not a thing, so while heat may result in changes to matter (stuff) it, isn't matter itself. When we add heat to stuff the molecules inside move faster and when you remove heat molecules move more slowly.

So no, heat cannot rise because heat isn't a thing. Hot air, on the other hand, does rise in colder air.

We see that when we heat the air in a balloon it will rise and float in the colder air around it, but why does this happen?

It all comes down to density and buoyancy and we see it all the time in water. When something is less dense than water it will float upward and when it is more dense than water it will sink down. Even water changes in density with changes in temperature and will sink or float depending on the differential of one mass of water compared to another.

Colder air is denser (more lbs per sq ft) than warmer air. So colder air “sinks” in warmer air and warmer air “floats” in colder air due to buoyancy just like hot air balloon floats in the air or a rubber duck floats in a bathtub.

When you add sensible heat to air the molecules in the air begin to move more quickly and they start to separate making warmer air less dense when the molecules are free to move. When you remove sensible heat from air the molecules slow down and the air becomes denser.

But that isn't the force at play in air and heat movement.

We ALSO know that heat tends towards equilibrium or “hot goes to cold” so that when a cold air mass hits a warm air mass and they start to mix the heat from the warmer air will start to enter the colder air creating an equilibrium.

Then we also see that pressures also tend towards equilibrium or “high pressure goes to low pressure” which also impacts air movement.

Because air is relatively free to move in a building you will observe all of these forces at play at once with some of the dominant forces being stack effect in the winter and reverse stack effect in the summer.

When you increase the temperature of air in a space through a heater the density of that warmer air leaving the register will be lower than the colder air around it. This will result in the warmer air “floating” in colder air and the colder air “sinking” below the warmer air.

As that warmer air continues to rise it will naturally create a lower pressure near the floor which will tend to draw in cold air from outside through any gaps lower in the home. This is what we often refer to as “stack effect”.

In the Summer when the air is cooled, the cooler air will sink in the warmer air creating a lower pressure near the ceiling that will tend to bring in heat from gaps higher in the structure such as can lights.

Again… there are many factors that impact the movement of air in a space and stack effect is only one of them and is based on buoyancy.

For example, imagine a roaring open fireplace on the first floor of an old leaky home. As that fireplace heats the air, air begins to rush up the chimney. This creates a low-pressure area in front of the chimney and air from all over the home pulls into that area to fill the void (high pressure goes to low pressure). At the same time, the fireplace is heating the room it is in (mostly through radiant heat) and the heated air starts to float in the colder air. In the meantime, the entire house is going under negative pressure compared to the outdoors and cold air is being drawn in from gaps and cracks all over the home.

In other words…. Air is tumbling and mixing all over attempting to balance the forces of pressure, temperature and buoyancy due to the simultaneous increase in room temperature and a decrease in room pressure caused by the open fireplace.

So heat doesn't rise, hot air floats in colder air and cold air sinks in warmer air and there are many other forces at play.

— Bryan

Vacuum Pump Oil

This article is written by Sal Hamidi, founder of, an innovative manufacturers representative agency that promotes great HVAC/R products through training and media. You can reach Sal at


If we are going to discuss vacuum pump oil it's important to understand what it is first.

Most HVAC application vacuum pumps are rotary vane pumps in single or double stage design.  A vacuum pump moves vapor molecules out of the system working in and out of the pump through the vacuum pump oil itself, making the vacuum pump oil one of the most critical variables in the evacuation process.


Would you like a single or a double they ask?  Depending on the level of vacuum you need to achieve, knowing whether to use a single or double stage vacuum pump is critical, as the stages determines the number of cycles the pump operates on with internal workings (chamber, rotor, and vanes) and exhaust, which determines the oils pressure in each exhaust stage.

Once you choose the correct pump for the job much of the performance and longevity of the pump comes back to the type and state of your vacuum pump oil.

“Oil” is a bit of a misnomer because modern pump oil technology has evolved well beyond the original distilled petroleum products.

There are now double- and triple-distilled oils available, as well as hydro-treated oils, low sulfur oils, silicone-based synthetic oils, and flushing oils used to clean the pump. Due to the wide variety of formulations available, these are often now referred to as pump “fluids” rather than pump “oils”.

These application-specific oils need to be refined for vacuum pump application and must be pure from additives, except for anti-foaming, anti-oxidation, or corrosion resistant additives.

Typical vacuum pump oil is mineral oil that is refined, has low vapor pressure and specific viscosity for the application it is being used in, high temperature or low-temperature applications. Oil selection for the average tech comes down to reading the literature from your pump manufacturer and matching it to the best oil.

What does vacuum pump oil do?

The primary role of most oils is mechanical lubrication and the vacuum pump is no different, additionally, the vacuum pump oil seals the vanes in rotary vane pumps, cools the pump through heat transfer between inner and outer parts of the pump.

The oil does the following:
– Provides a seal across the vanes and duo seal between the high pressure and low-pressure side of the pump in two-stage pumps
– Cools the pump by conducting heat from the stator to the outer casing where it can be dissipated.
– Protects the metal parts from corrosion caused by oxygen and vapor from the system
– Most Importantly, it lubricates the moving parts of the pump


Lubrication, the most important variable in our pump doing its job properly, as well as withstanding good ole natural wear and tear.  Why so important? Without the right viscosity properties (friction characteristics) or thickness the vacuum pump oil could lead to reduced efficiency or pump destruction due to lack of metal to metal protection.  

Not viscous enough? Seals don’t get created, and unwanted “leaks” begin to happen reducing pump performance and ultimate vacuum. Too viscous? oil fails to be properly deposited where it is needed which can once again lead to wear and poor performance. 

Creating the Seal

A seal in a vacuum pump between its high pressure and low pressure, or induction and exhaust sections of the pumping process, specifically for rotary vane pump application allows for separation of gas and oil within the pump reducing any unwanted contamination. If you look at the video above the seal needs to be created at the top of the rotor as well and in between the vanes and the chamber.

Vapor and Discharge

You have oil running into a pump where It is mixed with moisture and unwanted contaminants, this oil is then discharged through exhaust where it is exposed to pressures where it is either evaporated and discharged or it runs through the filtration process, and back through the pump.

Our vacuum pump oil is designed to have a low vapor pressure, much lower than the moisture in the HVAC/R system, ideally discharging it to the atmosphere.  Low vapor pressure means having lower evaporative properties where high vapor pressure has high evaporative properties. Because the vapor pressure of the pump oil is very low it can operate at extremely low pressures without evaporating. The deeper the vacuum the lower the pump oil vapor pressure must be which can become a factor in ultra-deep vacuums used in some industries and in scientific experimentation. 

Gas ballast

A gas ballast provides a way to allow air to enter into the pump during the initial stage of vacuum to help prevent moisture contamination and fouling of the oil on wet systems. 

In practice, techs are often taught to leave the ballast open until the pump is able to pull the system below a pressure (measured with a vacuum gauge at the system) below the boiling pressure of water for the ambient temperature and then you would close the ballast to pull the final deep vacuum. Vacuum Expert Howard Tring noted that leaving the ballast open until you get to the boiling point of water is a waste and that the goal is to leave the ballast open only when it is stalled at the moisture evaporation pressure/temperature point.

In reality, many techs will leave the ballast closed on a system that they have no reason to believe is wet and will only use this technique when they suspect the unit may be wet or when they have already had to change the pump oil once.

The proper use of gas ballast keeps contaminants out of the oil which can extend the life of the oil and the pump.

When should you change your vacuum pump oil?

Vacuum pump oil is substance inside the pump that lubricates the pumping apparatus while also collecting moisture and contaminants from evacuated systems. Simply put, it’s what keeps the pump pumping. This is why changing oil as soon as it needs to be changed is critical.

Because vacuum pumps don’t have filters, the oil inside will eventually become saturated with contaminants, which reduces the pump’s efficiency.

So, how often should you actually change it?
Best practices are….
– When the vacuum pump level is significantly higher than the rated blank-off vacuum. This is best tested with a high vacuum gauge mounted directly on the inlet port.
– Change whenever you observe a discoloration of the oil whether it is a change to creamy from moisture or if it's darkening from other contaminants or heat.
– Depending on your oil type, application and possible contaminants, the frequency of oil changes can range anywhere from daily to monthly but more often will extend pump life
– When the vacuum pump is not reaching the desired vacuum level. This is best tested with a vacuum gauge placed directly on the pump.

There is no exact answer here, an environment of use, proper use of pump aka proper use of gas ballast, and storage of oil all play a factor in how long your pump will last.
As soon as there is a notable difference from its original clear state is a good time to change your oil.  The signs above of contaminated vacuum pump oil defines for you whether your vacuum pump oil has altered its ability to keep its designed viscosity or its vapor pressure for maximum efficiency.

Change pump oil based on the manufacturer approved method but generally when the oil is warm. Make sure to dispose of the oil in a legal and approved location.

To Recap

• The most important factors in choosing vacuum pump oil are its viscosity and vapor pressure
• It is meant to seal the inner workings of your pump
• The Oil Lubricates your pump without blocking transfer areas of your vacuum pump oil
• It transfers contaminants from the pump to the atmosphere through pressure difference and evaporation 

— Sal


Triple Evacuation and Nitrogen FACTS

This article is written by longtime contributor and RSES CM Jeremy Smith. Thanks Jeremy!

Nitrogen doesn’t absorb moisture like many techs think that it does and I think that we, as technicians, need to reevaluate the reasons for the “triple evacuation” process.

OK.. Hold on, now. Put down the pitchforks and torches and give me a chance here.

Take a minute or two and read what I have to say and at least I’ll have a head start if you still want to come after me, OK. Fair enough?

First off, let’s start with what nitrogen is. Nitrogen is a chemical element with the atomic number of 7. An odorless and unreactive gas that forms about 78% of earth’s atmosphere.

Nitrogen is not truly inert, it is mostly inert, it is generally non-reactive but it can and does have some chemically reactive properties. They don’t affect much of anything that we work on in the HVAC/R industry so we can safely consider it “inert” for our purposes but it isn’t exactly inert. If it were inert, Ammonia (NH 3 ) would not be possible as nitrogen would not be able to react with the Hydrogen. But I kind of digress from the point I’m trying to make here.

My point is about the use of nitrogen to “absorb” moisture during the evacuation and dehydration of a refrigeration system using a process commonly called “triple evacuation”. Quite a lot of techs seem to think that nitrogen has some special properties because it’s “dry” and it will remove a bunch of water from a system. That’s not really the way things work and I’ll explain why that is if you’ll bear with me a bit. As we work through this together, I’m going to try to minimize the number of confounding
variables. This is both to keep everything consistent and for the sake of simplicity.

When we deal with nitrogen in our case, we will be referencing nitrogen at 70°F and standard atmospheric pressures. We are also going to assume that nitrogen is the primary agent in air that absorbs or holds moisture and that no other gasses in the mixture that is air have any contribution to the ability of air to absorb moisture. This way we can both simplify the thinking and the math involved and we will give nitrogen the benefit of the doubt. Increasing the pressure of nitrogen above atmospheric will reduce dramatically the amount of moisture the nitrogen is capable of holding, so we will keep it at atmospheric again, to give nitrogen the benefit of the doubt.

First off, let’s look at what “dry” means in this context. What is “dry nitrogen”? Industrial grade “dry” nitrogen is 5ppm moisture or lower. For convenience, we will use 5ppm as a reference to maintain constancy and evaluate the worst possible case scenario. This 5ppm gives us a dewpoint of -86°F or 0.0187% RH. Now, we have a definition of “dry” nitrogen.

Like air or any gas for that matter, nitrogen can hold a certain amount of water based on its temperature. For nitrogen, this relationship can be found in the psychrometric charts for air. Let’s look into the psychrometric charts and see just how much moisture nitrogen can actually hold.

Remember, in this case, we are giving nitrogen 100% of the credit for the ability of air to entrain and hold moisture even though nitrogen only makes up 78% of air. At our selected conditions of 70°F and atmospheric pressure, one pound (by weight) of nitrogen will hold 100 grains of moisture at 90%RH. For reference, one grain is 1/7000th of a pound, so that 100 grains is 0.0143 POUNDS of water. An ounce is 437.5 grains so one pound of nitrogen is capable of holding less than a quarter ounce of moisture at 90% RH at 70°F.

How does that apply to the practice of triple evacuation and using nitrogen to try to dehydrate a refrigeration system?

Let’s start with a couple of more reference points, then we’ll get into the nitty-gritty.

One pound of nitrogen, at atmospheric conditions, occupies 13.8 cubic feet. This means that a 40 CuFt cylinder of nitrogen that is completely full, holds approximately 2.9 pounds of nitrogen.

I like to use hypothetical examples that closely resemble what we see in the field, so let’s set up a hypothetical example as a way to think about this so that we can see how difficult a process removing moisture with nitrogen alone would be.

In our example, we will consider a system which conveniently holds exactly 1 pound of nitrogen at atmospheric pressure and 70°F. Keep in mind that the internal volume of that system is 13.8 cubic feet, so this is a very large system. As a point of reference, 100 feet of 1 ⅛” ACR copper has an internal volume of 0.69 Cubic feet, so this is a very large system we are using for this hypothetical example. Far larger than any residential system one will encounter. The results we get will be scaled down
accordingly on smaller systems.

Into this system, we will introduce one ounce of water. For a little perspective, this is 2 tablespoons full of water. That is not very much water given the size of the system.

What will it take to remove any significant amount of moisture using nitrogen?

The moisture, in our example, is simply a small pool or puddle of moisture, say at the bottom of a trap or simply laying along the bottom of a horizontal section of piping. A victim of poor practices more than likely. This will be a new installation job, so we don’t have any oil in the piping that could cause a problem during the evacuation and removing any other confounding variables from the issue. We just have a puddle of water and we need to get it out of the piping.

For the 1 pound of “dry” nitrogen to absorb the full 100 grains of water that it is capable of holding, it needs TIME. It needs to sit long enough for the humidity to diffuse throughout the entire system and raise the humidity of that entire quantity of nitrogen to 90%. If you only allow it to sit long enough to raise it to 30% or 40% for example, you only pick up between 30 and 40 grains of moisture which is only a fraction of the total maximum that same pound of nitrogen can

In a system of this size and complexity, the time to diffuse the humidity through the entirety of the system could be several hours or longer. Then, you have to repeat the standing nitrogen step once more, again waiting several hours (or longer) for the humidity to diffuse throughout the system, again. All for a measly 100 grains of moisture. Less than a quarter of an ounce.

Remember the basics here. Everything moves from “high” to “low” so the moisture moves from an area of high humidity to an area of low humidity. This process is a rather slow process. Imagine, for purposes of scale, what effect a small pot of water would have on the humidity of a room if that water were left at room temperature. How long would it take for the higher humidity level close to the water’s surface to have an impact on the humidity 20 feet away? Even if we lowered the humidity in the room to the levels of “dry” nitrogen, the humidity diffusion rate would be very low and very slow. Would it ever have a measurable, appreciable effect on the humidity of that room? This is key to why nitrogen doesn’t really have any impact on evacuation procedures.

While evacuating the system, there is a constant removal of moisture as the water will be constantly boiling, constantly absorbing heat from the surrounding piping and materials, resulting in a faster moisture removal because there are no interruptions in the process.

Well, wait, you say. I’m not saying that nitrogen “absorbs” water, but nitrogen DISPLACES moisture. OK, to a limited degree, this is a fair statement. IF you have a system filled with warm, humid air, a gentle push or ‘flush’ of nitrogen will displace that moisture. I’m not entirely certain how this displacement is expected to significantly accelerate dehydration. It will displace moisture one time. A vacuum pump will do exactly the same thing and do it just as fast.

Remember, the moisture content of air is measured in grains of moisture. The actual quantity of moisture is extremely small.

But wait, I’m not using nitrogen to “absorb” moisture and I’m not using it to “displace” moisture, I’m using it to prevent the moisture from freezing because the vacuum is being pulled so fast that I don’t want that to happen. If this is your position on the use of Nitrogen, I’d first ask you to listen to The HVAC School podcast with Jim Bergmann where he specifically addresses this issue with a resounding NO.

In a normal HVAC/R system under conditions that are not near or below freezing, it is extremely unlikely to freeze moisture due to the amount of heat available in
the environment that will keep any liquid water in liquid form and keep that liquid water boiling under vacuum. This simply is a non-issue.

But wait, Jeremy, you say. I KNOW nitrogen helps speed up my evacuation because I use it all the time and it makes my evacuation go faster. I’m not exactly sure what to say in cases like this. I think I’ve made a pretty convincing case that nitrogen doesn’t really assist the evacuation process. The math and the science are really on my side here. In many cases, particularly in the service world where systems are ‘contaminated’ with refrigerants and other gasses, nitrogen can just kind of recalibrate the micron gauge and cause it to show you a true reading rather than one that is distorted by a non-nitrogen based environment.

The last commonly used reason for triple evacuation is “the book says so” Yes, it generally does. If your installation manual, your customer, your employer, your foreman or manager or anyone else that has authority over the job you are doing or your job, in general, says to do a triple evacuation or any other process or procedure that you deem useless, unless that process is unsafe for your person, DO IT. In no way, shape or form am I encouraging technicians to refuse to do work as they are expected to do it because some random dude on the internet said so.

To conclude, yes, nitrogen does absorb some moisture. It really isn’t as much as most people like to think it is when you break it down and look at the math. It also takes a lot more time to have an appreciable impact than most techs are willing to give the process for what little benefit it can offer.

So, what is the takeaway here? Why did I spend a bunch of time working on this, researching the math and laying out these ideas in a way that is (hopefully) easy to read and to digest? Do I honestly expect manufacturers to change their processes and procedures based on my couple of pages? Nope. I don’t. If anything, my goal here can be stated in a single word.


Think about what you are doing. Think about why you are doing it. Evaluate every process, technique, procedure, and idea that is given to you. Don’t take something at face value just because you trust the person telling it to you or because you like him or he’s the boss. I’m not trying to organize some industry-wide revolution, so don’t tell your boss that I said he was wrong for making you do a triple evac. Don’t tell the Daikin or Mitsubishi rep on a VRF startup that I said that triple evac was stupid and that you shouldn’t do it. I haven’t said these things. What I hope you can take away is a little thoughtfulness about the deeper reasons you’re doing the things that you’re doing.

— Jeremy

P.S. – Bryan here. I was talking to Eric Mele about the subject of purging with nitrogen and triple evac. While it's pretty clear that purging with nitrogen or breaking with nitrogen is going to do very little to help with liquid water contamination it does help you get a more accurate reading on your vacuum gauge (like Jeremy stated). It has one other impact which is that it ensures that a larger quantity of molecules left over in the system will be “dry” nitrogen rather than Oxygen or water vapor (which are unwanted) simply through “homogenous dispersion”. This simply means that the more times you run nitrogen through the system the greater the concentration of nitrogen molecules vs. other stuff even under deep vacuum.

The undisputed conclusion is that proper assembly practices and deep vacuum are keys to keeping the nasty stuff out of a running system.

Pool Heater Water Flow Issues

When You first start servicing pool heaters the water flow circuit can feel a bit intimidating.

In this article I'm going to cover the pool circuit basics and common water flow issues that screw up our heaters.

How Pool heaters work

Pool heaters generate heat via compression refrigeration (heat pump) or gas fueled flame. This heat gets passed off into the water through the heat exchanger inside the pool heater. Heaters are equipped with a water pressure switch that opens on low pressure (I.e. pump off or flow restriction). However, just because the water has “pressure” doesn't necessarily mean there is adequate flow to remove heat from the exchanger.

Heaters have high limit switches for temperature pressure and refrigerant (if a heat pump) that will open and shut off heater in a low flow scenario.

In a heat pump when flow is low the system will run high head pressure. In a gas heater the exchanger can begin to cavitate and make a loud banging noise.

In my experience 1/3 of pool heater service calls are actually a water flow issue. Knowing how to diagnose and fix a water flow issue is a must for pool heater techs

The pool pump circuit

The pump pulls in from the pool/spa drains and skimmers. Drains are on the bottom of pool, skimmers are at the surface. Multiple valves are used right before pump to isolate different sections of the intake if needed. Pool, spa or skimmer can each be individually shut off from the pumps intake.

Water enters the pump at the clear sight glass where a large screen trap is positioned to catch any chucks entering the pump. This screen trap can be easily cleaned by turning off pump and removing the clear sight lid.

Water exits pump into the pools filter. This is a large cartridge that has a filter inside. If clogged, Filter can be removed by turning off pump, unscrew lid and lift out the filter. When needed you can leave filter out and replace lid temporarily for testing purposes

After filter, water goes into the pool heater or solar heat. Usually here, there is a water bypass valve right at heaters intake, this would allow water flow to go around heater (if not in use) instead of through it.

Next the water passes through the heaters heat-exchanger and out.

Water then flows through a back flow safety before entering the high chemical chlorinator. This back flow safety's is most commonly a one way spring loaded valve that closes when water flow stops. This protects heater from concentrated chemical flowing back into exchanger when pump is off.

Next in line are multiple vales directing water flow to the pool/spa jets and or waterfall/fountain fixture.

Spa mode

Pools with a spa can have two automatic actuators installed, one at the pumps pool/spa intake valves and one at the pool/spa jet return valves.

When in spa mode valve at intake closest of section that pulls water out of pool drains and skimmers, only allowing water to be pulled from the spa drain.

And the second auto actuator closest off jet supply to pool only feeding the spa jets. This is how the sap can be isolated and heated separately from the pool.

Common water flow issues and fixes

A dirty pool filter wins for most popular water flow issue on a heater service call.

This can be diagnosed by removing the filter and running pump again without filter in.

If the issue was in fact a clogged filter, you should have no water flow problems now.

Fix: Have owner get a filter replacement/cleaning.

Another common water flow issue is an open heater bypass valve. A bypass valve is put in place near intake, to stop water from flowing through the heater when it’s not in use. When in bypass, the heater’s water flow switch (I.e. pressure switch) should open and heater will not run.

Bypass valve is shown as closed in picture below.

Air in the water circuit is another common flow issue.

If the pool lever is low the pump will began pulling air in through the skimmer. You will notice low water flow and lots of bubbles from the jets.

Turn valve from skimmers off. This will pull water just from drains at bottom of pool/spa. In picture below the valve circled can be turned to block water intake from skimmers.

Fix: Let owners know pool needs to be filled before skimmers are opened again.

Most water pressure and flow problems can be easily diagnosed and fixed once there's a clear understanding of the pool circuit.

— Bert Testerman

Does a Furnace Decrease Humidity?

Does heating the air cause the humidity in the air to decrease? Yes and no

Heating air causes the RELATIVE humidity percentage to decrease but it does not change the overall moisture content in grains of moisture per lb of air.

Many old timers will swear a blue blaze that oversizing a furnace will directly result in lower humidity, cracking furniture etc…

The problem with that theory is that no matter how much you heat the the air you don't change the overall moisture content and when you blow that air into the space it quickly acclimates with the room.

But there still may be some truth in this oversizing dries stuff out theory

In the Winter during cold climates the moisture content is very low outside regardless of the relative humidity. When you use a larger furnace than you need you also tend to move more air than you need to.

When you move more air there is often greater negative or positive pressurization of the conditioned space due to zonal imbalance and duct leakage. This pressure imbalance will tend to drive more dry air into the space or more of the inside air out resulting in lower humidity.

Neil Comparetto also pointed out that when the appliance takes its combustion air from the space this can cause significant negative pressures which also draws dry air in from outside. The larger the BTU output the greater volume of air that must come in for combustion.

The other factor is the supply air temperature itself. If the hotter supply air is blowing directly on an object it will tend to dry it out more quickly due to the increased temperature of the object itself.

In conclusion –

Furnaces don't reduce air moisture quantity directly no matter how big or small

There are other reasons why oversizing can cause issues so don't do it.

— Bryan

Relative Humidity of Air Below Freezing

I was listening to someone talk about air relative humidity the other day while looking at a psychrometric chart and he commented that the chart ends down at freezing (32°F) because “all the water freezes out of the air at that point”

I think I made this Jed Clampett face

The psychrometric chart is designed to deal with typical air conditions (especially indoors) and it is true that as air gets cooler the grains of moisture per pound of dry air do drop significantly.

Take a look at the difference between these two air conditions where I hold the relative humidity to 45% and just change the temperature.

First at 40°F

And then at 25°F

So while the moisture content in relationship to saturated air is 45% in both cases, the total moisture content in grains (1/7000 of a lb of water) goes down.

This is why colder air is dryer in an absolute sense because it often contains significantly less water vapor than warmer air.

Relative humidity is only in relation to the maximum saturation of the air.

Moral of the story… air can (and does) still contain water vapor even below freezing and the fact that it may be “off the chart” doesn't mean there is no moisture in the air.

— Bryan


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