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 implementation of new building codes, they have become a kind of red-haired stepchild at the same time… the runt of the litter if, you will. We all know it's there, but nobody gives it too much attention. I have seen a few that still have the original filter in a couple of years later and likely haven’t worked right in a while. For the purpose of this article, I am only going to 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 it 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°F 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 not to over-cool 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 to create unwanted warm spots.


Why a dehumidifier?

The two main reasons in my experience are to pre-condition ventilation air that is needed 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 it 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. Still, there is great new research on the best way to connect a dehumidifier is by the Florida Solar Energy Center. Special care should be taken when ducting these, whatever the configuration may 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 it 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 the system's capacity performance, there's one major thing left to check—like almost all things HVAC, that would be the manufacturer’s specs rule. In this article, we are going to be following this one from Honeywell.

There are three 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 as 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 approximately 2.71 pounds per hour. 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: it 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 and to have enough collected at the drain pan so that 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.

That 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? It depends on how much water is 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°F 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 to be at 100 to 110°F dry-bulb and 15 to 20% RH. If, instead, our entering conditions were at 75°F 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°F and 35% RH (as the case may be in a “ventilated” attic). one can expect our leaving conditions to be closer to 130°F and 12 to 14% RH.

Disclaimer: These readings are based solely on my personal experience. As a matter of fact, I have on many occasions reached out to technical support reps of at least two different brands and asked them point-blank why 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 these numbers on properly functioning dehumidifiers for the very reason of being able to cross-reference them when working on one for 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 that 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 it 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 three methods have their merits and disadvantages. As 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 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



To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from