Dehumidifier Facts & Troubleshooting

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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.

 

Troubleshooting 

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

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