Should You Run A Dehumidifier And Air Conditioner At The Same Time?

Running a dehumidifier and air conditioner at the same time: Does it makes sense? Should you even do it? When to use a dehumidifier with air conditioner? There is some confusion about if you should run a dehumidifier and air conditioner at the same time. The answer here is not all that obvious at all. Let’s clear things up in order to best explain if a dehumidifier and air conditioner combo is a good idea:

A dehumidifier‘s primary and only function is to reduce indoor relative humidity levels.

An air conditioner‘s primary function is to lower the temperature and reduce indoor relative humidity levels.

Should You Run A Dehumidifier And Air Conditioner At The Same Time
Comparison of the inner structure of an air conditioner (mini split) and portable dehumidifier.

As we can see, there is a cross-section between the functions of a dehumidifier and an air conditioner. Namely, they both have the ability to reduce moisture levels.

Since an air conditioner already reduces humidity levels, why should you run a dehumidifier as well?

There are 2 specific situations when running a dehumidifier and air conditioner makes sense. We encounter both of these situations in the summer when the humidity levels are much higher than in the winter. Namely, we use the dehumidifier and AC combo when:

  1. Air conditioner alone cannot adequately reduce moisture levels. As we will explain further on, the AC usually has one goal: adequate cooling. Dehumidification is a side function of air conditioning. However, when the AC unit is unable to reduce the humidity levels below 50%, we need to run a dehumidifier simultaneously with an air conditioner to actually reduce the moisture levels down to 30% – 50%.
    running a combo of dehumidifier and air conditioner and impact on humidity levels
    In a normal situation, an air conditioner should reduce the humidity levels below 50%. In high humidity areas and especially during summer, AC units just can’t do the job alone.
  2. We want to optimize the energy efficiency of the AC unit. An air conditioner doesn’t work as well at high humidity levels. That means that it will struggle to reduce temperature, and thereby burn quite a lot of electricity to do it. If we use a dehumidifier as well as an air conditioner, the dehumidifier will reduce the humidity levels, allowing for the AC unit to run more efficiently and effectively. Of course, running a dehumidifier is also connected with a dehumidifier running costs (you can read more about those here).

As you can see, especially in high humidity areas, running a dehumidifier and air conditioner at the same time makes quite a lot of sense. Let’s look into both of these situations to see how a dehumidifier helps the air conditioner to lower temperature efficiently and how does an air conditioner helps a dehumidifier help to lower humidity levels:

1. Running A Dehumidifier And Air Conditioner To Reduce Humidity Levels

The most obvious case of when you should run a dehumidifier and an air conditioner simultaneously is when the AC unit cannot adequately lower the humidity levels by itself.

In their recommendations about moisture control, EPA cites that ‘… it is important that indoor relative humidity be maintained below 60%, ideally between 30% and 50%.

If you have above 60% humidity levels, the risk of mold growth is heightened.

humidity levels leading to mold problems

Now, every air conditioner works as a dehumidifier. You can read if an air conditioner removes humidity and by how much here. However, we need to be aware of one very important thing:

The goal of the AC unit is to lower the temperature. When then the temperature hits the set temperature (let’s say 72°F), the air conditioner turns off. That is – very important – regardless of what humidity levels are.

Example: A 2-ton AC unit reduces indoor temperature from 85°F to 72°F. While it is running, it also removes moisture; moisture levels fall from 80% to 65%. But when the temperature hits 72°F, the AC unit shuts off, and we are left with too high humidity levels (65% is more than the ideal 30% to 50%).

In this case, running a dehumidifier in parallel with an air conditioner is a smart move. Here’s what both of these units will do:

  • Air conditioner will make sure the temperature is 72°F.
  • Dehumidifier will make sure the humidity levels are 40% (or between 30% and 50%).

In this situation, it is quite clear that you should use both an air conditioner as well as a dehumidifier.

However, what about the situation where the air conditioner lowers the humidity sufficiently (with end moisture levels between 30% and 50%)? Even in this case, it might be smart to run a dehumidifier as well. Here is why:

2. Running A Dehumidifier + Air Conditioner Increases Energy Efficiency

Air conditioners are great at cooling (lowering temperature). They are not as great at reducing humidity levels. Dehumidifiers are superb HVAC units for reducing humidity levels.

Do you need a dehumidifier with an air conditioner?

Not necessarily; but it might be quite smart to use a dehumidifier. That’s because air conditioners work less effectively at high humidity levels. And a dehumidifier can effectively lower these moisture levels.

In air conditioners, the EER rating denotes how energy efficient a unit is. This is an efficiency metric that is – important – measures at 50% relative humidity levels. If you were to measure how energy efficient an AC unit is at 70% or 80% humidity levels, you would see a fall in the EER rating.

Example: Let’s say we have a 12,000 BTU window air conditioner with a 12 EER rating (at 50%). That means such a unit will be able to generate 12,000 BTU of cooling output with a 1,000W electric input. However, in a realistic situation, we might right such a unit at 70% humidity levels, where the EER rating might be closer to 10 EER.

In that is the case, that 1,000W electric input will generate less than 12,000 BTU of cooling output. If it falls to 10 EER, it will generate only 10,000 BTU cooling output.

Here’s the bottom line:

We can achieve better energy efficiency if we run a dehumidifier while also running an air conditioner.

What we need to be aware of, however, is that running a dehumidifier also costs us some electricity. Nevertheless, the running costs of a dehumidifier is often much lower than that of an AC unit. On top of that, if the dehumidifier helps the AC unit, the AC unit is not under such a struggle. This will likely increase the lifespan of an air conditioner as well.

dehumidifier in combo with air conditioner
Some portable dehumidifiers also have that unpleasant task you have to do; empty the bucket when it gets full of water.

If we live in a high humidity area, running a dehumidifier and an air conditioner at the same time is a smart move. It’s best to leave both units to do what they do best:

  • Air conditioner cools effectively.
  • Dehumidifier removes moisture effectively.

We hope this illustrates why a combo of a dehumidifier and air conditioner is a good idea and which factors come into play here.

Thank you.

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