Using a dehumidifier condensate to water consumable plants can be a heated topic among growers. Some say, “Yes, I use it and it works fine.” Others question the condensate water quality, wondering if contaminants accumulate in plants. Meanwhile, a small minority insist it is pure poison.

Along with questioning condensate water quality, many environmentally-aware growers are concerned about wasted fuel, electricity and water. Those who use an effective dehumidifier see a lot of water being pulled from the air. Dehumidifiers also consume electricity and condense considerable amounts of water. Many believe it’s a shame to dump potentially useful water down the drain.

Beyond the substantial amount of water produced by dehumidifiers, there is a multiplier effect that can drastically increase the potential water savings. This is because reverse osmosis machines waste from 1-3 gallons of water for each gallon of processed water, while every recycled gallon of dehumidifier condensate saves somewhere between 2 to 4 gal.

Growers are concerned --and have every right to know-- what their plants are drinking, but sorting through opinions reveals a glaring omission. Real data --like water analysis reports and controlled experiments-- is necessary before making the right call.

Let's have a look at the data regarding the water quality of dehumidifier water and find out the answers to these questions:

  • Do dehumidifiers add anything harmful to the collected water (i.e. heavy metals)?
  • Is it necessary to treat or process dehumidified water before recycling it?
  • Is dehumidifier condensate water safe for consumable plants?

Anatomy of Dehumidification

Before addressing these three questions, a basic understanding of dehumidification is necessary. A dehumidifier is a refrigeration system with two coils that resemble an automotive radiator. The coils are made from copper tubing surrounded by thin aluminum fins that help transfer heat.

There is a hot coil and a cold coil. The refrigeration system pulls heat out of the cold coil and gets rid of that heat via the hot coil. Most dehumidifiers use a single fan to blow air through the system, remove the water from the air and remove the heat from the coil. Water forms on the cold coil through condensation -- just like a can of beer does on a hot and humid day.

Inside the dehumidifier, water runs off the cold coil and is collected into a type of reservoir. Dehumidifier coils are not soldered—copper tubes are brazed. Brazing does not use lead because the pressure in a refrigeration system is too high for solder joints.

Finally, the aluminum fins are not soldered. Instead, they are held in place by expanding copper tubing after fins are slipped over the copper tubes.

Water Safety Standards

According to the Environmental Protection Agency's Safe Drinking Water Act, there is a defined safe level of contaminants that is allowed in drinking water. These levels, based on possible health risks and exposure over a lifetime with an adequate margin of safety, are called maximum contaminant level goals (MCLG).

The MCLG for copper is 1.3 ppm (1,300 ppb) and the MCLG for aluminum 0.2 ppm (200 ppb). Zinc does not have an official EPA limit, but many sources cite 2 ppm (2,000 ppb) as the limit.

Water Analysis

An independent and third-party water analysis company called Water Compliance Specialists, recently tested several samples of dehumidifier condensate. Two dehumidifiers were put to the test: a newer model and another one that was built in 1999 with all of its original parts.

Both dehumidifiers had been operating for several consecutive months prior to the water sampling. Water samples were evaluated for heavy metals (aluminum, copper and zinc), decaying organic material (such as nitrate nitrogen) and a standard, contaminant-indicating bacteria.

The water analysis revealed insignificant levels of heavy metals. Copper, aluminum, and zinc fell well below recommended levels. A nitrate test exposed the presence of decaying organic matter in water. Typical sources of these types of nitrates include decaying vegetable matter like leaves and trees.

The typical maximum allowable level of nitrate-nitrogen in public drinking water is set at 10 mg/L (10 ppm). The water analysis indicated a trace amount of nitrates in one water sample—a level significantly below acceptable levels.

The trace organic (nitrate) content was likely from dust that slipped by the dehumidifier’s MERV 8 pleated filter. Standard contaminant-indicating bacteria like coliform and E. coli were not present in either water sample.

Read: Getting the Right Ammonium to Nitrate Ratio

Water Analysis Conclusions:

Q. Does the dehumidifier add anything harmful to the collected water, for example, heavy metals?

No.

Q. Is water collected by dehumidifiers usable on plants?

Yes, dehumidifier condensate water is perfectly useable, even for consumable plants. There are almost no detectable metals. As long as the air entering the dehumidifier is filtered, there is no excessive organic decay.

When a dehumidifier runs without a filter, ambient dust may appear in the condensate water. Please note that you should never operate a dehumidifier without the recommended filter because dust will eventually plug spaces between aluminum cooling fins on coils and cause the compressor to burn out.

Q. Is it necessary to treat or process dehumidified water before reusing it?

No. There is no reason to handle fresh condensate water any differently than any other source of reasonably clean, uncontaminated water.

Through laboratory water testing, we found that normal and every day use of dehumidifier condensate water should not be a problem for a plant’s nutrient system. Lead isn’t an issue because no lead is used in the brazing process of the copper refrigerant tubes.

There is also no more organic material in the water than any other water that has been exposed to the environment of the growroom.

Read: The Best Temperatures for an Indoor Grow Room

Q. What are the best practices and recommendations?

It’s always a good idea (and most areas require it) to have the water source analyzed in any commercial garden. If a water sample analysis comes back positive for elevated levels of heavy metals, nitrates or bacteria, the reason for the contamination needs to be revealed and addressed. These issues aren't dependent on the amount of water collected by dehumidifiers.

If your dehumidifier has been idle, run it three days before recycling dehumidified water from it. A high-quality air filter keeps the interior of the dehumidifier clean and minimizes any organic content.

We did not see any mold growth as a result of using a dehumidifier on a daily basis. This is likely due to the constant washing action of the condensing water.

To store a dehumidifier for more than a couple days, every effort should be made to dry it out as much as possible because standing water, or even moisture, is an invitation for mold growth.

An old dehumidifier that has been well-maintained can be an effective piece of equipment that provides thousands of recycled water over a lifetime. One of the easiest ways to keep your dehumidifier working properly is to follow filter guidelines.

Test Your Water Sample

Taking your own water sample isn’t hard; you just have to be careful not to contaminate it Locate a reliable water-testing lab and follow their instructions. You will not need a complete analysis, just a test for heavy metals, nitrate nitrogen content and anything else that interests you.

Here are some sampling guidelines to help ensure valid results:

  • Wear powder-free latex gloves.
  • Use 16-oz. screw-capped containers (thick glass or polyethylene is best).
  • Use a new bottle for each test. If this is not possible, wash the bottle with phosphate-free detergent and water and thoroughly rinse the bottle and cap before use.
  • Flush any lines and valves so there is new water in them.
  • Rinse the container and cap with the water to be analyzed.
  • Fill the bottle almost full. Leave a small air space.
  • Mark the container with the source of the sample.
  • Send the sample to the testing lab. Follow their instructions.
  • A water sample can usually be stored in a refrigerator for up to 48 hours. The sample must be submitted to the lab within 48 hours of collection. Remember, your test results can only be as accurate as your sampling method.

Read next: Humidity 101: Basics for Your Indoor Garden