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Why Is There Mold in Water Heater? (Answered)

Wherever there is water, there is a possibility of growing mold. So you should always watch out for mold in the water heater. 

Reasons of mold in the water heater:

Water heaters are pretty quick in catching mold if you do not take care of them properly. If there are leaks, then the water heater will form molds rather quickly. In addition, old water heaters and ruptures from high water pressure may also cause mold. Clogged drains also cause mold. 

Water heaters form molds rather quickly. If you notice that your water heater has irregularities in its water pressure or has sustained any leaks, then you should watch out for molds. 

It is likely that your water heater will have sustained molds on the inside of the unit if there are irregularities in the water heater. Read below to learn about the reasons that may cause mold in the water heater. 


The most common reason why your water heater forms mold is leaks. If there are any leaks in the pipes of the water heater, it is likely to form molds rather quickly. This may happen in case of old water heaters. 

If your water heater is old and needs to be replaced, it is likely that there are some leaks inside the unit. This will most likely cause molds inside the machine. 

Higher water pressure: 

In addition, high water pressure can cause the water heater to sustain leaks as well. If you notice that your water heater has too high levels of water pressure, then this may be the reason behind a few leaks inside the unit, which will eventually cause molds inside. 

Also keep in mind that water pressure increases further when you change the temperature. When the water heater changes the water from cold to hot, the water pressure increases even more. So this will soon rupture the inside of your machine. 

This is why you will need to keep the water pressure in check. 

Using indoor unit outdoors: 

Moreover, if your water heater is designed to be used indoors but you end up using it outdoors, it is likely that the machine will sustain some leaks. 

The reason behind this is high wind speeds, rain, and other elements of nature that the machine is not designed to withstand. 

When your water heater is designed for the indoors, you should make sure that it is safely inside a cabinet where the elements of nature cannot hamper its activities. 

These machines are not built to be durable against nature, so they cannot withstand rain or heavy wind. As a result, the indoor units will sustain damage rather quickly if you place them outdoors. 

Trapped moist air: 

If the water heater has a lack of air circulation system inside it, it will cause the moist air to be trapped inside. As a result, the water heater will sustain mold much faster. 

So make sure that the unit has an active combustion fan inside it, otherwise the moist and humid air will be trapped inside. 

When there is too much moist air trapped inside the unit, the water heater will have mold. You should therefore ensure that the machine does not trap moist air inside it. 

Bad quality water: 

If the water supply in itself is bad in quality, it is likely that your water heater will catch mold. By bad quality water, we mean hard water which is too rich in minerals that are harmful for the metals inside the water heater. 

That is to say, if you are using water that is too hard, it will not take too long for the hard and high levels of minerals in the water to eat up the metal. In that case, your water heater will catch mold rather quickly. 

Clogged drains: 

Another reason why your water heater may cause mold is clogged condensate drains under the water heater. This will soon cause mold in your machine. 

Is mold in the water heater dangerous?

Yes, mold in the water heater is dangerous. If you notice that there is a possible formation of mold inside the water heater, you should stop using it immediately. Water that is exposed to mold can expose you to a range of diseases. 

Mold in water poses several hygiene concerns. You should not use water that has been exposed to mold, so you should definitely keep an eye on your water heater and check it out every once in a while. 

If you want to keep your family and friends free of health hazards, make sure that they are using hygienic water. 

When you use water exposed to mold, you are at risk of several diseases. You may catch respiratory diseases such as bronchitis, pneumonia, and breathing issues. In addition, you may experience fatigue and depression when you are exposed to moldy water for a long time.

Not only that but you may also face digestive problems when you are drinking water that has been exposed to mold.  

How to test water for mold?

There are several ways following which you may test your water for mold. One of the easiest ways is to check the water using an eyedropper. 

You will have to use a clean eye dropper for this purpose. You will find a range of mold testing kits in stores and online shops. Drop a few drops of water into the mold testing kit and place the mold testing kit in a dark place, where it will remain safe.

Make sure that your pets or kids cannot access the kit. 

You should keep the mold testing kit as it is in a safe place for at least seven days. Make sure that it does not get tempered with. No one should be able to touch or shake the water. After seven days, you should observe the sample that you had placed inside the mold testing kit. 

If you notice any mold growth inside the testing kit, it will indicate that the water has been exposed to mold. In such a case, you should immediately stop using the water. 

Water that has been exposed to mold will pose health risks for you, your family and guests who might be using the water. 

How to properly clean it?

There are some steps that you can follow in order to clean mold from the water heater. You can run these steps at the comfort of your home. Read ahead to learn more about it: 

Identify the reason: 

The first step would be to identify the problem. You will need to find what is causing the molds. One of the most common reasons why your unit may be forming molds is leaks. So you should firstly check for any leaks inside the machine. 

If you cannot find any leaks, you should watch out for other probable reasons. You should check the water pressure of the machine. Note that high water pressure can cause molds inside the unit as well. 

Fix the problem: 

Once you have identified the reason behind the mold, you should go on to fix the problem. This is a crucial step because if you do not uproot the issue, you will not be able to clean the machine properly. 

You should take professional help in this case, if you do not have any prior experience in fixing a water heater. This will help you ensure that the water heater issue, be it a leak, water pressure issue, or supply issue, is fixed properly and you will not have to worry about it for a long time. 

Stop water supply and clean: 

Now that the water heater is fixed, you should begin cleaning the mold that has formed inside the water heater. 

You should use undiluted vinegar and a sponge to get rid of any mold inside the water heater. You may also use bleach; the ideal proportion would be to use one cup of bleach with about a gallon of water. 

Solutions of mold in the water heater:

Here are a few solutions for mold in a water heater that we would like to suggest. Read ahead to learn more about it. 

Control water pressure: 

You should note that water, when converted from hot to cold inside the water heater, has a rising water pressure. So an already high water pressure level will cause an excessive pressure on the machine, which will eventually lead to mold formation. 

So make sure that you control the water pressure level that is being supplied to the water heater. If you do not control the water pressure level, it will soon start to eat up the metal inside the unit, which will eventually lead to a bad mold condition. 

Replace old water heater: 

Old water heaters are likely to catch leaks more easily than the new ones. And leaks, being among the most common reasons for mold formation, should be checked every once in a while. 

So make sure that your water heater does not have too old pipes or installments. Replace the entire machine if you deem necessary. 

Final thoughts

There may be several reasons why there is mold in your water heater. One of the most common reasons for mold formation inside a water heater is water leaks inside the unit. Besides, the machine may have irregular water pressure, hard water supply, etc. which may cause mold inside the water heater.