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The Scientific Guide to Sanitized, Germ-Free Water Heaters

Not long ago, I was at a weekend barbecue when the conversation took a strange turn. Suddenly, I found myself answering questions like, “Can my water heater give my kids Legionnaire’s (Legionella)?”, and “Will setting the water heater temperature at 140 °F make my hot water supply safe?”

That got me wondering: is it really true that water heaters are breeding grounds for disease? And what is the risk of contracting Legionella? I did quite a bit of research, and what I discovered was quite interesting.

Children make up less than 2% of the roughly 10,000 Americans affected yearly by Legionnaire’s disease (Legionella). Most pediatric Legionella cases are acquired in hospital, due to medical equipment or hospital plumbing systems, with risk of pneumonia highest in infants and babies under a year old.

That said, residential water heaters cannot be ruled out as a potential source of Legionella or other health risks. This post explains how your water heater could be contributing to your family’s health concerns, and what you can do about it.

Legionella, Mycobacteria, and other disease-causing bacteria can grow in water heaters and plumbing lines, even at high temperature settings. Chlorination, descaling, and thermostatic valves can prevent the growth of such germs.

Sources of Health Risks in Hot Water Heaters

If you’re questioning whether you should even be concerned about water heater health risks, you’re not alone. Not long ago, most people thought that a water heater was an appliance that you installed, then forgot about ‒ it just sat there quietly, providing hot water as needed.

However, it turns out that neglecting water heater maintenance could transform a water heater from a reliable, efficient source of hot water, to a health hazard, putting you and your family’s health and wellbeing at risk.

Here’s why:

Water heaters store water at elevated temperatures. The warm, ill-ventilated conditions in their tanks promote the growth of germs such as Legionella pneumophila (causes Legionella), Nontuberculous mycobacteria (MAC lung disease), Pseudomonas aeruginosa (Pseudomonas infections) and more.

Both, Legionella and MAC lung disease can be contracted through inhaling shower aerosols containing germs. In other words, these illnesses develop after the germs enter a person’s respiratory tract from the fine, misty spray created by a hot shower.

For most people, however, the chances are slim that inhaling either NTM or Legionella will lead to the severe, potentially life-threatening illness. Healthy adults are unlikely to develop any infection when exposed to NTM. As for Legionella, healthy adults may experience the milder, flu-like Pontiac fever that clears up on its own in two to five days. 

Given that children under age one (who are more vulnerable) are seldom exposed to showers, the germs from hot water systems pose a health risk mainly to:

  • older adults (age 50 and above)
  • individuals with chronic lung and respiratory allergies and conditions
  • those with immune systems weakened by illnesses like flu, mononucleosis or measles
  • people undergoing chemotherapy
  • individuals who are HIV-positive
  • persons with a history of heavy smoking or alcohol use

Factors Affecting Water Safety in a Hot Water Supply System  

1. Water Temperature

The question, “what’s the ‘safe’ temperature setting on a water heater?” is a hotly debated one. Scores of blogs and online articles have discussed the topic. The only problem is, there’s no consensus!

Some claim that setting the temperature at 140 °F is a surefire way to ensure that your water heater provides hygienic, germ-free water. Others declare that the cost savings of a lower temperature setting like 110 °F far outweigh the negligible health risks.

Even government sites don’t really clear up the confusion, with endorsing 120 °F1 and the Occupational Safety and Health Administration (OSHA) prescribing 140 °F2 for water heaters and 122 °F for outlets! (For more on the optimal temperature setting for water heaters that saves you money, see this post.)

So what’s the real McCoy? Is there a ‘safe’ temperature setting for a water heater? Well, it turns out that a water heater’s temperature setting is only part of the picture when it comes to getting safe, germ-free hot water. 

Research shows that many germs (for instance, Legionella pneumophila) thrive at temperatures between 75 °F and 120 °F.3 Besides the water heater, these germs can be found in different parts of the household plumbing, including pipes, faucets and showerheads. Now, even when water inside a water heater is heated to, say, 140 °F, it cools significantly by the time it reaches different outlets, so its effective temperature might be lower than 120 °F at a showerhead. 

Setting water heater temperature at 140 °F does not eliminate bacteria from faucets and showerheads, where temperatures are lower. Legionella bacteria enjoy an advantage at water outlets, as they can survive the ‘heat shock’ that kills other bacteria when hot water suddenly flows through the pipes.3

This problem is aggravated in homes with a recirculating line for the hot water supply, where cooler water is constantly reintroduced into the water heater, creating conditions suitable for Legionella.

2. Limescale and Corrosion

Aside from water temperature, germs need nutrients to survive. Though it’s difficult to imagine, many germs actually thrive on the foul-looking sludge that builds up at the bottom of the water heater. The iron, magnesium, aluminum and other minerals present in the sludge, or limescale as it’s commonly known, provide an ample food supply to germs like Legionella and NTM.

In addition to the limescale inside water heaters, the plumbing system itself represents another source of nutrition to germs. You may be aware that germs feed on the minerals released into water as metal pipes corrode, but what is less well known is that germs derive nutrients even from PVC and PEX plumbing! The organic carbon content that leaches from plastic pipes allows germs to flourish.4

So, if your water heater is more than a couple of years old, you’d be right in thinking that it could be putting you and your family at risk. 

Now that you know the worst, here’s the good news: It is possible to sanitize not only your water heater, but also your entire hot water supply. All it takes is a few simple steps. To find out how you can reduce the risk of Legionella (and other water heater-related illnesses) to almost zero, read on.

Sanitizing the Hot Water Supply System

The first method outlined below, if done right, should suffice to eliminate germs from your hot water supply. However, incorporating methods two and three will ensure that you keep your water heater and plumbing system sanitized in the long-term.

1. Shock Chlorination

This method helps you get rid of germs by infusing your entire hot water supply with a diluted solution of sodium hypochlorite, commonly known as household bleach. For best results, follow the steps below carefully:

  1. Turn off the electricity supply to your hot water heater. (Locate the mains panel, and flip the circuit breakers for the water heater to ‘OFF’ – there are usually two 30A breakers. Make sure to tag them, e.g., with a ‘LEAVE OFF’ sign taped over, until you’re ready to turn the power back on.
  2. Also turn off the cold water inlet to the water heater by turning the ball valve to ‘OFF’ or by rotating the dial valve tightly closed. Make sure that all hot water outlets – faucets, showerheads, dishwasher, washing machine are turned off.
  3. Next, extract the anode rod from the water heater through the opening by which it is anchored at the top (see video below).
  4. Drain the water heater of a few gallons of water, just enough to accommodate the liquid bleach that you will pour in. (For a quick guide on draining your water heater, see this handy post.)
  5. Wearing appropriate safety gear (gloves and a face shield), use a funnel to pour a measured quantity* of liquid household bleach into the water heater through the hole left by the anode rod. Be sure to use plain, unscented bleach. Plug the hole with a rag or a stopper once the bleach has been poured.
  6. Run each hot water outlet (including appliances) until you get water smelling strongly of chlorine. Allow it to flow for approximately two minutes, then turn off the water.
  7. Now, your hot water system is full of the diluted bleach solution. Let the chlorine in the bleach work on your hot water system for an hour.
  8. Turn the water heater’s cold water inlet valve back on.
  9. Run all hot water outlets in your home until you get water that has no chlorine odor. Take care to ensure that the chlorine odor is entirely eliminated.
  10. Turn the electricity supply to your water heater back on. It’s ready to use once again!

*Measure bleach in the following quantities:

  • 30-gallon water heater: 1 gallon bleach
  • 40-gallon water heater: 1.33 gallons (1 gal. 43 oz.) bleach
  • 60-gallon water heater: 1.67 gallons (1 gal. 85 oz.) bleach
  • 80-gallon water heater: 2 gallons bleach.

2. Descaling (Limescale Removal)

Although the shock chlorination method described above is an effective way of eliminating existing germs from your water heater, it’s not a long-term solution, since the effects of chlorine fade out in a few weeks. For your water heater to stay germ-free throughout the year, it’s important to ensure that there’s not much for bacteria or other germs to feed on. You can do this by eliminating the limescale build-up from your water heater. 

You can either use a store-bought descaling solution or make up one of your own, and use it to get rid of the sludge that’s accumulated inside your water heater. For a detailed description of how to do this, check out this step-by-step guide.

3. Thermostatic Mixing Valves (TMVs)

As discussed earlier in this post, a temperature setting of 140 °F cannot, on its own, suffice to keep your hot water supply safe. But, it can be used in conjunction with shock chlorination and regular descaling to effectively sanitize your hot water system.

However, in order to be really efficient, a water heater temperature setting of 140 °F should be combined with the installation of TMVs for showerheads and faucets. As you may know, a TMV’s built-in sensor measures incoming hot water temperature before mixing in cold water to provide water at the temperature specified for that particular outlet. This could be a showerhead or a bathroom or kitchen sink.

Using a thermostatic mixing valve ensures (1) hot water temperature regulation at outlets to prevent scald injuries, (2) optimal usage, such that hot water supply meets household needs, and (3) importantly, temperatures high enough in hot water plumbing to prevent growth of germs like Legionella.

Tip: For ideal results in keeping your hot water system impeccably sanitized, use method #1, then adopt method #3 for your household hot water supply, and follow method #2 at intervals of every six months.

With these strategies in place, you can have complete peace of mind where your home hot water supply is concerned. Whether it’s little kids, elderly folks or convalescents, you’ll know you’ve got them all covered!


  1. US Dept. of Energy. 2021. Savings Project: Lower Water Heating Temperature
  2. US Occupational Health and Safety Administration. 2021. Legionellosis (Legionnaires’ Disease and Pontiac Fever): Control and Prevention
  3. Ji, P., Rhoads, W. J., Edwards, M. A., & Pruden, A. (2017). Impact of water heater temperature setting and water use frequency on the building plumbing microbiome. Multidisciplinary Journal of Microbial Ecology. DOI: 10.1038/ismej.2017.14
  4. Cullom, A. C., Martin, R. L., Song, Y., Williams, K., Williams, A., Pruden, A., & Edwards, M. A. (2020). Critical Review: Propensity of premise plumbing pipe materials to enhance or diminish growth of legionella and other opportunistic pathogens. Pathogens. DOI: 10.3390/pathogens9110957

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