Water Heater Temperature and Sizing

Water Heater Temperature and Sizing
November 15, 2012 Lorelie
water heater temperature

Water Heater Temperature

Water heaters are important investments for any home which is why it is always important to properly maintain them.  If you have a water heater in your home, then you know just how convenient it is to have them although you only usually notice this when they have broken down and not running.

One of the main conveniences that modern water heaters provide, both gas and electric units, is the ability to change their settings, particularly the water heater temperature.  It may be surprising to know that the Environmental Protection Agency (EPA) recommends that the water heater temperature setting be set at 120°F, while the Occupational Safety & Health Administration (OSHA) recommends that you set the thermostat at 140°F under particular circumstances.  This is to prevent the formation or culturing of the bacteria that causes Legionnaires’ disease.

The Legionnaires’ disease is an infection of the lungs in the form of pneumonia and can be acquired by the inhalation of the water mist.  At low levels of contamination, the chance of getting the disease is very small.  However, this cannot be said when there is a high concentration of the organism in the water system.  The best way to prevent this OSHA claims is to maintain the water heater at 140°F.  If you have small children or elderly people at home, there may be a serious risk of scalding which is why it is a good idea to invest in a scald-prevention device.

Most water heater manufacturers recommend that you set the thermostat of your water to 120°F so as to prevent scalding as well as save energy.  Scalding is often a concern when there are small children as well as elderly people in your home.  Single handle faucets however can help reduce the chance of someone from getting scalded.

Should you have an old dishwasher that does not preheat water, setting the thermostat of your water heater to 140°F is recommended so as to ensure the cleanliness of your dishes.

If you have a bit of an understanding of physics, you know that hot air rises while cool air settles at the bottom.  This is also true with water heaters which is why the top can be much hotter than the bottom part.  For this reason, it can easily be argued that the settings of a water heater do not actually provide constant temperatures.  Even if the thermostat is set at 120°F, the unit may allow water to cool as low as 110°F and heat is by as much as 130°F.

How to Check for Temperature

Run your water heater unused for an hour before trying to check the temperature.  After an hour, go to the nearest hot water tap, run the water for about a minute to help preheat the pipes and give you a more accurate reading, then fill a coffee mug with hot water from the faucet and use a cooking thermometer to check the temperature.

If you have a gas-fired water heater, then you are in luck as adjusting the thermostat of gas-fired water heaters is done simply by turning the dial of the unit’s gas control valve.  Warm settings will be from 90°F to 110°F and hot settings will range up to 150°F.

If you have an electric water heater, you can adjust the thermostat by first turning off the circuit breaker for the unit.  Once that is done, locate that access panel and remove the screws using Phillips or screwdrivers.  If it does not have any screw holes, then flick the plastic covering open as it may only have a plastic locking mechanism.  Locate the thermostat adjusting screw and using a flat screwdriver, adjust the temperature as desired.  Normally, you will find temperatures from 90°F to 150°F and rotating it clockwise raises the temperature while counterclockwise lowers the temperature.  Once you have finished adjusting the thermostat of your electric water heater, make sure to place the access panel cover back and screw it if has screws on initially.  Finally, turn the power back on.

Water Heater Sizing

Sizing a water heater according to the needs of your household can be very difficult.  This is mostly because you wouldn’t want to heat a lot of water you do not really need, yet still making sure that everyone in your household gets enough hot water.  You can roughly size the amount of hot water you need using the chart below considering that the water usage is during the hour of its peak demand.  This would include showers, cooking, clothes washer, and dishwasher.

Approximate Gallons of Hot Water per Use:

Cooking                                       5 gallons

Hand Dishwashing                 6 gallons

Automatic Dishwasher       15 gallons

Clothes Washer                       30 gallons

Bath or Shower                       20 gallons


If there is something that you use hot water for that is not on the list, be sure to add it, particularly when you use it during peak hours.

Using the chart above is very easy.  For example, between 6 A.M. to 7 P.M., there would be two family members who will be taking a shower, will be cooking, and will be using the dishwasher, then the approximate amount used totals to 60 gallons which means the household needs a water heater that is able to heat up 60 gallons of water per hour.

Since you now have an idea of how much hot water usage during peak hours is used in your household, you can ready yourself should you need to upgrade to one that can cover your hot water needs.

If you are buying a storage tank water heater, try to look at its Energy Guide label.  Try to look for the unit’s First Hour Rating (FHR) as this shows how much hot water the unit can produce on a give hour starting with a tank full of hot water.  FHR is very important as it not only shows the efficiency of a water heater unit, but it also shows how much space you can save if you’ve done your research properly.  Say your hot water requirement during peak hours is 60 gallons, but then why go for a 60 gallon tank when a 40 gallon tank has an FHR of 72 gallons.  This is the very reason why researching your hot water needs as well as looking into the FHR of water heater units is very important.

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