Proper Water Heater Temperature

I have seen a lot of different temperatures on water heaters when I test them during a home inspection.  I test the temperatures at several places in the home for several different reasons.

1. Showers and bathtubs:  I am looking for a temperature of 120 deg F and NO HIGHER.  Why?
   In showers and tubs I am especially concerned about burns.  The plumbing code talks about the temperature of water provided from these two fixtures specifically.  Under 120 deg F is the magic number.  They deem this a safe temperature.  Honestly, 110 deg F is plenty hot for a hot shower.  More on how this is regulated in a minute.  Ironically, the maximum temperature in the code for a bidet is 110 deg F and for OBVIOUS REASONS.  Use your imagination and if you don't know what a bidet is - Google it. 
2. Kitchen sinks, laundry, bathroom sinks and other faucets:  I am looking for a temperature of more than 120 deg F.  Why?  In these fixtures, especially in the kitchen sink and laundry room and other hand washing areas over 120 deg F is what it takes to kill bacteria, especially Legionella bacteria growth (Legionnaires Disease) - The minimum temperature to prevent Legionella bacteria growth is 122°F. Above 122°F and up to 131°F, Legionella bacteria survives, but will not multiply. At 131°F, it takes about 5 to 6 hours for the bacteria to die. At 140°F, the bacteria dies in about 32 minutes. At 151°F, the bacteria dies instantly. The recommended minimum disinfection temperature is a few degrees above 151°F, which is 158°F for about 5 minutes. To prevent bacteria growth, a safety factor of a couple of degrees would require a minimum of 124°F at the coolest spot in the hot water distribution system (the pipes).  Now, this is why it is a good practice to let water that has sat in the pipes for awhile to just drain out and run down the drain.

Now, back to the shower and tub faucet temperature.  The plumbing code requires that these fixtures have a feature that prevents the flow of water over 120 deg F.  This is called an anti-scald device.  It mixes cold water with the hot water to keep it at a safe temperature.  If you have small children that use the tub or shower it is recommended that 110 deg F be the setting on these devices.  

So, with that in mind what should a hot water heater be set at? It is acceptable to turn your water heater as high as 140ºF if required but higher energy consumption will occur. Be aware that turning up the temperature on your water heater WILL increase your electric or gas bill every month. Adjusting the water heaters temperature higher is sometimes required if it is located far from the point of usage or if it is not properly insulated. If your shower is located on the other side of your home from the water heater, you may want to adjust the temp higher. If there are multiple showers in your home and they are all being used at the same time you can adjust the temperature to prevent running out of hot water.
Increasing your water heater temperature by just 10ºF from 130ºF to 140ºF may increase heat loss resulting in a larger electric bill from $30 to $75 dollars annually. Setting the temperature to 140ºF can also increase standby losses (cost of keeping water hot all the time) resulting in an annual expense around $500 dollars. One benefit from setting your water heater to 140ºF is that this temperature is hot enough to kill almost anything that could be growing inside the water heater itself. If you do decide to raise the temp on your heater for whatever reason, use caution when using faucets, bath tubs, or showers as severe scalding can occur. To minimize the scalding risk, you should ensure you either have or install mixing valves or other temperature regulating devices on any taps you use for washing or bathing.  If you turn up your water heater make sure to check the supply temperature at your showers and bathtubs so that the mixing valves are putting out 110 to 120 deg F water.  The mixer valves may need to be adjusted again.

With all of this being said I am sure you are asking where I set my water heater?  Well, I have mixing valves on my tubs and showers set to 110 deg F.  My water heater is set at 132 deg F because I want to kill the bacteria.  The water heater sits there all day usually doing nothing but just keeping hot for when I come home and need a shower so 5 to 6 hours of standby time is normal. 

So, remember we were discussing about water sitting in the pipes and cooling off? Well, that can be addressed too by installing a re-circulating pump.  This insures that hot water is regularly circulated through the distribution system and the amount of time you stand at the fixture waiting for hot water to come out is reduced.  Yes, this does use energy, but you may want to consider this with any plumbing upgrade.

I hope you found this helpful and educational.  Stay tuned for future posts on home issues.