What’s the best hot water temperature to save energy
What is the best hot water temperature – hotter or colder? If I turn the heater temperature up high, will I use less energy to heat my water? After all, less hot water will be coming out of the hot water tank since I’ll be blending in a higher proportion of cold water.
Answer from Green Energy Efficient Homes
It’s true that you’ll use less water from your hot water tank if the hot water temperature in the tank is hotter. For a given temperature coming out the tap, the higher the hot water temperature, the less hot water and the more cold water you’ll use.
However, the same amount of heat energy is coming out of the tap either way, which means the same amount of heat energy is coming from the hot water heater – whether the temperature is higher in a smaller amount of water, or lower in a larger amount of water. So the best hot water temperature from an energy savings point of view is the lowest temperature you can manage.
To understand this, imagine two burners of the same size on your stove. On one, you place a small pot with one cup of water in it. On the other, you place a large pot with five gallons of water in it. You turn on both pots to the maximum setting, and leave them on for five minutes.
Chances are the small pot will boil within that five minutes – it may even boil dry. The large pot, meanwhile, will heat up just a little bit. But you’ve used the same amount of input energy, since both burners were on high for five minutes.
For hot water heaters, the higher the temperature setting of the hot water heater, the more energy you use over time. This is because some of the heat escapes the tank through the hot water tank insulation, and through the pipes between the tank and the tap.
The amount of heat that leaks out of your tank, and out of the pipe leading from the tank, is determined by the amount of insulation, and by the temperature differential between the hot water and the outside. If the tank were contained inside a vacuum – near-perfect insulation – it would not matter how hot the tank was, there would be very little heat loss. But hot water tanks don’t have perfect insulation, and so the hotter you keep the tank, the more of the heat inside the tank escapes through the tank walls and the walls of the hot water pipes, instead of out the end of the tap.
So keep your hot water temperature lower and you’ll save more energy. The default setting on most water heaters is 60C or 140F, but the best hot water temperature is 49C or 120F – this will save you about 25% of your hot water heating costs. Water heating makes up about 20-25% of the typical fossil gas use in a gas heated house in a cold climate; lowering the temperature setting by that 11C or 20F can cut your overall natural gas use by 5-6%. (Of course I’d strongly recommend getting off fossil gas and installing a heat pump water heater – these are super efficient and don’t emit carbon dioxide!)
Don’t lower your hot water temperature below the best hot water temperature of 49C or 120F, because there is an increased risk of legionella bacteria in your water below that temperature. While legionella bacteria are harmless when ingested or when in contact with the skin, they can cause serious illness when inhaled – for example while showering.