Should I use my boiler or electric water heater in the summer?

“Every summer, my spouse turns off the boiler and our electric bill goes up dramatically. Should we be doing this to heat our hot water or would it be more cost effective to keep the boiler running?” asked a member recently.

Unless a boiler’s hot water heating efficiency is significantly lower than 60 percent*, it is more cost effective to use the boiler. Don’t be fooled, though; just because your boiler is 87 percent efficient when heating your house, it does not mean it is 87 percent efficient at heating domestic water.

Here’s why: based on average efficiencies – 99 percent for an electric water heater and 60 percent for an oil-fired boiler used only to heat domestic hot water – to heat 3,000 gallons of water from 40 to 100 degrees Fahrenheit would cost $84 using an electric water heater or $69 using the boiler.**

The main function an oil-fired boiler is to heat massive amounts of water to heat a home. When it is only heating small amounts of domestic water, its efficiency drops significantly.

Another reason to leave the boiler on: turning it off, especially an older model, during the summer months can cause the seals to dry out creating additional maintenance issues.

*As deduced from a study by the Energy Resources Division Department of Energy Sciences and Technology, 2007

**The math was done based on the following energy costs in Fairbanks, Alaska (May 2011):

  • $4.12 per gallon of oil
  • $0.19 per kilowatt-hour of electricity
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