Most business offices are open during the daylight hours, Monday through Friday. Allowing for cleaning crews after hours, the time during which hot water is needed is perhaps 60 hours a week. The typical hot-water heater is on all the time, 168 hours each week. A timer will reduce that energy use a lot. It can be set to turn on an hour or so before the beginning of the business day and still provide the same consistent service as though it were on constantly. (Turn the water heater off at the close of business and let the cleaning crews use what remains.) Also, with the extra downtime, the heater and its elements will have a longer useful life.
It’s very common, for example, for public schools to have the hot water system on an electromechanical timer. Physical education locker rooms need hot water during class hours (plus a few more hours in the late afternoon for extracurricular athletics). The school kitchen uses a lot of hot water as well, although they might benefit from a small steam boiler for industrial cleaning machines. (Electromechanical devices tend to be more dependable than digital timers.)
Is the average busy household that much different? Residential working hours are the converse: the house is empty all day, with peak busy (hot water) hours in the evening and early morning. No hot water is needed from 9 AM to 5 PM and from 9 PM to 7 AM. There are only six hours out of 24 when hot water is called for, so why keep the water heater going 24/7?
A tank water heater constantly loses heat from the tank. This is called “standby loss.” 140 F water cools off very fast, even with an insulated tank. The longer the water heater is off, the more heat it loses ~ but at a decreasing rate, as the water temperature drops. It takes less energy to warm the water back up to setpoint than is lost through diminished standby losses.
Businesses usually have a peak-hour charge, doubling the cost of electricity used during business hours. Some utilities offer variable rate schedules too – if so, a time on the water heater shifts all your hot-water electricity into a 50% rate profile.
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Excerpted from Going Green, available as an eBook at this list of eBook retailers online.
