We all know that residential heating load is highest in the winter and cooling load is highest in the summer. What’s a bit more subtle is how the seasons, and how we respond to them, change the loads on other household energy uses.
For example: this summer our basement temperature climbed above 70°F, and the incoming water temperature from the water pressure tank was in the mid to upper 60Fs. Here at year’s end, the basement temperature is in the mid-50Fs, and cold water temperature has dropped to as low as 48°F. Both of these changes since summer add load to the water heater, as it must provide more energy to get the lower temperature water up to 120°F, and the tank loses more heat to the colder basement. Here’s a graph of our domestic hot water (DHW) usage in gallons/day (GPD) and the corresponding kWh/day:
August through December DHW usage has been very constant at about 14 GPD, yet kWh/day has gone from just over 2 to just over 3.
Similarly, the 15-cubic-foot chest freezer is in the basement, and the refrigerator is on the main floor, both of which areas are cooler than they were in the summer. Here’s the kWh/month usage combined for both devices:
The load on the freezer and fridge drops because the spaces they are located in are cooler, so the temperature difference driving their heat gain is lower. In addition, the heat rejection components work more efficiently to reject heat to the surroundings when the air is cooler, so it takes less energy to meet the load.
Just as with the water heater, the two effects combine to change the energy usage, but in the case of the freezer and fridge, the energy usage goes down. That reduction isn’t as large as the increase in energy used by the heat pump water heater, unfortunately!
Again I find myself wishing for a fridge/freezer that rejects heat to a domestic hot water preheat tank, just as it’s done in commercial refrigeration. But energy will need to get more costly before that device appears on the market.
Marc Rosenbaum is director of engineering at South Mountain Company on the island of Martha’s Vineyard in Massachusetts. He writes a blog called Thriving on Low Carbon.
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