When I first wrote on GreenerHouse about dishwashers, I lamented that I could not find one with a hot-water inlet. For most of the year, I have an excess of solar-heated hot water sitting in tanks, so it is a waste for my dishwasher to be electrically heating cold water. At the time, I had been misinformed by a Bosch technical expert that the inlet to my dishwasher could handle a maximum of 40 degrees. Like an idiot, I hadn’t read the manual, which says that the inlet can take up to 60-degree water.
Water heated by flat-panel solar collectors does not tend to rise much above 60 degrees, and my kitchen is far—too far—from my water tanks to ever get to 60 degrees at the tap, according to my thermometer. (Vacuum-tube panels do often produce much hotter water, and may require a thermostatic mixer to keep temperatures at safe levels.)
So I recently asked my plumber to connect the hot water to my dishwasher. My hypothesis was that the thermostat inside the dishwasher would switch off the heating element more quickly with warm water entering the machine. Using a cold-water feed, this Bosch, A-rated model uses approximately 1 kWh per load at the 35 degree “Quick Wash” setting. After connecting the hot water, I recalculated the energy consumption, using the technique outlined here. It has fallen to 0.7 kWh per wash. Over the course of a year, this simple change should save about 100 kWh preventing some 100 kilograms of carbon dioxide from being released into the atmosphere.
If my dishwasher were located closer to the hot water, the savings would be far greater. In designing a new house, ideally the north-facing roof, solar panels, hot-water tanks, bathrooms and kitchen should all be as close as possible. If this is not possible, small-diameter Pex pipes can help overcome heat loss over long distances. (Combining Pex pipes and vacuum-tube solar collectors is asking for a meltdown, however.)
I compensate by running the hot water in the sink, usually while washing pots and pans, immediately before switching on the dishwasher. That way the water enters the machine hot for the wash cycle, though only slightly warm after the copper pipes have cooled the water for the two rinse cycles. One day I will try insulating the pipes and see if I get even better results. In the winter, when the sun does not always provide enough hot water, I will try to run the dishwasher in the late morning, so as not to deplete the evening supply of hot water.
Coincidentally, just days after my plumber had made the connection and before I had a chance to measure my results or write about it, a GreenerHouse reader published a comment here, reporting how pleased he was with his hot-water connection to his dishwasher. All green minds think alike.