One nice side effect of using solar energy is a heightened awareness about energy use in general. Not everyone will have the same reaction of course, but I enjoy the process of paying attention and making adjustments here and there as needed. It also a good way to take some personal responsibility for the state of our world.
The heart of many solar water heating systems is the heat-exchanger which transfers heat from the rooftop collector to the water in the tank. They are designed for maximum efficiency to capture as much heat as possible. Other parts of a home are also heat-exchangers, though often not intentional. Sometimes that can work against us in ways that are both unexpected and costly.
A Tale of Waste
My good friend and energy expert Les Lazareck once shared a good example of this. He was performing an energy audit on a home and while in the attic, he observed the hot and cold water lines running in parallel. This is common in almost every home but they should be separated to minimize the exchange of thermal energy. Les noticed that in this case the hot and cold lines were actually taped together, touching each other for most of the distance across the attic.
This created a linear heat-exchanger that warmed the cold water while cooling the hot water. It may have saved a bit of time for the installer who originally pulled in the lines, but the homeowner has been paying ever since through higher energy bills and reduced performance at the shower or sink. Imagine cold water that is never quite cold enough, hot water that is never quite hot enough and flushing costly hot water down the toilet. In that case, it probably happened every day.
Ideally, water lines should also be insulated but in residential construction it is rarely the case. The only exception is where insulation is needed to minimize the possibility of freezing. Wrapping water heaters with special insulating blankets can also reduce energy waste.
Think about all the places where hot water is available in your home and all the lines running through the walls, attic or floor. A hot water line acts like a stretched-out radiator, transferring heat to the surrounding environment, including the inside of your home. Often almost all water lines are inside the insulated envelope of the home. Their thermal energy is leaking into the living space year-round. In summer, the air-conditioning system must work harder to maintain comfort. So in many cases, uninsulated hot water lines can cost you twice: first to heat the water, and then to remove the heat they have released into the home.
These effects can be compounded by a device found in many homes: the automatic hot water circulation pump. They are intended to save water by reducing the wait time for heated water to flow from the tank. In that sense they work fine but excessive use can lead to energy waste and worse. A city plumbing inspector once told me that running them more than necessary can reduce the lifespan of water heaters and other plumbing components. Unfortunately, many homeowners run their circulation pumps 24/7. There is a better way.
Most circulation pumps have a control timer that can be easily programmed to fit your daily schedule. Ours is off from 8 PM until 7 AM when we rarely need hot water. For a two-hour period every morning and evening during periods of highest personal use, the pump alternates on/off every fifteen minutes. Thermal energy takes a while to dissipate so it is not necessary to run the pump longer than fifteen minutes at a time. We are home during the day but hot water is not a high priority so the pump only runs for fifteen minutes every two hours.
This program seems to be a good compromise for our particular needs. By adjusting your circulation timer, you can save energy and maybe even extend the usable life of your water heating system.
None of these issues are worth losing sleep over, but if you are interested in making your home just that much more efficient some of these ideas might help. Paying attention to energy habits and systems can reduce dependence on fossil fuels and save money too.