Did you know that when you use hot water in your home, you’re wasting 80–90 percent of the energy used to heat that water? Your water heater spends natural gas or electricity — and you spend money — heating water that shoots right out of a shower or a faucet and right down the drain, taking most of its heat energy with it. But, with the right drain-water heat recovery system, you can put that used hot water right back to work for you.
What is a Drain-Water Heat Recovery System?
Drain-water, or greywater, heat recovery systems capture the energy of hot water as it’s discarded down drains and use that captured energy to heat incoming cold water so your water heater doesn’t have to work as hard. That’s why drain-water heat recovery systems help you save money on energy costs.
How does a Drain-Water Heat Recovery System work?
Drain-water heat recovery systems work well with all kinds of water heaters but especially with tankless on-demand and solar water heaters. There are two kinds of drain-water heat recovery systems: storage-type systems and non-storage systems.
Storage-type systems typically have three parts: 1) a tank containing a reservoir of clean water, 2) a drain water coil at the bottom of the tank and 3) a water heater intake coil at the top of the tank. As hot drain water flows through the spiral tube at the bottom of the reservoir tank it warms the tank’s water. The warmed water rises to the top of the tank, which contains a preheating coil. The preheating coil feeds intake water from your house’s main line to your water heater. Before cold water in your main line reaches your water heater, it passes through the coil at the top of the tank, where it’s preheated. As a result, whenever you use hot water in your home, your water heater uses less energy to bring new intake water up to the desired temperature.
Non-storage systems typically replace a section of your main waste drain with a copper heat exchanger. As hot water flows down the waste drain, it passes through the heat exchanger. The heat exchanger is wrapped around the main water intake line prior to your water heater and warms the cold intake water before it reaches your water heater. And since the water is partially preheated by the time it reaches your water heater, just as it is with a storage-type system, your water reaches your desired temperature while using less energy.
Drain-water heat recovery systems also have the added benefit of extending water heating capacity, which can be a big improvement if you have an undersized water heater or several family members tend to take back-to-back showers.
What’s the Payback Period?
Based on information from the U.S. Department of Energy, prices for drain-water heat recovery systems range from about $300 to $500. Add the cost of professional installation by a qualified plumbing and heating contractor, and you’re looking at a payback period from about 2.5 to 7 years, depending on how much you use the system.
U.S. Department of Energy, Energy Efficiency & Renewable Energy, “Drain-Water Heat Recovery.”