In two previous Engineering Green Buildings columns (January and April 2007), I examined myths about water-efficient toilets and sensor-activated faucets. This month, I will attempt to invalidate a myth about tankless water heaters.
The myth: Tankless water heaters are water-efficient and use less water than storage-tank water heaters.
The reality: Not only is there increased water waste with tankless-water-heater systems, but evidence shows people take longer showers when a tankless heater is providing hot water. Those showers are negating any potential water savings that might have resulted from other water-efficient practices.
Tankless-water-heater systems must ignite burners and wait 8 to 15 sec for water to reach the desired temperature. During this ramp-up time, water flows through the heaters into pipes at a temperature lower than that demanded by end users. In most cases, this water flows directly to waste. A person taking a shower waits not only for the system to evacuate the cold water that was in the pipe before the shower was turned on, but all of the ramp-up water flowing through the tankless heater.
How can the water waste associated with tankless heaters be avoided? The most-apparent solution is to install tankless water heaters at or near bathrooms. Given the price of water in most jurisdictions, however, this solution usually is not cost-effective. Plus, the initial costs of such a strategy might be overwhelming.
A better solution is to insulate hot-water pipes. This may not be feasible in a retrofit situation, unless the pipes are accessible in a basement, crawlspace, or attic.
In new construction, a tankless system is only a little more expensive to install than a gas-fired storage-tank system. Replacing an existing storage-tank system with a tankless system, however, can be more expensive than making an even exchange.
Gary Klein of the California Energy Commission has identified a number of cost elements that need to be considered when replacing a storage-tank heater with a tankless heater in a retrofit scenario, including:
The gas-supply line from the meter to the water heater almost always needs to be replaced with a larger-diameter supply line.
The gas meter may need to be upgraded.
The larger burner in a tankless unit requires more makeup air and a larger-diameter flue. As such, venting requirements are greater. Making those changes can cost more than the heater itself.
A stainless-steel vent pipe may be required.
The water heater must have 110 vac available.
If the location of a heater is changed, plumbing will need to be rerouted.
An alternative to utilizing tankless water heaters in a retrofit is to install a condensing storage water heater with a larger-than-normal burner. This combines the benefits of having hot water at the beginning of a draw with the ability to keep up with the demands of one continuous showering experience. But whatever the technology or design, do not expect water savings without authoritative evidence from real-world installations.
John Koeller, PE, has extensive experience with water-efficient technologies and products. Nationally recognized as a specialist in these fields, he is a consultant to numerous North American water authorities and private-sector firms. He also serves with various standards organizations.