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Tankless Water Heaters

 

A conventional, storage-type water heater has an insulated tank and a relatively small gas burner or electric heating element (often two elements) to heat the water. Water is heated slowly and remains “thermally stratified” so that water drawn off from the top remains hot even after 90% of the hot water is used up. There is the risk of running out of hot water, however, because the rate of heating may not keep up with the hot water draw from typical uses, such as showers.

Tankless water heaters—often called demand or instantaneous water heaters—heat water as it is used. The main advantage is that hot water doesn’t sit in a tank all the time, losing energy through the tank walls, which occurs even with insulation.

As with storage water heaters, tankless models can be either gas-fired or electric. For very small loads, such as a remote lavatory that has only a sink with a low-flow aerator, an electric tankless water heater can make a lot of sense since it obviates the need for running a gas line. But for more extensive hot-water needs, such as an entire home, a gas-fired tankless water heater is almost always a better choice than electric because an electric model able to supply that much hot water requires very high current draw—often 40–60 amps.

With a whole-house, gas-fired, tankless water heater, the burner is very large—typically 150,000–200,000 Btu/hour (44–60 kW), compared with 40,000 Btu/hour (12 kW) for a typical gas-fired storage water heater. This means that larger-diameter gas lines are required (usually 3⁄4" instead of 5⁄8"), and a lot of combustion air is required, necessitating large ducts and potentially resulting in significant air leakage if not properly installed.

Gas-fired, tankless water heaters used to all have pilot lights, which burned gas all the time—as much as 5,000 Btu/hour (1,500 W), nearly equivalent to the heat lost through the insulated walls of a storage water heater. Today, most gas-fired tankless water heaters have electronic (pilotless) ignition. These models offer the highest efficiency of any water heater, except electric heat-pump and solar water heaters.

A few gas-fired tankless water heaters made by the Korean companies Takagi and Navien use condensing technology with an Energy Factor (a measure of efficiency) of up to .98. Non-condensing, electronic-ignition models have Energy Factors of .82 to .87, while conventional gas-fired storage water heaters have energy factors of .58 to about .67 (up to .80 for condensing models).

While providing significantly higher efficiency, electronic-ignition tankless water heaters are a lot more expensive than storage water heaters, and they are somewhat more prone to failure (especially with hard water). They also don’t work well in every application (with the most water-conserving lavatory faucets the burner may not turn on), and by providing unlimited hot water, tankless water heaters don’t encourage shorter showers as do storage water heaters.
September 1, 2009
DISCUSSIONS

Reader-contributed comments related to Tankless Water Heaters - EBN: 18:9. Comments are listed with newest at the top.

Other Considerations: Posted by Hernando Miranda on Sep 4, 2009, 01:41 PM  
These type of water heaters are best located on an exterior wall. As close as possible to hot water using sources (there is no storage tank to keep indoors); connected using a "Home Run" system, where small lines feed each point-of-use separately (flexible PEX works well); and with a bypass line near the connection points, which can be used to disconnect the water sources, hook up a small circulation pump, and run an environmentally friendly descaling agent through the water heater every 6 months or so.
Temperature Control and Mineral Scale Posted by Stephen Colley on Sep 4, 2009, 10:34 AM  
There remains much confusion among the different offerings of tankless water heaters with regard to temperature control relating to water flow, incoming water temperature, and outgoing water temperature. Some gas (and electric) tankless models address these problems better than others. I suppose that eventually all major manufacturers will adjust, but in the meantime, consumers could encounter frustration, not know how to make adjustments (for units that have the ability to do so) and will make acceptance of tankless water heaters more difficult. Oh, and then there's the problem refered to in the article with regard to high mineral content in the water. Until there's an environmentally appropriate way to minimize mineral build-up in plumbing lines, tankless water heaters would do well to provide an EASY means to remove mineral scale with regular maintenance. Some models address the problem, but sadly, most do not.
Groundwater temperature Posted by Heather DeGrella on Sep 3, 2009, 03:54 PM  
Another issue of which to be aware is sizing the water heater according to the groundwater temperature. We made the mistake of installing a gas-fired tankless water heater that was too powerful for the warm water in our central Texas location, and in the summers the water flow has to be very high (3 GPM or more)or it overheats and shuts itself off. Otherwise I love the heater, just wish we had been more aware of that issue when we first bought the tank.
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LEED Credits
Minimum Energy Performance: Design & Construction
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IMAGE CREDITS:
1. Illustration: Julia Jandrisits
DISCUSSIONS
Other Considerations:
Hernando Miranda
Sep 4, 2009
More comments
RELATED LEED CREDITS
Minimum Energy Performance: Design & Construction
EA Prerequisite 2
More related LEED credits
RELATED GREEN DESIGN
Hot Water Delivery to Fixtures
RELATED CSI LISTINGS
Fuel-Fired Residential Water Heaters
CSI Section 22 34 01