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Why Hybrid Water Heaters are Better

Second to heating and cooling spaces, water heating is the largest consumer of energy in most US households. Water heating consumes 18% of energy used in homes nationwide, as of 2009, and over 95% of all water heaters installed in 2006 were traditional tank heaters, that keep 40 or 50 gallons of hot water in the tank. Tank heaters make a lot of sense in large residential, commercial, and institutional buildings, where hot water is almost always being used. However, tank water heaters don’t necessarily make sense in homes.

In most single family homes, hot water is used primarily during two times of the day: morning and evening. A typical family might wake up at 6:00 in the morning and leave home for work and school by 8:00. During those two hours, they are using small to medium amounts of hot water constantly. If the family comes home at 6:00 PM and goes to bed at 10:00. Of the 24 hours that the water is kept hot, 18 of those hours are on “standby” and not actually being used. These standby losses account for a large portion of the energy used for water heating. Water heater energy efficiency is rated by Energy Factor (EF), which compares the amount of energy consumed to the amount of energy added to the hot water. Most standard gas tank water heaters have an energy factor of between 0.57 and 0.62, while gas tankless and hybrid tankless water heaters have Energy Factors between 0.82 and 0.95. The tankless and hybrid tankless heaters achieve up to a 40% improvement in efficiency through a combination of eliminating standby losses and improved combustion efficiency.

Though gas tankless water heaters have been around for over 20 years, in 2006, gas tankless water heaters accounted for only three percent of water heaters installed in the US. This is due to a number of factors, not the least of which is the nature of a typical water heater replacement. According to the American Council for an Energy Efficient Economy, the majority of water heaters are not replaced until they fail. Since the occupants need hot water immediately, and are faced with a large unanticipated expense, the water heater that gets installed usually ends up being whatever was cheap and available. You wouldn’t replace a car this way, but most households lose a cost-effective upgrade opportunity by waiting until disaster strikes to make decisions that will affect their home for the next 10 to 15 years and cost thousands of dollars in installation and energy to operate.

Another reason that on-demand water heaters have lagged in market share is that some early tankless water heaters were tough to live with. Standard tankless water heaters were often sized too small to meet the demand of simultaneous water use, such as two showers and a kitchen faucet running at the same time. They were also flow-restricted, so when the tankless heater couldn’t keep up, it would limit the flow of water out of the heater and reduce water pressure throughout the house. The burners were designed to modulate their output based on the amount of water that was moving through the water heater, but if the heater was providing hot water for a shower, and another person turned on a faucet, the water heater burner would shut off, recalibrate, and turn back on, allowing a “sandwich” of cold water to pass through the water heater and delivering a cold surprise to the person showering. Finally, traditional tankless water heaters use a burner that is only marginally more efficient than the burner of a regular water heater, with a combustion efficiency of about 82%.

Installed Hybrid Water Heater

In the last decade, tankless water heaters have become much more varied and sophisticated, as manufacturers work to improve efficiency and livability. The “hybrid tankless” water heater has succeeded in eliminating the livability problems of traditional tankless units, while simultaneously improving combustion efficiency from 82% to 95% or better. By installing a small tank inside the “tankless’ unit, a hybrid tankless water heater is able to modulate as additional taps are turned on or off without suffering the “cold water sandwich”. Hybrid tankless water heaters aren’t designed to restrict flow when their maximum output is reached, but to allow the water to flow at full capacity with slightly reduced temperatures. The efficiency gains are made by adding a second heat exchanger, which pulls heat from the burner’s exhaust and transfers that heat to incoming water. Hybrid tankless units are designed to self-clean, whereas traditional tankless units must be flushed out periodically to prevent scale from building up in the heat exchanger. Finally, since most of the additional expense of converting to a tankless water heater is associated with the labor of installing a larger gas line, a new venting system, and electrical supply, the cost to upgrade from a standard tankless to a hybrid tankless can be recouped in energy savings.

The highest capacity hybrid tankless water heaters can heat about 5.5 gallons of water per minute (GPM) in most US climates. This is enough water to provide 3 showers and a kitchen sink at the same time if low flow devices have been installed. The smaller capacity hybrids are designed for one-bathroom homes and provide closer to three GPM of water. Since a hybrid tankless water heater is designed to heat water instantly as it enters the heating unit, a hybrid will never run out of hot water, no matter how long a shower you take. Traditional tank water heaters “run out” of hot water faster than they can heat up incoming water, so after two 10-minute showers, they may take 20 or 30 minutes to heat the ever-cooling water in the tank back to the desired temperature.

If a home uses water for space heating or has a solar hot water system, traditional tanks are a good idea. But for the vast majority of households, where a gas furnace provides the heat and the family is gone most of the day, hybrid tankless water heaters provide energy savings of about 15% compared with standard tankless water heaters, and up to 40% energy savings compared with a traditional tank heater. Don’t wait until there is a puddle under your water heater to make the decision about what will replace it!

Sources:

Water heating energy consumption stat : EIA http://www.eia.gov/consumption/residential/

Tank market penetration stat: DOE http://www.energystar.gov/ia/partners/prod_development/new_specs/downloads/water_heaters/Water_Heater_Market_Profile_Sept2009.pdf

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