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Is Your Heater in Hot Water?

Considering a tankless (instantaneous) gas water heater? These heaters have recently grown in popularity due to their capacity to heat water to a specified temperature ‘instantaneously’, without having to store a volume of water like the traditional residential tank-type heaters. But before kicking your tank heater to the curb, there are numerous factors to think about, including initial cost, payback, location, maintenance, water quality, and demand.

“When weighing the costs, instantaneous heaters can be significantly more expensive to purchase than their tank counterparts, by as much as two to three times.”

When weighing the costs, instantaneous heaters can be significantly more expensive to purchase than their tank counterparts, by as much as two to three times. Although burner efficiencies are about the same between the two types, tankless heaters provide greater energy savings and payback, since no water storage is involved. With tank heaters, the stored water loses heat (wasting money and energy) through the tank shell, even though the construction, materials, and insulation have all been significantly improved over the past several years. Regardless of the type of heater you choose, high efficiency models of both tank and tankless heaters are currently available and eligible for tax credit.

Space requirements are another consideration. Tank heaters require a larger footprint and can’t typically be placed at the location of the highest demand for hot water. Instantaneous heaters, on the other hand, are small and can sometimes be placed much closer to where hot water is needed. Venting requirements may also determine where a heater can be located based on the routing or the construction of the vent. Instantaneous heaters require different venting than their tank counterparts, which can be much more expensive depending on material or routing. When locating water heaters, keeping the vent routing as short as possible helps to reduce costs.

Given their components and controls, instantaneous heaters often provide better self-diagnostic information than tank heaters, but they generally require slightly more maintenance. Some tankless models have controls that periodically flush the heat exchanger and/or ‘burn off’ mineral deposits; though, the units still need to be manually flushed with solution to de-scale the heat exchanger at regular intervals. Instantaneous heaters are also more sensitive to poor water quality, as the heat exchanger can be clogged by fouling. Typical rules of thumb – hardness above 10 grains may void the warranty, and water softening should be considered when hardness is above 5 grains. Tank heaters can withstand more neglect without affecting their performance, because they utilize a larger burner and more basic controls.

The potentially misleading idea of instantaneous heaters providing ‘unlimited’ hot water is that they are capable of doing so only when the peak flow rate is below a certain value. In other words, as more fixtures call for hot water, the heater will either limit (lower) the flow to each fixture in order to maintain a preset temperature or limit (lower) the water temperature in order to maintain the flow rate to each fixture. A typical residential unit can usually accommodate two major fixtures at once, such as two showers or one shower and a kitchen sink. Furthermore, some instantaneous heaters have a minimum flow that is required in order for the burner to fire. For example, the minimum flow required may be above the flow rate of a standard lavatory faucet, which means a higher flow aerator would be necessary. Clearly, water usage levels are a key consideration.

Deciding whether to use a tank or a tankless water heater is not easy. Choosing the water heater best suited for your application requires careful assessment of all the above factors.

– Jarod F. Stanton, PE, LEED AP