What Webs We Weave: How the Water–Energy Nexus Gets Us in Hot Water

April 21, 2015

As many of you know I edit both Water Efficiency and Business Energy magazines. From time to time, articles in one or the other title have relevance in both. These are the features that fall into the “water-energy nexus” category. Our May issue of Business Energy has just been sent to the printer. It will feature an article by Carol Brzozowski on tankless water heaters, when and why they are a fit sometimes, and upgrades some companies have made to the equipment.

As many of you know I edit both Water Efficiency and Business Energy magazines. From time to time, articles in one or the other title have relevance in both. These are the features that fall into the “water-energy nexus” category. Our May issue of Business Energy has just been sent to the printer. It will feature an article by Carol Brzozowski on tankless water heaters, when and why they are a fit sometimes, and upgrades some companies have made to the equipment. [text_ad] Gary Klein, one of the members of our Water Efficiency Editorial Advisory Board is very active in the hot water space. His work looks at both water and energy efficiency around hot water technologies. I thought I would share Klein’s thoughts about tankless water heaters from a separate interview with Brzozowski. In this interview Klein points out the obstacles and disadvantages he believes exist with tankless water heaters. I invite readers with experience to please comment. Is Klein taking an overly negative view? Are the points he brings up as problematic as he suggests? If you read to the end, you will see that his ideal prescription for heating water does incorporate tankless on-demand water heaters, but as a small part of a solar system of heating. Below is the write up from Brzozowski’s discussion with Klein:
Gary Klein heads up Affiliated International Management, which provides global sustainability consulting with a focus on water/energy carbon footprint issues and an emphasis on hot water. The company specializes in how to integrate a high performance hot water system into buildings to achieve sustainability, working with national codes, standards, and incentive programs to develop internally consistent provisions for hot water systems in all buildings. Klein would like to see more market penetration of solar water heating systems, which heat the water when the sun is out and store it in a tank for demand use. In a two-part series Klein wrote in 2010 for Solar Today magazine, Klein points out that he views hot water as a system within building science that includes delivery, use, and cool-down. The delivery speed depends on the water heating method and the location of the water heater in relation to the hot water outlets, he says. While hot water use can be more efficient with water efficient equipment such as aerators, showerheads, dishwashers, and washing machines, tankless water heaters do not fire at low-flow rates, notes Klein. Klein notes that there are “real and perceived” drawbacks to tankless water heaters. Case in point: The retrofitting expenses of increasing the gas line size and changing the venting for gas tankless water heaters. Electric tankless units require space in the distribution panel, with the unit’s capacity limited by the panel’s amperage rating and wiring that needs to be of larger diameter than is used for storage electric water heaters, he says. The typical recommendation for the relationship between panel size and storage volume is 2 gallons of storage per square foot; Klein advocates moving the needle to up to as much as 4 gallons per square foot. There’s the increased maintenance necessary with incoming hard water for both gas and electric tankless units. In the overall delivery, use and cool-down system, water wasted in the delivery phase can account for up to 30% of the total hot water consumed, says Klein, adding that energy is wasted as well. In reducing piping volume, piping insulation also is important, Klein notes. Piping configuration depends on the layout of the hot water outlets relative to the water heater location and may include a backup system. In addition to maximizing water and energy inefficiencies from a hot water system, end users then must size and select a supplemental water heater for the solar water-heating system, Klein says. Sizing the supplemental heater depends on part on the amount and temperature of stored solar-heated water, says Klein, adding that the supplemental heater must also meet the entire load demand when the sun hasn’t shined significantly for a long time period. Choices include a storage tank to accommodate enough hot water, a burner or element large enough to sustain any desired flow rate or a combination of some stored volume and a more modest burner or element (60,000–120,000 BTU per hour or 15–30 kW), which Klein prefers. While storage water heaters accept any given temperature and activate only when the thermostat registers cold water, standard gas-fueled tankless water heaters do not favor high-temperature inlet water, Klein notes. However, electric tankless water heaters appear to be able to do so, he says, adding that they don’t activate when the temperature is above the set point and heat only the amount needed when it’s below the set point. Supplemental heaters need to provide small amounts of additional energy for boosting, which is a challenge for most tankless water heaters, particularly gas units without built-in storage, Klein points out. It takes about 40,000 BTU per hour (10 kW) to keep up with a flow rate of 1 GPM at a 70°F temperature rise, says Klein. There must be a combination of flow and temperature rise to absorb the heat. If only a 1°F temperature rise and a flow rate of 0.25 GPM is needed, the water needs to be supplied with only 125 BTU per hour before accounting for the efficiency of the water heater, Klein says. The capacity of the burner at the low end of a modulation range of a gas tankless water heater is too big, Klein says. Gas tankless water heater manufacturers recommend increasing the flow rate so the device fires, but that makes it almost impossible to use low-flow rate outlets, Klein adds. Even when the solar-preheated water is colder than the desired set-point temperature, a high cold input water temperature will give a gas tankless heater problems unless a tempering valve has been installed between the cold inlet and the solar storage. Activating and sustaining operations at very low flow rates is also a challenge for most electric tankless water heaters, with most needing at least 0.5 GPM to operate, Klein adds. Some electric tankless units can modulate, providing only the incremental heat needed to match the flow rate and temperature rise. Klein says he believes the optimal match for solar preheating is a water heater that will heat the water on-demand and only as much as needed to reach the set-point temperature. He has historically advocated for the use of a fully modulating electric tankless water heater, but also gives a nod to a condensing gas tankless water heater rated EF = 0.95 with 2 gallons of built-in storage.

Gary Klein, one of the members of our Water Efficiency Editorial Advisory Board is very active in the hot water space. His work looks at both water and energy efficiency around hot water technologies. I thought I would share Klein’s thoughts about tankless water heaters from a separate interview with Brzozowski. In this interview Klein points out the obstacles and disadvantages he believes exist with tankless water heaters. I invite readers with experience to please comment. Is Klein taking an overly negative view? Are the points he brings up as problematic as he suggests? If you read to the end, you will see that his ideal prescription for heating water does incorporate tankless on-demand water heaters, but as a small part of a solar system of heating. Below is the write up from Brzozowski’s discussion with Klein:

Gary Klein heads up Affiliated International Management, which provides global sustainability consulting with a focus on water/energy carbon footprint issues and an emphasis on hot water. The company specializes in how to integrate a high performance hot water system into buildings to achieve sustainability, working with national codes, standards, and incentive programs to develop internally consistent provisions for hot water systems in all buildings.

Klein would like to see more market penetration of solar water heating systems, which heat the water when the sun is out and store it in a tank for demand use. In a two-part series Klein wrote in 2010 for Solar Today magazine, Klein points out that he views hot water as a system within building science that includes delivery, use, and cool-down. The delivery speed depends on the water heating method and the location of the water heater in relation to the hot water outlets, he says.

While hot water use can be more efficient with water efficient equipment such as aerators, showerheads, dishwashers, and washing machines, tankless water heaters do not fire at low-flow rates, notes Klein.

Klein notes that there are “real and perceived” drawbacks to tankless water heaters. Case in point: The retrofitting expenses of increasing the gas line size and changing the venting for gas tankless water heaters. Electric tankless units require space in the distribution panel, with the unit’s capacity limited by the panel’s amperage rating and wiring that needs to be of larger diameter than is used for storage electric water heaters, he says. The typical recommendation for the relationship between panel size and storage volume is 2 gallons of storage per square foot; Klein advocates moving the needle to up to as much as 4 gallons per square foot.

There’s the increased maintenance necessary with incoming hard water for both gas and electric tankless units. In the overall delivery, use and cool-down system, water wasted in the delivery phase can account for up to 30% of the total hot water consumed, says Klein, adding that energy is wasted as well. In reducing piping volume, piping insulation also is important, Klein notes. Piping configuration depends on the layout of the hot water outlets relative to the water heater location and may include a backup system.

In addition to maximizing water and energy inefficiencies from a hot water system, end users then must size and select a supplemental water heater for the solar water-heating system, Klein says. Sizing the supplemental heater depends on part on the amount and temperature of stored solar-heated water, says Klein, adding that the supplemental heater must also meet the entire load demand when the sun hasn’t shined significantly for a long time period.

Choices include a storage tank to accommodate enough hot water, a burner or element large enough to sustain any desired flow rate or a combination of some stored volume and a more modest burner or element (60,000–120,000 BTU per hour or 15–30 kW), which Klein prefers.

While storage water heaters accept any given temperature and activate only when the thermostat registers cold water, standard gas-fueled tankless water heaters do not favor high-temperature inlet water, Klein notes. However, electric tankless water heaters appear to be able to do so, he says, adding that they don’t activate when the temperature is above the set point and heat only the amount needed when it’s below the set point.

Supplemental heaters need to provide small amounts of additional energy for boosting, which is a challenge for most tankless water heaters, particularly gas units without built-in storage, Klein points out.

It takes about 40,000 BTU per hour (10 kW) to keep up with a flow rate of 1 GPM at a 70°F temperature rise, says Klein. There must be a combination of flow and temperature rise to absorb the heat. If only a 1°F temperature rise and a flow rate of 0.25 GPM is needed, the water needs to be supplied with only 125 BTU per hour before accounting for the efficiency of the water heater, Klein says. The capacity of the burner at the low end of a modulation range of a gas tankless water heater is too big, Klein says. Gas tankless water heater manufacturers recommend increasing the flow rate so the device fires, but that makes it almost impossible to use low-flow rate outlets, Klein adds.

Even when the solar-preheated water is colder than the desired set-point temperature, a high cold input water temperature will give a gas tankless heater problems unless a tempering valve has been installed between the cold inlet and the solar storage.

Activating and sustaining operations at very low flow rates is also a challenge for most electric tankless water heaters, with most needing at least 0.5 GPM to operate, Klein adds.

Some electric tankless units can modulate, providing only the incremental heat needed to match the flow rate and temperature rise.

Klein says he believes the optimal match for solar preheating is a water heater that will heat the water on-demand and only as much as needed to reach the set-point temperature. He has historically advocated for the use of a fully modulating electric tankless water heater, but also gives a nod to a condensing gas tankless water heater rated EF = 0.95 with 2 gallons of built-in storage.

About the Author

Nancy Gross

Nancy Gross is a former editor of Business Energy and Water Efficiency magazines.

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