Rooftop installation with 64 solar-thermal collectors and 18 tankless units meets domestic hot-water needs.
When the 182-unit Avalon Hayes Valley apartment complex in San Francisco was awarded LEED Platinum Certification in the Multifamily Midrise category, it was only the most recent expression of the ongoing commitment of AvalonBay Communities to environmental sustainability and corporate responsibility in its properties. The latter now number 282 developments, containing nearly 83,000 apartment homes in 11 states and the District of Columbia, with another 27 communities under development.
“This is a highly intentional culture,” explained Mark Delisi, senior director of corporate responsibility. “We act like owners, and as such, we must consider the life-cycle impact of our decisions. When deciding whether to do something, we will evaluate the opportunity based on its contribution to the future health of a property: Is it the right thing, long term? As property owners and managers, that’s a priority for us.”
Earning LEED Platinum for the East and West buildings at Avalon Hayes Valley was no small achievement. The project is a podium-style structure with five levels of residential living in each tower situated over a single-level parking garage.
“Building in San Francisco is kind of experimental, so you must always be on your game,” said senior project manager Bryan Moore, LEED AP, who served on the Avalon Hayes Valley Asset Team with senior development director Joe Kirchofer and property manager Alyssa Perry. “Of course, that challenge also puts AvalonBay on the cutting edge when it comes to green building.”
AvalonBay recently announced a new “Building Certified” construction standard, requiring all of its new high-rise and mid-rise construction projects to achieve third-party certification of environmental and energy efficiency from external rating programs such as LEED or Energy Star.
A major contributor to the impressive LEED scores at Avalon Hayes Valley is the buildings’ use of a solar-thermal-supported, tankless water-heating system, manufactured by Noritz America, Fountain Valley, CA, for all domestic hot-water needs.
Like most proponents of tankless, AvalonBay management appreciates the energy savings of on-demand water heating, as opposed to constantly reheating and storing hot water in large tanks in anticipation of its use. A dramatically smaller footprint also comes with eliminating the storage function. But while these attributes were apparent from the outset, AvalonBay still had questions about tankless—primarily having to do with the long term.
“Because we hold onto the properties we build for many years, the maintenance and the operations sides of all these products and standards are very important to us,” said Susie Maglich, LEED AP BD+C, director of design resources, AvalonBay Communities. “So we wondered how the move from conventional tank water heating to tankless would impact our Residential Services group.
“For instance, we’ve learned that we need to flush these tankless units every year, and that flushing routine is now part of our maintenance procedures. As a result, tankless has become one of our preferred methods when we are pursuing LEED or Energy Star or wherever the local energy codes are particularly stringent. We encourage our teams to go tankless to help us achieve our goals.”
Integrating Tankless and Solar
The installation at AvalonBay Hayes Valley uses 18 Noritz NCC1991-DV condensing tankless water heaters to meet domestic hot-water needs, including apartment sinks and showers, janitorial sinks, and even a dog-washing area in the parking garage. The system is a rooftop installation, capitalizing on San Francisco’s year-round mild climate. The municipal water feeding the tankless units is preheated by a solar-thermal system to lighten the load, saving still more energy as well as wear and tear on the heaters.
The twin-tower construction at Hayes Valley required the heaters be split into a pair of nine-unit systems. “A fully centralized system would have required extending the water-heater return lines from one tower to another,” explained design engineer Jeff Koch, CPD, LEED AP, ACCO Engineered Systems, San Leandro, CA, that did the design and installation of the tankless units, plus the waste and vent systems, storm drainage, and natural-gas and fuel-oil lines.
“By confining each system to its own structure, we minimized the amount of return piping, reducing our labor and making the overall system more manageable and efficient,” Koch explained.
When a resident turns on a faucet or a shower, the tankless system at either tower senses the subsequent water flow and activates the burner in one of the nine rooftop units. If demand exceeds the capacity of that initial unit, the system controller will trigger a second tankless unit to fire, then a third, a fourth, and so on until the need is fully met. As demand subsides, the units shut down one by one.
The water supply itself comes through the solar-thermal system, which was designed and installed by Sun Light & Power (SLP), Berkeley, CA. Also located on the roof, the system consists of 32 solar collectors on each of the two structures at Hayes Valley. Each collector setup is connected to a pressurized, 1,950-gal. storage tank where the heat energy is stored, awaiting a call for hot water.
When a demand for hot water arises, the solar-thermal system preheats the water supply through a pair of heat exchangers: the first between the collectors and the pressurized tank, the second between the tank and the municipal water that actually enters the tankless units. The latter operation is controlled by a variable speed pump, a Grundfos MAGNA3, that cycles on and off with demand in response to the building’s DHW demand. The glycol-water solution in the collectors never comes in contact with the buffered water in the storage tank, and the latter never infiltrates the DHW supply, thus eliminating any potential concerns over Legionella bacteria.
The job of the solar collectors is to boost the temperature of the incoming water supply, so that the tankless units need not work so hard to meet the set-point temperature of 120 F, according to Martin Morehouse, solar-thermal engineer and estimator at SLP. “The actual temperature boost the system provides varies, depending on the available heat in the storage tank,” which itself depends on the available sunlight.
“During periods of low-to-moderate demand when the tank is hot, the solar pre-heat is able to meet 100% of the demand without the tankless units activating.” [Note: A tempering valve caps the tank water at the set point of 120 F.] During the cooler months, the solar collectors boost the incoming water by only 10 to 20 deg., with the system providing 30% to 40% of the heat energy required to meet demand. In summer, that “solar fraction” jumps to 80%, for a year-round average of 64%. Greenpoints become available when the renewable energy fraction reaches 40%, which is why AvalonBay was interested in the SLP system.
“The system is designed to produce 7,250 therms of energy,” Morehouse continued. “With the water heaters having a thermal efficiency of around 96%, the system should offset approximately 7,552 therms of natural gas, or the equivalent of 40 metric tons of carbon dioxide saved per year.”
What if the solar collectors cannot provide any temperature boost, as might be the case when the fog rolls in or during an extended rainy period? “The tankless system simply compensates for what the solar system cannot provide,” said Morehouse. Even extensive maintenance is not a concern, according to Koch: “We have designed the tankless system with a bypass. If the collectors must go offline, the heaters will still function on their own to meet demand.”
The Inside Track
The ACCO organization is as enthusiastic about tankless as their counterparts at AvalonBay. Koch cites the energy savings, of course, stressing the high turndown ratio of a multi-unit system. At Hayes Valley, each heater has a minimum output of 16,000 Btu/hr. at a flow rate of 1/2-gal./min. “If a tenant turns on a hot-water outlet at 3:00 a.m.,” he explained, “we can comfortably and efficiently meet that isolated demand without firing all nine units.” But a few hours later when the entire building is in the shower, the heaters’ maximum outputs—199,900 Btu/hr./unit, or nearly 1.8 million Btu/hr. for all nine — should be more than sufficient to meet demand.
The modular configuration also streamlines installation and maintenance, minimizing cost and inconvenience. All 18 tankless units at Hayes Valley were installed by two ACCO plumbers in two days, said Koch. Nor was an expensive crane needed to lift the 70-lb. units onto the roof for installation onto steel screens: “The units are so light with no storage, the rooftop weight load was never a concern,” he commented. “Plus, the screen mounts provide our installation and maintenance crews accessibility to the front and sides of each unit.”
Likewise, if an individual tankless unit needs servicing, it can be easily isolated and removed for repairs without depriving the entire building of hot water. “Instead of using a crane to place a large piece of equipment onto the roof,” said Koch, “you simply wheel the replacement model on a hand cart up five floors using the service elevator.”
The lone problem ACCO has encountered with tankless relates to the increased amounts of silt in the water because of the drought in California. “Lower water tables throughout the state have resulted in silt in the plumbing,” said Koch. “So ACCO now equips all our tankless installations with strainers that filter the incoming water before it enters the heaters. Problem solved.”
So tankless technology has the inside track for any commercial project involving ACCO? That is certainly the case when the company functions as the design-build contractor from the outset of a project. In such instances, “we will likely evaluate that project in a way that will lean toward tankless,” said Koch.
Morehouse is excited about the prospects for solar —and tankless — in California, as the state drives toward a net-zero energy goal for new-residential buildings by the year 2020. In fact, he expects low-rise and mid-rise structures to be seriously considered for inclusion on the new net-zero stipulation.
“Getting as efficient as possible is the central reality for every developer, designer, and installer in our current world,” he noted. “Tankless is definitely something that will be seen as a great benefit to the overall efficiency of buildings, and we believe our solar-thermal system is an excellent complement to that technology.”