Eighteen tankless water heaters combine with a solar hydrothermal pre-heating system to deliver a steady flow of economical hot water to tenants in this 273-unit LEED Gold apartment complex.
Tankless water-heating technology has demonstrated its workability in a wide range of commercial-building projects over the past decade. The financial attractions of saving energy and space—the units are frequently wall-hung and thus have a zero footprint—are the obvious drivers. In addition, many commercial-building owners and managers have found multiple, redundant tankless units to be an ideal strategy for applications, such as hotels and restaurants, that simply cannot afford to be without hot water.
This technology and its benefits also apply to large, high-rise, multifamily projects. One example is the system installed in the AVA 55 Ninth high-rise apartment building in San Francisco’s Central Market. The facility contains 273 apartments on 16 levels, including a 3,300-sq.-ft. fitness center, plus another 26,600 sq. ft. on the ground floor, including 4,800 sq. ft. of retail space. The system combines tankless water heating with an array of sustainable features that recently enabled the property to earn a LEED Gold certification. Among the features are:
- low-flow, drip-irrigation systems that reuse rainwater collected in a cistern housed beneath a ramp leading to the building’s parking garage
- high-efficiency toilets and low-flow showerheads, and bath and kitchen faucets in all apartments
- waterless or low-flow urinals in the common areas on the ground floor.
AVA 55 Ninth also includes a number of informal “chill spaces,” designed to encourage tenant socialization. Besides the fitness center, the property offers an outdoor barbecue and dining area with a movie screen, a social-media trending wall, a utility center to oversee resident electronics, and a 10,000-sq.-ft., landscaped courtyard. Bicycle parking stalls, a bike-repair room, a pet-washing station, and a dog run round out the amenities package for the building.
Less visible, but no less crucial to the quality of tenant life, the innovative domestic hot-water system combines 18 separate, gas-fired tankless water heaters—all high-efficiency, condensing, 199,900 btu/hr. models with an energy factor of 0.95—into a single cohesive operation.
Manufactured by Noritz America Corp., Fountain Valley, CA, the NCC1991-DV units are hung and piped together in a 16 x 7-ft., custom-fabricated, Unistrut rack system located in a 1,000-sq.-ft., mechanical room on the roof of the building. Direct-vented with 4-in. PVC piping through a nearby wall, each water heater ramps its outputs up and down with the building’s changing demand for hot water throughout the day.
The heater package was designed and installed by ACCO Engineered Systems, San Leandro, CA. Lead plumbing engineer Kevin Hunt did the design. While Hunt has designed several such systems, the 18 units at AVA 55 Ninth, remains his largest system in terms of number of heaters.
Building owners often have the final say on the domestic hot-water system, and the driver is usually the sustainability goals of that same owner. In 2013, owner AvalonBay Communities Inc., Arlington, VA, announced it would take a more comprehensive approach to “environmentally sustainable development.” Among its initiatives: provide tenants with energy labels for their apartments and energy-usage benchmarking in 48 of its communities “to enable more proactive decisions about building performance and maintenance needs.” As of March 31, 2015, AvalonBay Communities owned or held a direct or indirect ownership interest in 279 apartment communities, containing more than 81,600 units in 11 states and the District of Columbia.
“AVA 55 Ninth community is an excellent example of our deepened commitment to sustainability in our properties,” said Tony Joyce, project engineer for several AvalonBay properties, including AVA 55 Ninth. “Tankless technology was a good fit for this building, and it helped us achieve our goal of LEED Gold certification.”
Tankless was, in fact, the first choice at AVA 55 Ninth from the earliest stages of the design process, said Hunt. “We did look at a storage system for domestic hot water, but a number of managers on the project were actively pushing for tankless.”
More Than Tankless
The impetus, of course, was the anticipated energy savings and how these savings would help AvalonBay toward its goal of achieving LEED Gold. Other considerations played key roles in the decision-making process:
Solar System. The ACCO team also liked the way tankless technology would work with the building’s solar hydrothermal pre-heating system, installed to enable the structure to meet California’s stringent Building Energy Efficiency Standards (Title 24). By trimming the energy draw of the tankless water heaters, “the solar pre-heating system reduces the building’s overall energy consumption,” explained Joyce.
Here is how the system works, as designed by Sun Light and Power, Berkeley, CA:
Thirty-eight rooftop solar panels use a glycol-and-water mixture to collect and transfer heat to a 2,250-gal. basin, a closed system filled with water and located inside the mechanical room. A second pumped loop transfers the heat in the basin water to the water entering the 18 tankless units nearby. As a consequence, the incoming water temperature is approximately 20 deg. warmer than it would be if it came straight from the ground.
The upshot? “The tankless system doesn’t have to work so hard bringing the incoming water to the set point temperature to meet demand,” Hunt remarked. “That temperature boost of about 20 degrees [Fahrenheit] saves even more energy and, we expect, extends the life cycles of the individual units.”
Redundancy. An apartment complex the size of AVA 55 Ninth typically uses hot water much like a large hotel: peak demand, or close to it, first thing in the morning, as tenants prepare for their day; low-level usage throughout the day while most tenants are away; and a second, smaller spike in the evening when tenants return to do food preparation, dish washing, and bathing.
That ever-fluctuating demand is why 18 heaters were used. The large number of units permits a more precise response to the hot-water usage habits of 273 different tenants. When all 18 are fully operational, total consumption is 199,900 btu/hr. x 18, or 3,598,200 btu/hr. Total flow rate is 104.6 gal./min.
Of course, actual demand is usually well short of that maximum, and sometimes (the middle of the night) there is comparatively little demand at all. But when a tenant does turn on a tap, shower, or washing machine, they expect the hot water to flow all but instantaneously.
The AVA 55 Ninth system responds to that right-now need by having at least one tankless unit operating at all times, keeping water at a minimum temperature constantly circulating to all 273 units. As demand grows, the other 17 heaters fire in whatever sequence is required to meet the current demand.
A single control unit, mounted on the rack and connected to all 18 heaters, smoothly orchestrates this sequencing process, making sure demand is fully met with minimum energy expenditure, i.e., no heater firing that unnecessarily heats water. “It’s not quite plug-and-play,” said Hunt, but the system controller (which can handle 24 units) “made it easier to organize the hot and cold water lines, as well as the gas and condensate connections in the rack, and then to set the system.”
Each unit has a minimum consumption of 16,000 btu/hr. That calculates into a turndown ratio of 225 to 1 (3,598,200 divided by 16,000). The higher the ratio, the greater the ability of a system to adjust to the wide-ranging demand of a large, multi-family community.
“The high turndown ratio allows the system to maintain a steady water temperature, even with a single unit running at only its minimum,” said Hunt. “This approach is much more efficient than a system that’s constantly cycling on and off. That kind of ‘short-cycling’ would result in significant and noticeable fluctuations in water temperatures for the tenants.”
Meanwhile, the solar-thermal setup allows this always-on system to function efficiently by minimizing the energy needed to raise the temperature of water coming into the heaters. Absent any storage tank, there is no wasted energy beyond the heat lost through the building’s supply piping as the water circulates through it.
One additional benefit of this redundancy cannot be overestimated. If any heater in the system needs maintenance, the other 17 will combine to pick up the slack. Except for absolute peak-demand conditions, tenants should be unaware that any servicing is happening, let alone experience an interruption in the water flow.
Other Cost Savings
Whatever initial concerns AvalonBay and ACCO had regarding the ability of a tankless system to meet the considerable hot water needs of AVA 55 Ninth, the system sizing and design phase addressed the concerns to everyone’s satisfaction.
“Sizing the system correctly was a major emphasis,” stated Hunt, “so we took extra care—speaking with the local rep, working with the design team at Noritz, and using their online sizing calculator on top of our own calculations, based on ASPE [American Society of Plumbing Engineers] recommendations.”
One significant area of focus, because of the sheer size of the system, was the diameter of the supply lines needed to feed gas to the water heaters. Early on, ACCO arranged for medium-pressure service, or 2 psi. “As a result, we cut the diameter of the gas piping from six inches to two and a half,” said Hunt, “which was a substantial cost savings.”
Tankless technology also provided another key cost reduction. Because of their comparative small size and weight, there was no need for a crane to place the water heaters on the roof. The units were simply delivered to the building’s parking garage and then taken up on the service lift. “Eliminating the need for a crane was a major savings,” said Hunt.
From design to delivery to installation, to its subsequent performance for the tenants of this property, the tankless system has proven to be a solid choice.
“For ACCO, tankless is now an option on all projects,” stated Hunt. “The big influences are the building owner’s priorities, of course, as well as what we think we can do with the system design. But, if the specification calls for a storage tank, we might still assume tankless as an alternative. In any event, we increasingly lean toward tankless.”