by arslan_ahmed | October 27, 2022 10:00 am
By Derrick Paul
Packaged Multizoneasaurus thrived primarily between 1960 and 1980 A.D. Habitat for this large heating and cooling beast typically consisted of rooftops in cool and temperate regions of North America, where the length and severity of warm and cold seasons are similar. Many can still be found in place today. Notably, its present-day counterparts consume far less energy, a process which took decades of evolution.
Usually, packaged multi-zone air handlers were installed at institutional facilities, schools, hospitals, or banks and were selected based on their ability to provide simultaneous heating and cooling to different zones within the structure.
As for comfort, the huge units did a good job. However, from an energy standpoint, there was plenty of room for improvement. These hot deck/cold deck air handling units use a large supply fan (and often a return fan as well) to force air through a chilled water or refrigerant coil and a hot water coil. On the supply side of the stacked coils, zone dampers are placed on each take-off control, and they mix air for each zone.
Heritage building, vintage system
Victoria, B.C., has a climate well suited for multi-zone, rooftop air handlers. The area is known for temperate winters and mostly tolerable heat and humidity during the summer. Long shoulder seasons also mean heating and cooling requirements between the north and south side of a building can vary substantially. This is ideal for a system which can serve dissimilar zones.
One of the oldest structures in Victoria, the Rithet Building, was constructed in the late 1860s. Its prominent external cast iron columns make it one of British Columbia’s finest examples of West Coast Iron Front architecture.
In the mid-1970s, ductwork was added to the 2322 m2 (25,000 sf) brick building, and three multi-zone packaged units, each roughly 9.1 m (30 ft) long, were installed on the roof. One of these multi-zone units was installed on the upper floor and was recently replaced with seven packaged rooftop heat pumps. The other two multi-zone units remained in service, each serving seven zones.
Installed in 1993, four boilers downstairs provided ample hot water for the big heating coils, and each unit had an integral direct expansion (DX) coil served by a multi-stage compressor. Avalon Energy Management, a mechanical engineering and energy consulting firm, was approached by the property owner about replacing the remaining two multi-zone units.
“The Rithet Building now contains offices with some retail space below, and it needed to remain fully occupied for the duration of any HVAC retrofit,” said Tom Pedersen, engineering technologist at Avalon. “The fact that it’s a Landmark Heritage Building further reduced our options when it came time to select a new system.”
Instead of overhauling the existing multi-zone rooftop units or replacing them with new versions of the same technology, the firm designed a multi-zone air handler which employed air-to-air heat pump technology exclusively.
“After looking at a few different options, we determined that new, custom-built air handlers would be best,” said Pedersen. “Unlike the originals though, which had only two coils, the seven fan coils inside the new units are served by variable refrigerant flow (VRF) condensers with a heat recovery feature for load sharing. The cost increase over a new factory multi-zone unit was marginal, but the difference [reduction] in energy consumption was impressive.”
Once settled on VRF equipment, Pedersen’s biggest design hurdle was static pressure. Could the fans on the evaporator coils provide the air movement needed? Much of the existing ductwork was concealed and the mechanical “as-built” drawings were outdated and unreliable. After calculating the pressure loss based on these old drawings, the firm took the extra precaution of hiring an air balancing company to determine the true pressure drop. In each ducting run, both the calculated pressure drops and the measured pressure drops fell within the manufacturer’s published capabilities for their high pressure fan coils.
Avalon narrowed their search for VRF manufacturers to three as they were the only locally available manufacturers to provide systems which could meet the static pressure requirements in the fan coil sizes required. Ultimately, the coil dimensions made the decision simple. The chosen evaporator coils, which range from 2268 to 4536 kg (2.5 to 5 tons), were narrower and higher than the other two. This profile better suited the existing, roof-level duct takeoffs Pedersen needed to transition into.
Avalon then produced the initial design for the two air handler “boxes.” Each box contains seven independent evaporator coils and they draw from a common return air plenum. The plenum includes a single return air fan powered by a variable frequency drive (VFD), simply to maintain neutral pressure behind the coils. Each of the two big multi-zone units is served by three, 8165 kg (9 ton) VRF condensing units. The entire system fits directly onto the existing roof curb.
Writing the book
“I’m not aware of any other retrofits quite like this one, said Pedersen. “We issued a conceptual design bid, knowing that the fabricator would need to improvise on many of the details.” Capital City Refrigeration, based in Victoria, won the bid. They worked closely with JB Sheet Metal during the ground-up build of both new units.
“This was a bid-spec job, but it progressed like a design-build,” said Tim Sykes, who co-founded Capital City Refrigeration with Tyler Little. “We like a challenge though.” The company’s focus is commercial and institutional HVAC, refrigeration, and large residential work. With four technicians in the field, they describe themselves as lean and mean.
Work on the new air handlers began in March 2022 at JB Sheet Metal. The timeline was tight in order to have both units ready to install during the spring shoulder season.
Owner Jay Bull and other fabricators at JB Sheet Metal built the frame, cabinet and duct takeoffs, while Capital City technicians filled the boxes. With the return fan and filters on one side and ductwork on the other, coils, power components, controls and refrigerant lines made for a tight fit in the middle.
Despite the adequate static pressure capability of the units, Avalon wanted an insurance measure which would eliminate the risk of inadequate flow. A small box was installed just above each duct elbow leading down through the roof. Here, a knockout could facilitate the addition of a booster fan in the future.
“Now, after completion of the project, we realize it was an unnecessary precaution, but it was cheap insurance,” said Pedersen. “With the systems up and running, there’s even better flow than before the retrofit.”
The units were tested and commissioned off-site with a generator before being delivered and craned onto the Rithet Building; the first one was placed in April and the second a month later.
Before delivery, building control and power rough-ins were completed as well. Once on the roof, it took about a day-and-a-half for Capital City Refrigeration to put the units into service.
The new air handlers tied into the existing direct digital control (DDC) control system with the building automation and control (BAC) network hardware and software, allowing off-site control and monitoring. A 3D traffic interface was also installed.
In the spirit of maintaining the historical integrity inside and outside, the building’s old cast iron radiation system remains in place, staged as a secondary heat source through the DDC system. The radiators, which line only the perimeter, run at a low temperature when outdoor conditions drop to freezing temperatures.
The building’s old boilers served both the radiators and the multi-zone hot water coils before the retrofit and were grossly oversized once the VRF system assumed the role of supplying primary heat. At the time of the project, the old boilers were replaced by a condensing boiler.
“All told, this was not a cheap retrofit alternative,” added Sykes. “But given that the building couldn’t be disturbed, and that the exterior had to meet historical guidelines, I think it was the very best alternative, especially considering lifecycle costs.”
According to Sykes, they have since had three inquiries from other companies about similar retrofits. Both Capital City and Avalon say the process will go more quickly next time though.
From everyone’s perspective, the project was a resounding success. The property owner did not have any building downtime, the systems are operating as designed, and energy consumption is expected to be comparable to a geothermal heat pump system.
Derrick Paul is the director of variable refrigerant flow (VRF) sales at Fujitsu General America. He is a graduate of The University of Alabama, where he earned a mechanical engineering degree. Paul began his career at Fujitsu as a sales engineer in the southeast.
Source URL: https://www.constructioncanada.net/rithet-building-reborn/
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