At this Denver office building, the building owner wasn't ready to replace both of his outdated multizone units, so the failed unit was replaced and the other unit was refurbished. Now the new rooftop unit supplies cooled air to volume dampers, which regulate the volume of conditioned air to each zone in response to their individual requirements.
Owners of older buildings today are sometimes faced with inefficient, worn, 25- to 30-year-old HVAC systems. It can be costly and time-consuming to replace whole systems, especially when system replacement presents the potential problem of having to renovate the building's roof to replace older curbs. Many owners have relied on retrofitting their building's existing system to save money and time.

Linton Towers, a 20,000-sq-ft, three-story office building in Denver, houses insurance agencies and real estate agents, among other tenants. The building had two multizone units on the roof. They were classic, 1970s-vintage multizone units, simultaneously running one stage of mechanical cooling and one stage of gas-fired forced air heat. Typical of multizone units, they were mixing heated and cooled air to satisfy the varying requirements in each zone - an unnecessary waste of energy.

Climate Engineering in Denver is responsible for maintaining the HVAC equipment in Linton Towers. When one of the multizone units needed replacing last year, the company undertook the job of renovating the new system.

"Last summer, all three of the compressors in one of the units went out," said Dale Moody, a Climate Engineering mechanical technician. "We were running the last part of the summer on one replacement compressor. Then the heat exchanger went out early this fall."

The building owner wasn't ready to replace both units, so only the failed unit was replaced. A Lennox 30-ton L Series(r) rooftop unit replaced one of the old multizone units. It now supplies cooled air to volume dampers, which regulate the volume of conditioned air to each zone in response to their individual requirements. There is no wasteful mixing with heated air to maintain desired temperatures in the different zones.

Some of the parts from the multizone unit that was replaced were used in the existing one to refurbish it. Since that unit's heat exchanger still works, they will continue using it, which muddies the energy savings analysis of the L Series unit. However, with savings already at about 4% and zero service calls or complaints reported, the solution has already provided benefits.

Curbing Tenant Disruption

Replacing the curbs was another task. The owner stipulated the building was to remain occupied with no interruption to the tenants' normal operation during the retrofit. The ThyCurb Company was called in to design a retrofit curb - a Retro-Mate(tm) adapter curb. Retro-Mate is an insulated, structural steel base that adapts the differing configurations of the new unit to the existing ductwork below.

This part of the job was simplified by completing a major portion of the work off site. Climate Engineering sent ThyCurb sales engineer Doug Hinton drawings, measurements, and serial numbers from existing curbs. When the curb was completed, it was simply hoisted onto Linton Tower's roof and fitted over the existing curb.

Honeywell's RapidZone(tm) zoning dampers and controls were factory installed in the curb. With no roof patching, or sheet metal modification required, and with controls in place, the Multi-Zone Retro-Mate curb significantly reduced installation time and costs.

On Monday, the installation team finalized the job details. Tuesday morning, Climate Engineering hoisted and set the adapter curb and the unit. The finishing piece of the job was the new L Series rooftop unit.

"We planned properly, and once we set the equipment, all we had to do is terminate the control lines and reconnect the gas line," said David Hoogendyk, mechanical technician. "We completed those tasks by Thursday and put the system online that day. The whole changeout went very smoothly."

The Sound Of Silence

Each of the building's eight zones is controlled by a sensor and thermostat, which keeps the system operating at a comfortable temperature. Lennox territory manager David Garver said, "The tenants joked the system must not be working, since they couldn't hear it; the new one is quieter. Each zone of the building is always within a degree of the desired temperature."

"The building occupants also feel a big difference in their climate," said Benny Benson, automation technician of Climate Engineering. When the system comes out of night setback, the temperature is brought up slowly so the whole building is thoroughly warmed without overshooting the setpoint and wasting energy.

"Built-in intelligence in the automation system determines when the warm-up will start each morning, depending on outdoor and indoor temperatures," said Benson. "Night setback sensing in each zone will maintain a uniform unoccupied temperature throughout the building. All of this saves energy - days, nights, weekends, and holidays."ES