C.D. Howe Renewal
March 1, 2010
The C.D. Howe Building was built in 1977, and has a gross area of 14,200 square metres. Located two blocks from Parliament Hill, the building is one of Ottawa's premier office facilities and is a defi...
The C.D. Howe Building was built in 1977, and has a gross area of 14,200 square metres. Located two blocks from Parliament Hill, the building is one of Ottawa’s premier office facilities and is a defining feature of the city’s skyline.
It consists of two 11-storey interconnected towers. They house 3,800 office workers, as well as 60 retail and service outlets on the lower three levels, and an underground parking garage.
At 30 years of age, the building’s comfort systems had reached the end of their useful service lives. The primary chilled water distribution and air treatment systems were no longer serviceable. They had become obsolete from an energy performance perspective, and were frequently plagued by failures which caused disruption for the occupants. Dessau is the prime consultant on a project to totally reconfigure the base-building mechanical and electrical systems.
District cooling challenges
The C.D. Howe Building is one of many customers of a district energy system (the Cliff Central Plant) that provides chilled water and steam to facilities throughout the parliamentary precinct and downtown core.
The district energy plant can only operate efficiently when there is a steep differential between the temperatures of the supply and return water. Otherwise the district plant must pump excessive amounts of water to the receiving building. This extra pumping consumes energy and affects the efficiency of the central plant’s chilling equipment.
In order for it to operate efficiently, therefore, the central plant’s contract terms require that in the cooling mode building users must produce a 9 C temperature rise so that the minimum temperature of the returning water is 15.6 C.
Since initial commissioning in the 1970s, the C.D. Howe building has always been one of the worst performers on the district network, with quite a poor temperature differential on the chilled water. It produced a temperature rise of only 3-4 C during peak summer demand.
One of the most challenging aspects of the design was therefore to improve the temperature rise in the C.D. Howe Building to make it comply with the requirements of the district energy supplier.
The retrofit needed to triple the building’s temperature rise in its cooling mode. The goal was to redesign the system so that each gallon of chilled water from the district energy system would deliver three times the cooling energy. The temperature of the return water would therefore increase from approximately 4C to 15.6C.
This change required a major technical intervention. Instead of simply replacing deteriorated components with new ones, a complete re-design was required.
Energy recovery the solution
The solution was to interconnect the chilled water piping system serving two major central outdoor air handlers with the compartmentalized air handlers on every floor. The result is a sophisticated energy recovery scheme that pre-cools the incoming outdoor air with cold extracted from the floors during the summer. Simi- larly, during the winter the system uses heat recovered from the tower floors to preheat the outdoor air for the entire facility.
The phasing of the work is being carefully scheduled since ventilation, cooling and all other comfort control services to the tenant spaces must remain operational and uninterrupted.
Dealing with humidity and air quality
The building had long suffered from excessive humidity levels during Ottawa’s muggy summers. The simple damper-style of controlling outdoor air to the floor air handlers had been impossible to balance. As such, several floors received more than the correct amount of outdoor air, while others suffered from the opposite condition. Tenant complaints of stale air were commonplace.
Pressure-independent constant volume boxes now precisely meter the correct quantity of outdoor air to the office units’ air handling equipment. Furthermore, the floors are individually scheduled to halt outdoor air delivery during unoccupied periods, according to the needs of tenants.
The floor air handlers originally relied on outdated fan capacity controls as a means of controlling temperature. Inverter-style variable speed capacity controls were added to all the fans (aggregate motor size 1,200 HP), with a demand-based comfort control strategy which is far more energy efficient.
Temperature control by variable pumping
The design also incorporated variable speed drives as a means of controlling the supply temperature of fluid systems. This approach is rarely implemented on large, high flow systems — typically HVAC pumped fluid systems use control valves to regulate flows in order to control temperatures. In several instances in the C.D. Howe retrofit control valves were eliminated entirely; the pump speed was varied in response to the control signal from an energy management and control system. The result is a reduction in capital costs, maintenance costs on the control valves, and lower fluid friction, generating energy savings.
Another innovation was the use of small circuit balancing valves to create a small but constant flow across the temperature sensors.
The new design has resulted in excellent thermal comfort. Indoor humidity levels now remain within the recommended comfort zone throughout the year.
Tenant server rooms cooled more sustainably
The tenant server rooms on every floor were previously supplied from a dedicated chilled water riser to ensure that the rooms were continually cooled during periods when the district energy provider carried out maintenance and shut down the main system. However, this practice of using once-through cooling with domestic water for the server room equipment was both unsustainable and very costly.
The retrofit approach entirely abandoned the use of chilled water for this purpose. New rooftop fluid coolers and a glycol circuit reject heat without interruption, while also providing preheating of the outdoor air via heat recovery.
Maintenance, and cost and energy savings
The retrofit has alleviated a number of maintenance problems. With the original configuration, minor failures would result in the dehumidification coils in the penthouse freezing and causing occasional flooding damage. These problems have been eliminated.
The retrofit of the base building’s mechanical and electrical systems and equipment is almost 90% complete, while the tenant floor system retrofits are 70% complete.
The combined energy and demand savings from the base-building systems’ renovation are projected to be approximately $400,000 annually. With project costs of $4,000,000, the overall payback is therefore 10 years, which is impressive for a projbuildings ect predicated on life-cycle renewal.
Taking into account the thermal efficiencies for the district energy supplier’s plant, the project is estimated to reduce greenhouse gas emissions equivalent to 733 tons of C02 per year.
Greenhouse gases from commercial buildings represent almost 20% of all North American GHG emissions. While sustainable design and programs such as LEED represent the future of new construction, it’s clear that much of the existing commercial building stock will remain in service for at least the next 30 years. Innovative projects for renewing existing buildings like the C.D. Howe Building are therefore crucial.
Owner: C.B. Richard Ellis
Prime consultant, mechanical/electrical: Dessau (Louis Landry, P. Eng., David Landsberg, P. Eng., Roch Nault, P. Eng., Jean-Luc Thomassin, T.P., Jacques Rioux, T.P., Jacques Deschnes, ing., Claude Lauz, T.P.)