J. Sager (NRCan)
R. Glazer (NRC)
F. Szadkowski (NRCan)
T. Strack (Strack and Associates)
Publication date: September 2014
Zoned high-velocity combination systems combine the provision of space heating and domestic hot water heating in one system. Due to their small equipment footprint, small ductwork and zoned distribution, the systems are a good fit for newer row units, townhomes and multi-unit residential buildings where mechanical system space is limited and maintaining consistent comfort conditions between upper and lower floors is a challenge. A previous investigation (Sager, J., et al, May 2013) determined that the high-velocity zoned combination system tested consumed less energy for space and water heating and provided equivalent heating comfort when compared to the standard system. This assessment focuses on the cooling performance of the high-velocity zoned combination system.
The high-velocity zoned combination system tested was a packaged (not “site-built”) system consisting of an air handler with high efficiency blower motor and two zone integrated supply dampers. This was combined with a condensing gas tankless water heater (Energy Factor 0.83) for space heating and a central air conditioning condenser (SEER 13) for space cooling. The system’s two-zone ductwork was divided into one second-floor, bedroom level zone and a second main floor and basement living-area zone. The two zones were independently controlled by wall mounted programmable thermostats.
The standard system that it was compared against was a low-velocity non-zoned gas furnace with condensing level performance in space heating, a central air conditioning condenser (SEER 13) for space cooling, and power-vented tank type gas water heater. The furnace was equipped with a high efficiency blower motor and a fan controller which adjusted air-flow volumes. The standard system was controlled by a wall mounted programmable thermostat located on the main floor.
The test results were modelled in order to predict the electricity usage by a range of residential cooling systems equipped with different circulating fan motor technologies and cooling air-flow settings. The high velocity combination system with two-zone ductwork used less on-peak electricity for cooling than any of the non-zoned cooling systems evaluated. The average on-peak demand savings for systems with typical air-flow settings ranged from 280 to 590 watts per household for cooling systems using continuous air circulation, and from 280 to 430 watts per household for cooling systems using automatic fan settings. The higher values in each case were associated with cooling systems that used permanent split-capacitor (PSC) motor technology to drive the circulation fan in the furnace or air-handler. The high-velocity zoned cooling system used less daily electricity than non-zoned cooling systems equipped with PSC motors using continuous air circulation, with daily savings ranging from 3.3 to 7.9 kWh per household for low-velocity and high velocity systems respectively.
In spite of using less on-peak electricity, the high-velocity, two-zoned system used more total daily electricity to provide cooling to the test house than the standard system. The average daily usage by the high-velocity zoned cooling system was 16.6 kWh, while the average daily usage by the standard cooling system was 10.6 kWh. Most of the additional electricity usage occurred during the off-peak period (i.e. 7 pm to 7 am) of the day. More than half of the additional usage was attributed to extra work by the circulation fan to move air through the smaller diameter duct system used by the high-velocity combination system. The balance was used to provide additional cooling, which significantly improved comfort on the second floor of the zoned test house during the overnight period.
The high-velocity zoned combination system consistently cooled the second-floor of the house to the desired temperature during the overnight period, while the standard system under-cooled the second floor and was unable to achieve the desired temperature at any point during the overnight period on most days during the evaluation period. On average, the overnight second-floor temperature in the zoned house was 24oC. The overnight second-floor temperature in the standard house was on average 1.4oC warmer with peak differences as high as 2.4oC warmer than the zoned house. Comfort conditions on the main floor and basement zones were equivalent in the zoned test house and the standard, non-zoned reference house, with no significant differences noted.
A set-forward thermostat schedule was employed in the zoned house in an effort to explore the potential energy savings and comfort benefits of eliminating cooling to unoccupied spaces and focusing delivery to occupied spaces throughout the day. This served to eliminate unnecessary cooling on the second-floor zone during the daytime and to the main-floor/basement zone during the overnight period. The daytime set-forward schedule of the second-floor thermostat contributed to an on-peak reduction in electricity usage by the zoned cooling system, while the main-floor thermostat maintained the desired temperature conditions on the main-floor and basement zone of the house.
For access to the full publication, please contact the CanmetENERGY-Ottawa Business Office.
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