DECARBONIZATION SOLUTIONS FOR EXISTING HOT WATER SYSTEMS
YONI BUTBUL P . E ., CPD
THE ABILITY TO COMPLETELY REPLACE AN EXISTING HEATING SYSTEM WITH AN ALL-ELECTRIC SYSTEM IS OPTIMAL FOR REDUCING OPERATIONAL CARBON BUT NOT FOR LIFE-CYCLE COSTS .
Aggressive decarbonization targets set by government agencies and some owners are requiring the industry to formulate strategies to reduce the embodied and operational carbon in the built environment . Decarbonizing the existing building stock is critical to meeting these carbon-reduction goals . The building sector is a significant source of global carbon emissions , with buildings ’ operational energy accounting for 27 % of energy-related carbon emissions in 2020 ( IEA Paris , 2021 ). Most of the hydronic space heating systems in existing buildings were designed around high heating hot water supply temperatures ( 180 ° -200 ° F ). Reducing the operational carbon in existing building heating systems designed for these higher water temperatures presents a considerable challenge , as replacing the entire hydronic heating systems is not economically viable .
There are a few rare intervention points in a building ’ s life cycle where a complete heating system replacement can be financially viable . Oftentimes , a partial upgrade of a heating system ’ s generation equipment and some ancillary components is more likely to occur . As designers , we need to analyze the driving factors for conversion and evaluate the financial and carbon impacts while reviewing options for conversion . In the near term , the most economical solution to reducing operational carbon can be applying all-electric heating solutions to supplement traditional natural gas-fired heating equipment . When evaluating existing system upgrades , short-term building decarbonization strategies should focus on reducing as much annual carbon emissions as possible , utilizing the mechanical infrastructure in buildings that still has significant useful life to offer . Long-term solutions should aim to nearly eliminate all-natural gas heating in buildings .
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Q2 / 2022
Heat pumps are a proven , all-electric heating technology that will play a significant role in decarbonizing existing and new buildings . Advances in heat pump technology present an opportunity to convert fuel-burning equipment into fully electric or hybrid gas-electric options in many climates . Airsource heat pumps have limitations for heating in cold climates . More research and development will be needed to develop heat pumps to work in very cold climates .
Heat pumps are significantly more efficient than natural gas and electric-resistance heating equipment but are limited in the quality of the heat they can produce . Most heat pumps can only produce hot water temperatures as high as 140 °. Utilizing a lower supply temperature from the heat pump will improve the operating efficiency of the heat pump . Utilizing 140 ° ( or lower ) to heat buildings that were initially designed for higher water temperatures ( e . g ., 180 °) can be challenging .
This article focuses on possible solutions to provide most of the annual heating with heat pump technology for low-load conditions using lowtemperature heating hot water on existing buildings that were designed with higher heating hot water supply temperatures .
Traditional Hydronic Heating Systems
One of the biggest challenges in reducing the operational carbon of existing buildings is determining how to do so cost-effectively without replacing the entire mechanical system . Reducing the heating loads through energy efficiency measures can provide better returns on investment . When considering a water heat pump to replace fossil fuelbased hydronic heating , it is critical to understand how the original system was designed and the assumptions that went into its sizing . In projects that only involve the replacement of heat-generating equipment , an engineer is typically limited by the existing pipe sizing and available coil heat transfer surface area . Designers are generally limited to the original design flow rates without increasing pipe velocities and pump head . Many times , the heat