22 will eventually decarbonize via the proliferation of renewable energy carriers and/or nuclear energy (CDDI-1). The choice to electrify all heating by say switching from a natural gas boiler to a heat pump may initially seem "easy" since the latter has a coefficient of performance of around 3 compared to a natural gas furnace of around 0.95. As discussed in CDDI-1, the 2016 greenhouse gas (GHG) emissions of grid electricity and natural gas in New England were 0.256kgCO2e/kWh and 0.231kgCO2e/kWh, respectively. This means that for each kWh of heating load a building with a heat pump generated 0.256kgCO2e/kWhelectricity ÷ 3.0 = 0.085kgCO2e/kWhthermal load Equ 1-5 compared to 0.243kgCO2/kWhthermal load for the natural gas boiler. This means that the heat pump systems would have reduced carbon emissions by 65%.However, given the cost difference between electricity (US$0.225/ kWh) and natural gas (US$0.043/kWh) at the time, operating the the heat pump would have been 66% higher. To economically break even, the local carbon price would have had to be 178US$/tCO2e (CDDI-1). In the absence of a carbon tax, design teams and their clients need to agree on a project-internal carbon evaluation to avoid GHG emissions and/or install an adequately sized photovoltaic system. This type of analysis is highly context-specific. Up-to-date electricity emission numbers are provided by the Electricity Maps platform.9 By selecting the fossil fuel and electricity or all-electric system option in the HVAC selector form, the number of viable systems is further filtered down. The final choice is concerned with the building's heat delivery medium. Heat Delivery Medium While an all air system meets any space conditioning needs via heated or cooled air, minimum outside air systems only satisfy fresh air requirements. This means that any space heating or cooling loads have to be met separately, for example via a fan coil unit as in Fig 1.10. As shown in the diagrammatic section in Fig 1.11, a fan circulates room air through the unit across a heat exchanger coil. The temperature of the coil is controlled via hot or chilled water that is pumped via two separate piping systems through the building, leading two pairs of supply and return pipes. A fifth pipe collects any condensed water that drips from the heat exchanger coil during the cooling season. For the fan coil unit in Fig 1.10, heat is delivered via water which is why the system is called "hydronic." Part three provides photo/ section pairs as in Fig 1.10 and 1.11 for a variety of HVAC components. Why does the heat delivery medium matter? Given that the volumetric heat capacity of water is 4000 times higher than that of air, water pipes take up
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