transfer coils used in higher-temperature hot water systems , like those found in terminal reheat boxes , were typically specified as 1-row .
The complete decarbonization of an existing highertemperature hydronic heating system would require the replacement of nearly all system components , including the boilers , pumps , heating piping , and all 1-row heating coils .
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 . The economic cost and the embodied carbon implications resulting from a near-term complete system replacement make the proposition unjustifiable for most building owners . A hybrid solution that seeks to reduce the majority of the annual heating emissions with heat pump technology for lower-load conditions is a worthwhile idea to evaluate .
Building Heating Loads
Building heating loads can be significantly overestimated , especially in mild climates . Figure 1 shows an annual heat load distribution profile for a 125,000-square-foot building at a local university constructed in 1960 . The building heating system was designed to cover a peak load of 2,700 MBH using 180 ° water at a 20 ° ∆T . The reality is the building heating load peaked at 1,620 MBH , 60 % of the design load , and 80 % of the annual cumulative therm usage occurs when the load is ≤30 % of the design load . On a square-foot basis , the peak design load was 21.5 Btu per square foot , while the actual peak load was only 12.9 Btu per square foot . The author has seen design heating plants sized for up to 60 Btu per square foot in this climate zone . In considering any heating system decarbonization project , it is critical to understand the true peak heating load and at what load bins the system emits most of the annual emissions . A building ’ s annual heat load distribution profile can be obtained through direct measurement or most commercially available load calculation programs . Trend data is preferred over simulation data , as heating load calculations do not usually take into account internal heat gains and result in overestimated heating loads .
FIGURE 1 : An example of an existing building ’ s annual heat load distribution . Images courtesy of P2S Inc .
After recognizing the wide variability in building heating demand , it begs the question as to why a boiler system would need to operate at 180 ° F to meet the heating load throughout the year . Hydronic hot water reset strategies are common energy savings measures that reduce a building ’ s hot water supply temperature as a function of the outdoor temperature . Traditionally , hot water supply temperature is reset downwards under low-load conditions to minimize piping heat losses , improve controllability , and maximize condensing boiler operation . A typical heating hot water reset curve for a mild climate is shown in Figure 2 .
FIGURE 2 : A sample heating hot water temperature reset based on outside air ( OSA ) temperature .
Heating hot water reset in a hybrid heating plant allows a heat pump to serve the building heating load when the required heating hot water supply temperature set point is below 140 °. Heat Pump