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.

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. Air-source 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 low-temperature 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 fuel-based 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 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 higher-temperature 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 Technology and Hybrid Heating Plants

Heat pumps operate by moving heat from a lower-temperature area to a higher-temperature area. They typically take advantage of an electrically driven compressor and a closed vapor compression cycle. Commercial-grade heat pumps come in many forms and can provide useful heating, useful cooling, or simultaneously provide useful heating and cooling. Heat pumps are rated using their coefficient of performance (COP), which measures their useful output to the required energy input. The COP for heating (COPH) is the ratio of the useful heating energy divided by the energy input. The COP for cooling (COPC) is the ratio of the useful cooling energy divided by the energy input. For dual-mode heat pumps that can provide useful cooling and heating, the unit will have a combined COP — the ratio of the sum of heating energy and cooling energy to the energy input.

There are many ways to integrate a heat pump into an existing building heating system. The designer should consider the actual building heat load and the potential to apply the useful cooling available from heat pumps. Buildings with concurrent heating and cooling loads can be good candidates for heat pumps, such as water-to-water heat recovery chillers (HRC). It’s important to understand the presence of simultaneous heating and cooling loads in a building can indicate building inefficiency. Designers should first reduce wasteful simultaneous heating and cooling before sizing any heat pump system. Large buildings and campuses made up of many buildings will have simultaneous heating and cooling requirements that justify using an HRC. Heat recovery chillers can be used to cover the majority of a building heating load while supplying chilled water to the campus chilled water loop. To illustrate this concept, the simplified heating hot water piping diagram in Figure 3 shows a modular HRC piped in series with a variable-flow, gas-fired, condensing boiler plant to provide preferential loading of the HRC (Peterson, 2019).

FIGURE 3: A sample preferential loading heat recovery chiller (HRC) heating hot water (HHW) piping diagram (Peterson, 2019).

Heating Coil Performance at Reduced Water Temperatures

As discussed, the maximum capacity available from existing heating hot water coils will be limited by the original design building pipe sizes and the construction of the coil, i.e., rows, fin spacing, circuiting, and the quantity of coil passes. Heat exchanger effectiveness can be a useful concept in assessing the capacity of an existing coil against lowering water inlet temperatures. Heat exchanger effectiveness is the ratio of the actual heat transfer rate to the maximum possible heat transfer rate in a counterflow heat exchanger of infinite surface area with the same flow rates and inlet temperatures (ASHRAE, 2021). Suppose performance data of a heating coil, i.e., data from a drawing schedule, is known. In that case, the effectiveness of the heat exchanger can be calculated at its scheduled air and water flow conditions (cfm and gpm). The effectiveness could then be used to determine the capacity of that coil against varying supply water inlet conditions. Figure 4 shows the capacity reduction realized from reducing the inlet water temperature on a 1-row coil for several standard variable air volume (VAV) box sizes. The capacities below are based on heating cfm that is 50% of the recommended maximum (Taylor & Stein, 2004) box cooling cfm. The heating water flow rate was determined by locking in the heating coil heating hot water (HHW) ∆T at 30°.

FIGURE 4: A single-row variable air volume (VAV) reheat capacity versus entering water temperature (EWT).

Figure 4 illustrates the concept of heat exchanger effectiveness in that the capacity will vary linearly with inlet temperature for a given airflow rate and water flow. Figure 5 shows the percentage of the total heating capacity achieved at 180° entering water temperature (EWT) verses lower EWT temperatures, assuming the water flowrate remains constant. The graph gives the reader a quick way to determine the capacity of any commonly available VAV box reheat 1-row reheat coil for a given fixed air and water flow rate. The 130° EWT still provides 60% of the original 180° heating capacity. To explore the effect of changing the airflow and water flow rates through a heating coil, an engineer would need to determine the details of the construction of the coil and rerun the coil performance using commercially available coil software.

FIGURE 5: Tracking variable air volume (VAV) reheat percentage capacity versus entering water temperature (EWT).

Assessing Existing Buildings

Determining the annual heating load profile for a building's system-level preheat coils and zone-level reheat coils is critical to right-sizing a heat pump as well as determining the minimum allowable water temperature capable of satisfying a substantial amount of annual heating load.

If possible, measure the actual heating load for a heating season (or year). Annual historical load and outside air temperature trend data can be used to assess the financial feasibility of a heat pump or HRC. Measured actual heating demand for an existing building is instrumental in right-sizing a supplemental heat pump system. An alternative to measuring actual heating load is to do an 8,760-hour load simulation on the system using energy modeling software. This method is feasible but not as accurate as direct measurement.

On occasion, a building owner may agree to test a building’s heating system to determine the minimum heating water supply temperature at various outdoor conditions necessary to satisfy the building’s heating loads. This data could be used to generate an HHW reset curve.

While we have illustrated that building heating systems are usually oversized, there may be zones in an existing building that are right-sized or marginally oversized for the actual heating load and would not respond as favorably to aggressive hot water reset temperatures. This is analogous to zones that were improperly designed for their cooling loads and end up driving the building’s chilled water system. Replacing a few heating coils in a building to help increase the amount of time a heat pump boiler could support the entire building load is a worthwhile investment. Figure 6 compares capacities available from 1- and 2-row heating coils on a 16-inch VAV box. In this example, replacing the 1-row with a 2-row heating coil allows the air terminal to meet the 180° design load with 125° water. For most commercial VAV boxes, there is no difference in physical size between a 1- and 2-row coil, making the possibility of a coil replacement at a few critical zones feasible.

FIGURE 6: A variable air volume (VAV) 1-row and 2-row reheat percentage capacity versus entering water temperature (EWT).


The decarbonization of existing buildings presents a challenging but vast opportunity to reduce the built environment's operational carbon. The application of heat pumps to supplement traditional heating equipment is an economically viable strategy that can drastically reduce the amount of time combustion-driven heating needs to occur. HHW reset strategies applied to hybrid heating plants allow all-electric heat pumps to serve the building heating load when the required HHW supply temperature set point is within the operating range of a heat pump. 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. Designers should keep in mind that a partial upgrade of a heating system's generation equipment and some ancillary components is more likely to occur than a complete heating system replacement. In these situations, the maximum capacity available from existing heating hot water coils operating at reduced water temperatures will be limited by the original design building pipe sizes and the construction of the coil.


ASHRAE. (2021). Heat Transfer. In ASHRAE Handbook of Fundamentals (pp. 4.1-4.36). Atlanta, GA: ASHRAE.

IEA Paris. (2021, March 2). Retrieved from IEA: 

Peterson, K. (2019). Heat Recovery Chillers In Campus Chilled Water Distribution Systems. ASHRAE Journal, 61(11), 68-71.

Taylor, S. T., & Stein, J. (2004). Sizing VAV Boxes. ASHRAE Journal, 46(3), 30-35.