This is the last of a three-part series focused on net zero energy (NZE) performance in multifamily and office buildings. Part 1 addressed the building science and thermodynamics considerations for NZE performance (https://bit.ly/4eVwFBy), while Part 2 described ventilation best practices for implementing radiant floor cooling (https://bit.ly/4bA5SI1).

This part will focus on dramatically improving heat pump efficiencies with optimized pumping configurations and source process heat exchangers and conclude with a review of the overall NZE architecture encompassing the technologies discussed in this series. NZE systems architecture uses off-the-shelf appliances in unique configurations to double the energy efficiency of any one appliance. 

The proven architecture discussed here not only delivers high performance but enables the timely transfer of heat from any source to any need. Optimal sources of heating or cooling fluids are used to provide superior comfort and indoor air quality with a minimal carbon footprint. 

Radiant Infrastructure

A radiant heating and cooling architecture needs circulating hot or chilled water supplied optimally from heat pumps or the direct use of these fluids without the heat pumps. PEX tubing is embedded in high-mass, thermally conductive structures such as concrete slabs or gypcrete, referred to as a thermally active building structure (TABS). 

Since radiant floor cooling (RFC) cannot meet all the cooling needs for a conditioned space, supplemental cooling and ventilation are delivered via dedicated outdoor air system (DOAS) unit(s) with enhanced air distribution or distributed water-air heat pumps.

The first step for NZE performance, as described in Part 1 of this series, requires a high-performance building envelope oriented to take advantage of or reduce solar gains based on climate zone. The next step is to install the PEX tubing in the slabs, reducing the “lift” required of the heat pumps; e.g., delivering the coolest hot water to meet heating demand and the warmest cool water to meet cooling demand. 

The optimum configuration considering capital and operating costs is to use 5/8-inch oxygen-barrier PEX-A at 6 inches on-center with maximum circuit loop lengths at 300 feet. The tight tubing spacing is required to optimize radiant floor cooling due to the low-temperature differential of the supply water (58 F) versus slab temperature (65 F). 

While this density is not required for heating, the tighter tubing spacing provides the added benefit of reducing the heating supply water temperature required from a typical 110 F to 90 F. This lower lift increases heat pump efficiency by 12% to 15% based on 50 F entering the source water temperature. It also provides more precise floor temperatures within the controlled space. 

The supply and return piping to each zone is sized to optimally deliver the flow rate required to meet the maximum heating or cooling load for that zone. After considering the energy used by compressors to heat or cool the supply water, the next highest energy is consumed by the hydronic circulators, which circulate the water in the TABS.

Optimized System Pumping

Pumping configurations must be considered for circulation within the TABS and for the heat pumps that heat and cool the water for the structure. This creates opportunities for optimized pumping in three applications that will be discussed: 

1. Circulation of source-side heat transfer fluids in ground heat exchangers (GHEX) to the heat pumps in a central plant or distributed throughout a building; 

2. Circulation of load-side fluids from water-to-water or air-to-water heat pumps (air-source heat pump) to thermal storage;  

3. Circulation of the heat transfer fluid from thermal storage to the TABS for primary heating and cooling and to hydronic heat exchangers in DOAS or fan-coil units for supplemental heating and cooling. 

Assuming that these applications would use the same variable-speed pumps is correct; however, the piping configurations and control sequences are substantially different to meet NZE requirements. This fact is often overlooked by mechanical engineers designing circulation systems for these three applications.

As a matter of training and experience, engineers use traditional design processes based on past successes. Reducing technical risk was a driving factor, not optimizing energy efficiency. 

For example, a traditional approach to designing a pumping system required sizing distribution piping to support required flow rates to not exceed 5 feet/second and calculating a resultant pump head pressure based on that flow and pipe size. Next, the pumps were sized to deliver that flow rate at the required head pressure. 

Controls were quite simple. Turn on the pump when there is a call for heating or cooling the TABS, or heating or cooling a thermal storage (often a tank) using a heat pump. In the past decade, engineers have improved efficiency using variable frequency drives to modulate the flow of a fixed-speed pump based on heating or cooling loads. 

This configuration providing adequate flow does not deliver optimum performance to meet NZE requirements without advanced controls and piping configurations.

Optimized Ground Heat Exchanger and Heat Pump Circulation

Certified GeoExchange Designers certified by the Association of Energy Engineers are taught to ensure that the GHEX has turbulent flow for adequate heat transfer. As a rule of thumb, flow rates through GHEX piping are calculated to maintain a Reynolds Number over 2,500 at peak load conditions. 

What is not considered in these calculations is that ground-source heat pumps operate at partial loads 87% of the time. Therefore, most commercial geothermal systems installed today waste energy through excess pumping. 

Another design flaw related to NZE performance is that the flow rate through the GHEX is the same as the flow rate through the building, considering the same pump(s) are used for GHEX flow and building circulation through a common manifold. The worst case is a fixed-speed pump that turns on with a call for heating or cooling. 

This is improved by modulating flow rates when multiple water-source heat pumps in a building share a common GHEX, and the heat pumps are equipped with zone valves to open when the heat pump turns on, enabling source-water flow. Setting the system pump on constant pressure control varies the flow rate, which is the same for the building and the GHEX based on the number of heat pumps calling for heating and cooling.

For NZE performance, optimized flow hydraulically separates GHEX flow from building circulation using a primary/secondary piping configuration. This hydraulic separation enables the GHEX fluids to circulate at a different flow rate from the building source water flow rate to the distributed heat pumps. 

A best practice is to set the pump control to maintain a maximum differential temperature (delta T) across the GHEX. At low-load conditions, GHEX flow is minimal, matching the actual heat extraction to or from the GHEX. As loads increase, the temperature differential across the GHEX increases, and the pump speeds up to reduce the delta T. The building system pump flow rate is set on constant pressure, as previously described. 

For higher performance, the on/off source-side zone valves installed on each heat pump are replaced with modulating zone valves. With heat pumps equipped with two-stage or variable-speed compressors, the valves modulate with the compressor speed to meet minimum heat pump flow (ranging from 2 to 3 gallons/minute/ton, based on GHEX configuration). 

The highest energy efficiency is achieved when these valves are modulated based on delta T between the entering (EWT) and leaving water temperature in the heat pump. This modulates water flow to the minimum required to meet the source flow rates within the heat pump based on actual performance. 

One means of implementing this functionality at a reduced cost is to procure premanufactured pump stations for GHEX and building distribution. Geo-Flo Corp. is the largest manufacturer of flow centers in the United States and builds components that engineers can specify to minimize installation and maintenance complexity and costs. 

Figure 1 discloses a 40-ton pump station achieving this objective with fluid connection and hydraulic separation between the GHEX and building heat pumps equipped with modulating zone valves. Two Grundfos Magna3 pumps operate in parallel (the bottom two pumps) to circulate GHEX fluids to the hydraulic separator (at the left in the photo.) Three Grundfos Magna3 distribution pumps (on upper manifolds) circulate this source water to the 30,000-square-foot building complex. For optimal mixing in the hydraulic separator, the GHEX source fluids to the load circulators enter at top left and return to the GHEX via source circulators at lower right.

Note the two building pumps in a series configuration on the second row to meet the higher head requirements to serve heat pumps in a remote mechanical room on campus. This pumping station was installed on-site in two weeks with two technicians covering only 50 feet of floor space in the main mechanical room.

The same principles used for GHEX pumping apply to load-side flow through water-to-water or air-to-water heat pumps. Rather than pumping load-side fluids to thermal storage at a constant rate, the flow rate is modulated to maintain an optimum flow rate through load-side HEX based on delta T. 

Load-side performance is optimized with predictive controls that model energy use in real time and then determine flow rates based on historical flow rates versus environmental conditions (building loads, outdoor air temperature, conditioned space humidity), and aligned with heat-pump-specific performance under partial and full loads.

For TABS pumping applications, similar predictive controls can modulate flow rates through the thermally conductive structure based on the same environmental factors. The engineer has the option to design the system using a constant temperature with variable flow rates, variable temperature with constant flow rates, or a combination of the two. 

The effect of supply temperature on heat pump efficiency must also be considered. As discussed in Part 2 of this series, heat pump efficiency increases with lower lift between supply and ambient temperatures. 

For campus applications, where supply water is maintained at a constant temperature, the temperature of the thermally conductive structure can be maintained by modulating the mixed radiant supply fluid by mixing the supply water with the return water, pulsing fixed-speed circulators or directly modulating flow rates with variable-speed pumps based on delta T. 

To reduce the complexity of installation and controls, Energy Environmental Corp. designed a modular radiant distribution panel that serves up to 10,000 square feet. As shown in Figure 2, this panel is also manufactured by Geo-Flo Corp. and reduces installation space, time and costs versus site-built components. 

Hot or chilled water is supplied at the lower right from thermal storage tanks heated and cooled by heat pumps. The panel is equipped with a hydronic circulation pump set on differential pressure to regulate flow in the TABS. An injection pump and three-way mixing valve work in tandem to regulate supply temperature based on delta T between the supply and return circuits and to prevent condensation by maintaining mixed radiant supply fluid above the conditioned space and thermally conductive structure dew points.

Heat Pump and TABS Configurations Required for NZE Performance

To achieve building performance with a reasonable on-site electrical requirement to achieve NZE status, heat pump and TABS performance must be increased significantly beyond Air-Conditioning, Heating, and Refrigeration Institute (AHRI)-certified efficiencies. This is achieved by configuring heat pumps for dual-process heating and cooling, passive use of ground fluids for TABS applications, and integrating source process heat exchangers in the systems architecture.

The highest efficiency cooling and heating performance is attained by using source process heat exchanger fluids already at the desired supply temperature from ground and source process heat exchangers. One application is using GHEX water directly at or below the required 58 F supply water temperature for RFC without using a heat pump to chill the water. 

A water utility in Denver employs this strategy by using a potable water main as a heat exchanger to supply chilled water. The Coefficient of Performance (COP) of this application is 50 (for example, delivering 120 tons of cooling with an 8,000 watt circulator). Similarly supplying heat from a solar thermal array or rejected heat from a data center for use in radiant heating deliver comparable energy efficiency.

One means of doubling a heat pump’s COP is found with reversing chillers. A typical geothermal heat pump rejects heat to or extracts heat from a GHEX, so it delivers three units of heat using one unit of power. A reversing chiller operates without a GHEX and heats a hot water supply by extracting heat from a cold water supply. 

In this simplified example, the reversing chiller produces four units of useful hot water and four units of useful chilled water using one unit of power at a COP of 8.0 — double the manufacturer’s COP. While this application works well for balanced heating and cooling loads, supplemental heating or cooling is required when the loads are not balanced. 

Recently, WaterFurnace announced technology with modular water-to-water heat pumps configured as reversing chillers and integrated with an air-source heat pump to enable this high efficiency with unbalanced loads. For large custom homes with wine cellars, providing hot water with an air-source heat pump water heater (cooling the air in the mechanical room) is symbiotic with split-unit wine coolers (which heat the air in the mechanical room), thus improving the efficiency of both appliances. 

This process has been used in community centers for simultaneously heating a pool while cooling an ice rink.

When the source process heat exchanger fluid temperature is not optimal for direct use, it can be the source water for ground-source heat pumps to improve their efficiency. Using GHEX source water at 50 F for heating water applications, a heat pump delivering 110 F load water has a COP of 3.7. 

The same heat pump efficiency increases 127% when supplied with a warmer source of water at 90 F, such as that from a snow-melted patio during the swing seasons, and the heat pump delivers 110 F water with a COP of 4.7. For solar thermal applications and heating-dominated climates, segmenting the GHEX into two or more independent GHEX allows one heat exchanger to operate as a source/sink for cooling in the summer while the other serves as a heat sink for the solar thermal array. 

This reduces the required storage tank requirement for the solar thermal array while supercharging the ground loop for heating in the fall. Offsetting the heating and cooling loads in this manner also ensures that GHEX is not degraded over time due to unbalanced loads.

A cost-savings measure for providing cool water to distributed water-to-air heat pumps in a building is using the same chilled water at 58 F for RFC with TABS to provide source water for heat pumps. This configuration eliminates the need for and cost of dedicated GHEX source piping in the building for distributed heat pumps.

Another means for providing supplemental heating or cooling to a conditioned space is to use the return water from the TABS directly in ceiling- or wall-mounted fan coils. In this application, the fan coils can be operated without condensate drains when in cooling mode. These ceiling-mounted fan coils are referred to as chilled beams, whereby passive chilled beams use convection alone and active chilled beams are equipped with circulation fans. Heat pump systems without a cooling tower conserve water and reduce operating costs by eliminating the costs of water and maintenance of the cooling tower.

The final part of the NZE puzzle is to add thermal storage to the system to improve overall energy efficiency. A 200,000-square-foot NZE office building in Denver uses two 30,000-gallon thermal energy storage (TES) tanks with the capability to store hot or chilled water. Since the dominant need year-round is cooling, the tanks routinely operate as chilled water storage. 

During nighttime operations when the reversing chillers are heating the hot water storage and there is no cooling load, the reversing chillers extract heat from the TES tanks to heat the hot water storage in a dual process configuration, thus doubling the COP. This configuration is often referred to as heat recovery chillers. The chilled water in the TES tanks is used for cooling as required to meet peak cooling demand when cooling exceeds the capacity of the installed appliances. 

The same capability can be used to provide passive cooling midday to reduce peak demand charges, which are a higher cost than the electrical usage for a typical office or multifamily structure.

Net Zero Energy Architecture

With limited editorial space, the implementation details for these NZE concepts may not be obvious. However, those details are disclosed in the patent specifications for seven patents issued by the U.S. Patent and Trademark Office to the author. 

Figure 3 is a conceptual view of this architecture implementing the NZE concepts discussed in this series. This patented net zero energy heating and cooling systems architecture achieves the stated goal of efficiently and timely moving any heat source to any heating need to provide superior comfort and indoor air quality with a minimal carbon footprint. 

The core system is a high-mass TABS receiving fluids from hot and chilled thermal storage tanks or campus supplies. A heat pump can heat the hot or chilled storage simultaneously in a dual-process configuration or heat and cool separately using a split GHEX or source process heat exchangers that provide EWT at optimal temperatures for heat pump efficiency. 

A bypass around the heat pump delivers fluids from the source process heat exchangers for passive direct use by the radiant system. A direct digital control network using a client/server architecture such as BACnet that incorporates the sensors and processors is required to achieve the predictive controls required for ZNE performance. 

For more information on this architecture and a detailed explanation of the thermodynamic theory behind it, search Google Patents for “hydronic building systems control.” These patents are US9,410,752, US10,072,863, US10,330,336, US10,907,848, US11,287,152, US11,644,214 and US11,796,210, with 20 allowed claims in application 18/242362.

Albert Wallace serves as president of Energy Environmental Corp. He is a Certified Geoexchange Designer and Certified Energy Manager. He is a member of ASHRAE, an American Institute of Architects associate member, and a certified trainer and installer by the International Ground Source Heat Pump Association.