Important things to know when designing and applying AWHP

AWHP insights

• Although air-to-water heat pumps (AWHPs) resemble an air-cooled chiller, an AWHP differs significantly.

• Successful application requires attention to details that distinguish this technology from traditional heating solutions.

Various technologies are available to provide heating in facilities without using on-site fossil fuels, which can help meet the decarbonization agenda. Air-to-water heat pumps (AWHPs) are an approach growing in popularity and this technology does not have the limitations associated with other nonfossil fuel burning technologies such as solar, geothermal or electric heating.

Coefficient of performance (COP) for AWHPs exceed the 1.0 COP associated with electric heating. AWHPs do not have the associated cost and land area needed for geothermal bore fields and unlike solar heating, AWHPs may not need backup heating equipment.

However, owners of facilities with AWHPs commonly experience startup and operational issues due to misapplication of equipment. While AWHPs have been increasingly used in Europe over the past decade, the technology being applied in North America is a new practice leading to misinformation and limited application knowledge.

Proper application of AWHPs starts with the expected building load. Figure 1 shows a typical building load profile where the blue line represents the cooling load over the course of a year and the red line represents the heating load over the same period. Once the load profile has been determined, equipment can be selected that meets the peak heating and cooling load. Oversizing the equipment is not recommended.

The next step in the design process is when AWHP operating modes should be considered. In addition to providing heat for a building, AWHPs can also provide cooling. There are two distinctly different technologies available to achieve this. Sometimes referred to as a simultaneous or multipurpose unit, these AWHPs can provide cooling, heating or both cooling and heating simultaneously.

Alternatively, a more simplistic unit is available that can produce either heating or cooling, but not both at the same time. It is important to evaluate the building load profiles to determine how many units should be simultaneous versus a more traditional AWHP for the equipment to operate effectively and efficiently while also being cost conscious at the time of installation.

AWHPs are available as a packaged type and a modular type. From an output perspective, packaged and modular type AWHPs can be used interchangeably (i.e., a packaged type could be installed in place of a modular type and vice versa), however building constraints will have an impact on which type of AWHP is best suited for the application.

AWHP building constraints

Figure 1: In this sample building load profile, the blue line represents cooling load, and the red line represents heating load. Courtesy: Metropolitan Equipment Group

If the AWHP will be installed in an existing building, a review of the building’s structural capacity is necessary. A structural engineer should identify the load-bearing capacity of the building’s structural system. This load bearing knowledge may limit the equipment selection to a specific AWHP type or indicate that reinforcement of the existing structure is required if either AWHP types are to be considered.

Physical space availability also impacts AWHP selection. AWHPs resemble air-cooled chillers in their basic configuration, however, an AWHP has a larger footprint for a given capacity than a comparably sized air-cooled chiller.

Airflow restrictions are another similarity between AWHPs and air-cooled chillers. Each AWHP manufacturer has minimum distance requirements between the perimeter of their AWHP and surrounding building elements as well as minimum distance requirements between the perimeters of adjacent AWHPs. Installing an AWHP in a “pit” where it is surrounded on all four sides by building elements brings additional minimum distance requirements into consideration. If an AWHP is installed in a pit setting, most AWHP manufacturers will recommend installing the top of the equipment at the same elevation as the top of the surrounding building elements. During equipment evaluation and selection, clearances should be requested from the manufacturers.

Although AWHPs can be installed within a building, for the purposes of this article, we are focusing on AWHPs being provided as standalone, outside of the building envelope. It should be noted that within the UK and Europe, AWHP are typically located and installed within central utility plants..

A packaged AWHP provides a single large machine capable of a greater heating and cooling output when compared to the modular machine. Packaged machines typically have smaller footprints than modular types providing the same capacity, providing a benefit for overall space consumption and weight distribution. Packaged types also come with fewer components which impacts reliability and regular maintenance.

For example, a typical packaged AWHP might only have a maximum of four compressors where an equivalent modular type could have as many as 16 compressors.

Projects with tight space constraints such as retrofit or a project with limited rooftop or surface areas are good candidates for modular type AWHPs due to their flexibility for design. Modular AWHPs are provided in incremental sizes as low as 25 tons and up to 80 tons. The ability to take a 150-ton building load and split it up among six AWHPs provides the flexibility to space out the modules across multiple locations, versus having to be stacked together on the roof and occupying a large single footprint.

If a split approach is applied to modular machines, additional connections will be required. Each of the individual module typically has two compressors, so in this example, there would be 12 compressors to maintain for the 150-ton machine versus the single large, packaged machine requiring only four compressors. End users and designers must consider the positive and negative aspects of a packaged approach versus a modular approach when preparing to design an AWHP project.

Building load profile when AWHP is specified

Upon selection of an AWHP approach for a building’s heating and cooling demand the next critical step is identifying the building load profile (see Figure 1). If the load profile indicates a simultaneous heating and cooling load all year, it is typically beneficial to use a simultaneous heating and cooling AWHP that allows for production of chilled and heating water and doesn’t require switch over from heating mode to cooling mode.

In this approach, the simultaneous heating and cooling AWHP would be paired with supplemental reversing AWHPs that can handle the additional heating load in winter and the additional cooling load in the summer. If the building load profile indicates minimal simultaneous heating and cooling throughout the year, an AWHP that can reverse from cooling mode to heating mode depending on the time of year and the demand should be selected.

A simultaneous heating and cooling AWHP should be selected so that the machine is always running close to 100% of its maximum capacity for both heating and cooling. The optimal operating point of the simultaneous heating and cooling AWHP is a balanced load at maximum capacity.

However, a balanced load is typically not achievable year-round and therefore the simultaneous AWHP should be selected close to the simultaneous heating and cooling demand that the building has throughout the year. Avoid using simultaneous heating and cooling AWHPs for heating only or cooling only applications Instead, use a reversing AWHP that either provides heating only or cooling only.

Simultaneous AWHPs typically need to be supplemented with reversing AWHPs to efficiently meet the building load. Select the simultaneous AWHP(s) to accommodate the shoulder season heating and cooling simultaneous demand. The heating and cooling differential between shoulder season and peak heating and cooling loads will be covered by the reversing supplemental AWHPs. Simultaneous heating and cooling AWHPs are capable of handling unbalanced heating and cooling loads unlike heat recovery chillers (HRCs).

For example, if a building’s load demand is only 80% of heating maximum and 40% of cooling maximum, a simultaneous AWHP will be able to provide the building with the high demand for heating and still be able to provide the decreased cooling demand. HRCs only provide their maximum heating capacity while the machine is providing the maximum cooling capacity. As the cooling demand drops so does the heating capacity of an HRC.

Regulatory, operating and design limitations of AWHPs

Jurisdictions throughout the United States are adopting regulatory requirements that may dictate the application of AWHPs. For example, since 2022, the Washington State Energy Code requires a percentage of water be heated using heat pumps. An early step in the design of AWHPs is researching the codes in the jurisdiction of the project.

Once the relevant equipment has been selected for a project, the next step is to examine the operating limitations and design capabilities of the AWHPs along with the intended project design conditions. Consultation with AWHP manufacturers typically starts at this point in the design process.

One of the most critical items when designing an AWHP system is the ambient air temperature due to its importance when the AWHPs are in heating mode. The ambient air temperature will dictate the leaving heating water temperature the AWHP will generate. As the ambient air temperature decreased, there is an operating point where the leaving heating water temperature starts to decrease (see Figure 2).

If the ambient air temperature drops below the AWHPs minimum operating point, the AWHP will no longer operate reliably requiring backup heating.

Figure 2: This shows sample ambient temperature versus leaving heating water temperature. The green line represents a standard scroll compressor’s operating envelope and the red line represents a vapor injection scroll compressor’s operating envelope. Courtesy: Metropolitan Equipment Group

Heating water system temperatures associated with AWHPs differ from typical design practice. In the past many projects used boilers capable of generating heating water from 160° to 180°F. Heating water temperatures of 160° to 180°F will not be possible with the AWHPs currently on the market. A leaving heating water temperature for AWHPs is recommended from 90° to 120°F as good practice.

Another important consideration is the difference between the supply and return water temperatures known as the temperature differential (delta T). Boilers can use a 20° to 30°F delta T. AWHPs either have difficulty or are not capable of handling a 20° to 30°F delta T. A good practice is to design for AWHPs is to have a primary pumping loop delta T of between 10° to 15°F paired with a secondary building side pumping loop that operates at a higher delta T.

The points presented above are for AWHPs using standard scroll compressors. However, there are AWHPs available that use vapor injection scroll compressors. Vapor injection compressor machines can provide additional lift to either provide a warmer heating water leaving temperature up to 160°F or provide the standard AWHP leaving water temperature at much lower ambient temperatures down to as low as -0°F ambient. It’s important to consult the AWHP manufacturer on the ambient operating limits. Scroll compressor machines and vapor injection compressor machines are not interchangeable as the system design required is different for each technology.

AWHP selection

After the project criteria and design conditions have been identified the AWHP selection begins. There are a few critical items that must be understood and evaluated before finalizing the equipment selection. AWHPs require a minimum and a maximum system water volume to operate properly, particularly the primary pumping loop system volume.

A good pumping arrangement for AWHPs is to incorporate a constant volume primary loop paired with a variable secondary loop. There are exceptions to a primary/secondary pumping layout where AWHP manufacturers are recommending variable pumping primary loops. This should be reviewed and understood before selection of the machines and consultation with the AWHP manufacturer it is recommended.

System water volume is the most important requirement that an AWHP system needs to operate properly. Failure to meet the active system water volume requirements will result in the AWHP machine being unable to provide the proper design leaving water temperatures and could result in premature failure of the machine’s components. Depending on the manufacturer and the type of AWHP used, the gallons per ton (gal/ton) will vary.

However, a basic rule of thumb is to provide a minimum of 6 gal/ton for reversing AWHPs and 10 gal/ton for simultaneous heating and cooling AWHPs. The recommended gallons per ton, which represents the total system volume, should not be confused with the system flow, which is represented in gallons per minute, or gpm.

The system volume given in gallons per ton that is required for the AWHP machine is for “active” system volume only. Active system volume is the water that is always readily available to the AWHP system regardless of the load condition on the building. Consult with the AWHP manufacturer to ensure that the active system water volume is sufficient for the application.

AWHPs located outside in colder climate zones can potentially require a glycol solution to offer freeze protection. As with other system types, adding glycol to the system causes less heat transfer to occur in the system and, therefore, there is a sacrifice in overall system performance referred to as “glycol performance penalty.” Providing the AWHP manufacturer with the required glycol percentage is critical to being able to account for the glycol performance penalty.

When using glycol in an AWHP system it is good practice to make the primary pumping loop the glycol side and use a heat exchanger to separate the primary side from the secondary side, keeping the glycol from circulating through the entire building. Using the heat exchanger prevents derating any indoor equipment’s capacity. AWHP manufacturers may also offer an option for heat tracing and an electric heater for freeze protection. This is an option that should only be considered if the project is in a milder climate and or the project cannot use glycol.

Defrost in AWHPs

When an AWHP is in heating mode, the coils can begin to ice up. To remove this ice buildup, it is necessary to reverse the machine into defrost mode. When using AWHPs for heating, it is very important to look at the defrost cycle along with the defrost penalty for capacity that all AWHPs have between 25° and 40°F ambient air temperature.

In defrost mode, hot refrigerant will pass through the coils to heat them and melt the ice. Reversing the AWHP introduces neutral to cool water into the heating loop, which diminishes the leaving heating water temperature. The active system volume becomes critical because having the proper amount of system volume will lessen the impact of this cooling effect on the overall system.

Without enough active system volume, having an AWHP go into defrost mode could result in a runaway heating water loop. A runaway heating water loop occurs when the leaving water temperature is decreased and as a result the returning water temperature is much lower than design. This cycle continues until the machine leaves defrost mode at which point the return water temperature is too low and the AWHP cannot provide enough lift or heat to get back to the required leaving water temperature.

It should also be understood that in North America, it is not required for AWHP manufacturers to provide their machine’s heating performance with a defrost penalty. There will be derated heating performance of an AWHP while it is heating in defrost mode. The AWHP manufacturer should provide the heating performance with the defrost penalty included otherwise there is a risk of having a machine short on heating capacity and undersized by up to 33%.

European manufacturers are required to include the defrost penalty in their reported performance. It is best practice to request defrost penalty information from AWHP manufacturers during the design process.

Ongoing AWHP operations

AWHP systems should include the ability to monitor return water temperatures to verify that the system is functioning properly. The return water temperature sensor is a remote device that is not manufacturer provided. When finalizing AWHP selections, it is a good practice to explore the additional accessories that can be provided by the manufacturer.

• Request that the AWHP manufacturer provide its machines with integral constant volume primary pumps. Most manufacturers can provide integral pumps including a redundant pump for N+1 redundancy.

• Explore the option for an integral buffer tank. Otherwise, the designer will either need to provide an external buffer tank or enlarge system piping to provide the minimum system volume.

• Designers should consider the AWHP manufacturers’ integral control panel when using multiple AWHPs. These multimachine control panels can operate all the AWHPs evenly and at their most efficient operating point when compared to controlling the AWHPs with a building automation system.

As an alternative to the manufacturer-provided accessories listed above, some of the components can be provided by the installing contractor and field installed in the hydronic distribution system. For example, a four-port buffer tank can be used to mechanically separate the primary and secondary pipe systems. With this approach, the four-port buffer tank can be provided with the ability to install the return water temperature sensor in a factory provided opening rather than in a field installed opening located in the return piping.

When retrofitting AWHPs in existing systems, the original operating parameters need to be considered. If the system was originally designed with coils using 180°F supply heating water and 50°F return heating water, the coils in the entire heating water system may require replacement. Alternatively, measures to raise the supply heating water temperature may be implemented.

In conclusion, AWHPs are an easy solution to provide heating, which helps decarbonize a facility if the above measures are observed and incorporated into the design of system. However, application of AWHPs requires a rethink of previous system parameters. When using AWHPs, they system can heat with lower heating water temperatures and cool with higher chilled water temperatures than the industry has used in the past.

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