Domestic Hot Water (DHW) Production with Air to Water Heat Pumps
Air-to-water heat pumps provide an efficient and reliable solution for domestic hot water production, particularly in applications where reduced energy consumption and consistent performance are key priorities.
Introduction
Domestic hot water production has traditionally relied on electric resistance water heaters or gas-fired boilers, both of which are widely used because of their simplicity, availability, and proven performance. However, as building codes and energy standards become increasingly focused on reducing energy consumption and greenhouse gas emissions, conventional DHW systems are facing new challenges.
Electric boilers typically have high operating costs due to direct resistance heating, while gas boilers depend on fossil fuels and may make it harder for projects to meet energy-efficiency and decarbonization targets. In this context, air-to-water heat pumps offer a modern alternative for domestic hot water production by transferring heat from outdoor air to water with significantly higher efficiency than traditional electric heating. Their ability to reduce energy use while maintaining reliable hot water supply makes them an increasingly attractive solution for code-compliant, high-performance buildings.
Safety Requirements
When domestic hot water is produced indirectly through a heat exchanger, CSA B214, Section 16, requires the air-to-water heat pump to incorporate an internal double-wall heat exchanger with a visible means of leak detection between the potable water and the heat-transfer fluid. This requirement is particularly important for air-to-water heat pump systems because the refrigerant pressure on the hot side of the system is typically much higher than the domestic cold-water pressure.
Refrigerant pressures can commonly be in the range of 400–600 psi, while domestic water pressure is typically in the range of 30–70 psi. In the event of a heat exchanger failure, the pressure difference could allow leakage from the high-pressure refrigerant or heat-transfer side into the lower-pressure potable water side.
The intent of the code requirement is therefore to provide a physical safeguard that prevents contamination of domestic water and protects occupant health and safety.
Most air-to-water heat pumps currently available in the Canadian and U.S. markets are equipped with single-wall heat exchangers. While these units may be suitable for space-heating applications or for DHW systems using appropriate external separation, they may not directly satisfy the CSA B214 requirements for indirect domestic hot water production where potable water must be protected from the heat-transfer fluid.
Our air-to-water heat pumps are available with both single-wall and double-wall heat exchanger configurations, allowing the system design to be selected according to the application and code requirements. It is important to note that the piping schematic differs between single-wall and double-wall configurations, since each arrangement requires a different method of separating, transferring, and protecting the potable water circuit.
Proper selection of the heat exchanger configuration and corresponding piping layout is therefore essential for code compliance, system efficiency, and safe DHW production.
DHW Production with Single-Wall Heat Exchanger Air to Water Heat Pump
The below arrangement uses air-to-water heat pumps for both space heating and DHW preheating, with a water/glycol heat-transfer fluid loop, a domestic hot water preheating tank T-01, and a conventional DHW heater downstream delivering 140°F domestic hot water. Because potable water does not pass directly through the heat pump, the indirect coil of the DHW preheat tank provides separation between the glycol heat-transfer loop and the domestic water. This can offer a practical pathway for using a single-wall heat pump in DHW preheating applications to meet applicable CSA B214 requirements.
- The heat pump preheats incoming domestic cold water before it enters the conventional DHW heater. This reduces the temperature lift required from the electric or gas water heater, which can lower energy consumption and operating cost.
- Another limitation of this configuration is the achievable DHW preheat temperature. Since the domestic water is heated indirectly through the internal coil of the preheat tank, a temperature difference is required between the heat-transfer fluid and the potable water for heat transfer to occur. This results in a temperature approach loss, meaning the domestic water leaving the preheat tank will typically be lower than the heat-transfer-fluid supply temperature. Therefore, the air-to-water heat pump should be considered a DHW preheating source rather than the final DHW heating source in this arrangement. The downstream conventional DHW heater is still required to raise the water temperature to 140°F or to the final temperature required by the project.
- Because the preheated water tank feeds a conventional DHW heater, the system still has a familiar final heating stage capable of delivering 140°F domestic hot water. This is useful when the heat pump cannot fully satisfy DHW temperature requirements during cold weather or high-demand periods.
- The same air-to-water heat pump system contributes to both space heating and DHW preheating. This can improve equipment utilization, especially in shoulder seasons when space-heating demand is lower.
- A single-wall coil or heat exchanger arrangement is generally simpler and less expensive than a double-wall configuration.
DHW Production with Double-Wall Heat Exchanger Air to Water Heat Pump
The below arrangement uses air-to-water heat pump with a double-wall heat exchanger is connected directly to a DHW storage/buffer tank, with domestic cold water entering the tank and domestic hot water leaving directly from the tank. The backup boiler is connected through the indirect coil, rather than the heat pump using the coil to preheat DHW
- Because the heat pump is connected directly to the DHW tank, the stored domestic water can be heated directly by the air-to-water heat pump. This avoids the temperature approach loss associated with heating DHW through an indirect coil. As a result, the heat pump has a better opportunity to raise the tank water closer to the required DHW storage temperature.
- This arrangement allows the heat pump to perform the main DHW heating function instead of only preheating incoming domestic cold water. The backup boiler becomes a secondary or supplemental source, which can reduce fossil-fuel or electric resistance energy use depending on the project design.
- Since the heat pump is not transferring heat through the tank coil, there is no additional coil temperature loss between the heat pump circuit and the stored domestic water. This can improve the achievable DHW tank temperature compared with the previous schematic where the heat pump heated DHW indirectly through a coil.
- The schematic identifies the heat pump as double wall, which is important because potable water is being heated directly by the heat pump circuit. A double-wall heat exchanger with visible leak detection is typically the safer and more code-appropriate approach where potable water and heat-transfer fluid must be separated.
- The backup boiler is connected through the tank’s indirect coil. This means the boiler loop does not mix directly with the domestic water stored in the tank. The boiler can add heat when required while remaining separated from the potable water circuit.
- A double-wall heat exchanger heat pump is typically more expensive than a single-wall version. The added safety barrier and leak-detection design may also increase product complexity and service requirements.
Conclusion
The single-wall heat pump configuration is suitable for DHW preheating, where the heat pump heats a water or water/glycol loop and transfers heat through the tank coil before a conventional DHW heater raises the water to final temperature. This approach can reduce energy use but may not achieve 140°F DHW directly. The double-wall heat pump configuration allows the heat pump to heat the DHW tank directly, reducing temperature losses and making the heat pump the primary DHW source. However, it requires verified double-wall construction, visible leak detection, proper controls, and approval by the authority having jurisdiction.