idronics #27:  Air-to-Water Heat Pumps


The system shown in Figure 8-2, is another “heating only” application, but with  extensive zoning of the heating distribution system.
The split system air-to-water heat pump operates to keep the water temperature in the buffer tank at a temperature based on outdoor reset control. This water supplies a homerun distribution system in which a single manifold station supplies six independently controlled panel radiators, each equipped with a thermostatic valve operator. Each panel radiator is sized to provide design heat output to its associated space when supplied with water at 120ºF. As the outdoor temperature increases, the target temperature in the buffer tank decreases. This significantly increases the heat pump’s seasonal COP.

The buffer tank is piped in a 3-pipe configuration, which allows heat to flow directly from the heat pump to the heat emitters, as required, when the heat pump is operating. The balance of the flow leaving the heat pump passes through the buffer tank. All flow returning from the panel radiators passes into the lower portion of the tank. This helps ensure that the thermal mass of the tank is well “engaged” in the energy flow processes.

The heating distribution system is very simple. A single variable-speed pressure-regulated circulator operates continuously during the heating season. It’s speed automatically increases and decreases to maintain a constant differential pressure as the thermostatic valves on the panel radiators open, close or modulate.

The thermostatic valve at each radiator allows it to operate as a separate zone, maintaining the desired comfort level in each space. The split system heat pump allows the hydronic system to operate without need of antifreeze. A magnetic dirt separator protects the heat pump’s condenser. A combined air/dirt/magnetic particle separator protects the permanent magnet motor in the ECM circulator for iron oxide. It also provides high-efficiency air  separation for the system.

A spring check valve prevents reverse thermosiphoning between the buffer tank and heat pump. This reduces extraneous heat loss through the piping when the heat pump is off. The heat pump can be isolated from the balance of the system for service if necessary. This system also has very simple control requirements. A single switch can be used to “enable” the system at the start of the heating season. The heat pump and its associated circulator turn on and off as necessary to maintain the target water temperature in the buffer tank. The distribution circulator (P2) operates continuously but is always tracking the differential pressure present based on the status of the thermostatic radiator valves. With current ECM circulator technology and proper component sizing, this distribution system could precisely deliver over 1,500 Btu/hr per watt of electrical energy supplied to circulator (P2). This is much higher distribution efficiency than what could be attained with forced-air delivery.


21st century refrigeration techniques now make it possible for air-source heat pumps to operate in cold winter climates, with significantly improved performance compared to earlier-generation heat pumps. By combining low-ambient air-source heat pump technology with the versatility and high distribution efficiency of modern hydronics, designers can create systems for unsurpassed heating and cooling comfort, as well as domestic hot water production. Those systems are ideally suited for use with renewably sourced electricity, carbon reduction goals and “net zero” building projects. When properly selected and applied, low-ambient air-to-water heat pumps can approach the performance of geothermal heat pump systems at significantly lower installation cost and complexity. Air-to-water heat pump systems offer hydronic heating professionals a means of responding to evolving market trends and constraints without compromising quality, comfort or efficiency.