idronics #27:  Air-to-Water Heat Pumps


This and the related articles combines many of the details from previous sections into complete systems. These systems are configured around monobloc as well as split system air-to-water heat pumps. Some are “heating only” systems. Others include domestic water heating and chilled water cooling. Different approaches to details such as buffer tank piping, domestic water heating and auxiliary heat input are mixed into a variety of systems. Many additional combinations of these details and subassemblies are possible.


This system shown in Figure 8-1 is a single zone “heating only” application where a monobloc air-to-water heat pump supplies a heated floor slab. This simple arrangement would be ideal for a garage or shop environment.

The high thermal mass of the slab provides a substantial buffering effect, eliminating the need for a buffer tank. This system assumes that the heat pump can monitor the temperature of the fluid supplied to the floor heating manifold station. If the heat pump has a fixed-speed compressor, it would turn on and off to keep the supply water temperature within some differential above and below a target temperature. If the heat pump had a variable-speed compressor, it would modulate to keep the supply temperature as close to the target value as possible. The target temperature could be a fixed value, suitable to meet design heating load, or it could be based on outdoor reset control. The latter has a significant advantage because it allows the system to operate at lower fluid temperatures as the outdoor temperature increases. This would significantly improve the heat pump’s seasonal COP.

The power to operate circulators (P1) and (P2) is supplied through the heat pump. For the highest seasonal COP, both of these circulators should have electronically commutated motors (ECM).

A SEP4 hydraulic separator provides high-efficiency air, dirt and magnetic particle separation for the system. It also provides hydraulic separation between circulators (P1) and (P2), allowing for stable, but potentially different flow rates. This configuration also allows continuous operation 

of circulator (P2), while circulator (P1) turns on and off with the heat pump. Continuous operation of the load circulator helps reduce variations in slab surface temperatures.

This system would operate with a solution of propylene glycol antifreeze. It is equipped with two bidirectional filling/purging valves to quickly fill the system and flush out bulk air during commissioning.

The spring check valve near the upper left connection on the SEP4 prevents reverse thermosiphoning through the heat pump when it is off.

The entire system can be operated from a simple wall thermostat.

Figure 8-1

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.