Air-to-water heat pumps offer new opportunities for hydronic pros.

Figure 1.


Many hydronic professionals are aware of geothermal water-to-water heat pumps. They extract low temperature heat from the earth using a stream of water as the “conveyor belt.” That heat is absorbed from a buried earth loop, water well or a pond, and carried to the heat pump’s evaporator. A vapor compression refrigeration cycle then raises the temperature of the absorbed heat and delivers it to a hydronic distribution system.

I’ve designed several radiant panel systems around water-to-water geothermal heat pumps. Some have been in operation for more than 25 years. More are on the drawing board.

Figure 2.

Getting out of the dirt

Although a properly designed geothermal water-to-water heat pump can provide high thermal efficiency and long life, it’s not a universal solution for all buildings. The logistics and cost of installing an earth loop or other source of ground water can be formidable and expensive. A potential installation has to consider land area, soil composition, ground water regulations, available excavating or drilling equipment, required pipe-fusion tools and flushing devices and long-term thermal stability of the soil.

Installing a geothermal loop field at an existing building usually requires major disruption of landscaping. In some locations it’s just not practical, cost-effective or even legal to consider such an installation.

Until a few years ago, geothermal water-to-water heat pumps were about the only readily available option if you wanted to combine hydronics and heat pumps. Today, more options are available. Several companies now offer “air-to-water” heat pumps in the North American market.

In the heating mode, these heat pumps extract low-temperature heat from outside air. A vapor/compression refrigeration system upgrades the temperature of this heat and transfers it to a stream of either water or water-based antifreeze solution.

In the cooling mode, heat is extracted to produce chilled water for use by an indoor distribution system. Heat is rejected to outside air forced through the outdoor unit by a fan.

Figure 3.

The most common air-to-water heat pump systems now available in North America have scroll compressors and operate with R-410A refrigerant. The Altherma heat pump from Daikin is one such unit. It has a variable-speed scroll compressor to modulate both heating and cooling capacity between 20% and 120% of its nominal rated capacity. It’s available in two configurations: monobloc and split system.

The monobloc configuration keeps all components of the heat pump within a single outdoor enclosure. Two pipes carry either water or a mixture of water and antifreeze between the outdoor unit and the interior portions of the overall system.

A split-system configuration uses refrigerant lines to connect between the outdoor unit and the indoor unit. The outdoor unit contains the compressor, air-to-refrigerant heat exchanger and outdoor air fan. The indoor unit contains the refrigerant-to-water heat exchanger, circulator, expansion tank, controls and, in some systems, an electric-resistance element for auxiliary heating.

Figure 1 shows the monobloc configured for a cold-climate application where it’s wise to use an antifreeze between the outdoor unit and a brazed-plate heat exchanger inside heated space.

Figure 2 shows a split-system configuration.

Both configurations have their pros and cons. For example, the monobloc configuration arrives with a fully charged refrigeration system. Thus there is no need to connect refrigerant lines and add refrigerant on site. However, in cold climates, the presence of water in the outside piping and condenser presents the possibility of freezing. Although the Daikin Altherma can be configured to automatically operate its circulator and auxiliary electric heating element if freezing is imminent, a sustained power outage during subfreezing weather could still lead to a hard freeze.

The compromise is to use an antifreeze solution between the outdoor unit and an indoor brazed-plate heat exchanger. This adds costs and reduces thermal performance, but provides positive protection against freezing.

Because no water is contained in the outdoor components of a split system, it is inherently freeze-proof. However, like any split system, it requires a refrigerant technician to connect the line sets and ensure the refrigeration system is operating correctly.

Figure 4.

The lower the better

Like any heat pump, the greater the temperature difference between the media from which heat is being absorbed and the media to which the heat is being transferred, the lower the heat pump’s heating capacity and coefficient of performance. A typical variation in heating capacity is shown in Figure 3. Figure 4 shows a typical variation in COP.

When designing a hydronic system for a heat pump, it’s imperative to keep the required water temperature as low as possible. In the case of an air-to-water heat pump, this minimizes the temperature lift between the outside air and the supply water temperature. The lower this temperature lift, the greater the heating capacity and the higher the COP.

Plenty of low-temperature heat emitters are available in the North American market. They range from radiant floor, wall and ceiling panels to panel radiators and even “low-temperature” fin-tube baseboard as discussed in my January 2012 column (“Baseboard makeover”). My recommendation is to design your distribution system so it can supply design heating load using a supply water temperature no higher than 120º F.

Hundreds of combinations are possible for air-to-water heat pumps and hydronic distribution systems. Some general classifications would be space-heating-only systems, space heating and domestic water heating, heating and cooling, as well as systems that integrate an auxiliary heat source such as a mod/con boiler. Other possibilities include systems with solar thermal input, and thermal storage tanks that allow the heat pump to operate under the most favorable ambient conditions or to take advantage of time-of-use electrical rates.

Figure 5.

Figure 5 shows how a split-system air-to-water heat pump can be combined with a zoned radiant panel distribution system and provide domestic water heating. The heat pump has an internal circulator that can provide sufficient flow through either the “primary loop” that serves the space-heating subsystem or the heat exchanger in the indirect water heater. A diverter valve determines if the heat from the heat pump’s indoor unit goes to the domestic-water-heating load (which typically has priority) or to the space-heating load.

The domestic water tank has an upper electrical element that provides auxiliary heating if necessary. It also serves as a backup in the event the heat pump is off for servicing.

The space-heating subsystem is hydraulically separated from the heat pump loop by a pair of closely spaced tees. Downstream of these tees is a pressure-regulated ECM-based circulator that modulates its speed in response to how many of the zone valves are open at any given time.

Figure 6 (page 22) shows how zoned cooling can be added to the system. This arrangement does not allow the heat pump to provide simultaneous heating and cooling. Domestic water heating is still the priority load. Once it is satisfied, the heat pump can operate in either heating or cooling mode, supplying heated or chilled water.

Some readers may wonder why there is no buffer tank in these schematics. It’s because of two criteria. First, if the Daikin Altherma heat pump is used, its variable-speed compressor can modulate heating and cooling output down to about 20% of maximum. Second, the zoning is designed so the minimum zone heating or cooling requirement is matched to the minimum output of the heat pump. If you’re doing a “zones gone wild” system with many small zones, a buffer tank is still a good idea to prevent short-cycling.

Just like the circulator that ships with some boilers, the circulator in the monobloc heat pump (or within the indoor unit of the split system) is only designed to move flow through a modest amount of external piping. Be sure to check its “net” flow and head ratings when designing the system. Larger systems, such as those with high head-loss components, or those with hydraulic separation of subsystems (such as in Figures 5 and 6) will require additional circulators.

Likewise, the expansion tank housed within the heat pump may have to be supplemented with an additional tank, depending on the total volume of the system.

Figure 6.

Running some numbers

Here’s a common question when people are introduced to air-to-water heat pumps: How does the performance of an air-to-water heat pump compare to that of a geothermal heat pump? The answer will vary with many factors such as load, climate, type of earth loop used, local soil conditions, design water temperature of the distribution system and available credits/rebates.

The following is a comparison I ran for a modest home in Syracuse, N.Y. The performance results were obtained using software from a manufacturer of air-to-water heat pumps, as well as a manufacturer of geothermal heat pumps.

Example house: 36,000 Btu/hr. design load at 70º inside and 0º outside temperature
Location: Syracuse, N.Y. (6,720 heating degree days)
Total estimated heating energy required: 49.7 million Btu/season
Average cost of electricity: $0.13/kwhr
Distribution system: Radiant panels with design load supply temperature = 110º

Air-to-water heat pump option:
Based on software simulation, a split-system air-to-water heat pump supplying this load has a seasonal average COP of 2.8.
Estimated installed cost = $10,600 (not including distribution system)

Geothermal water-to-water heat pump option:
Based on software simulation, a water-to-water heat pump supplying this load from a vertical earth loop has a seasonal average COP of 3.28.
Estimated installed cost = $11,800 (earth loop) + $8,750 (balance of system) = $20,550 (not including distribution system)
Deduction for 30% federal tax credit (geothermal only): $ -6165
Net installed cost: $14,385 (not including distribution system)

Annual space-heating cost:
Air-to-water heat pump (COPave= 2.8) = $676/year
Geothermal heat pump (COPave = 3.28) = $578/year
Difference in annual heating cost: $98/year
Difference in net installed cost: $3,785

Although the geothermal system has a higher seasonal average COP (3.28 vs. 2.8 for the air-to-water unit), its significantly higher installed cost, even factoring in the available 30% federal tax credit, makes for a long payback on the additional savings. Assuming that the cost of electricity inflates at 4% per year, it would take 24 years for the lower operating cost of the geothermal heat pump to cover the higher installation cost. I’m not convinced that’s an acceptable return.

Air-to-water heat pumps aren’t limited to vapor compression refrigeration systems. They also are available as gas-fired absorption heat pumps. We’ll discuss them in an upcoming column.

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