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Air-To-Water Heat Pumps And The Important Role They Play In Heating

January 8, 2018

As technology changes, so do we. Hi Valley Supply is proud to offer the most advanced and high-quality options available on the market. We continue to expand our product offerings, which now includes Air-to-Water Pumps. These advanced heat pumps are going to have in integral role in the future trends of heating.

 

Below is an in-depth look at Air-to-Water Pumps, written by mechanical engineer John Siegenthaler, P.E., providing you all you need to know about these pumps. Contact us to learn more or to place your order. We can also answer any questions you have.

 

GETTING STARTED

Electrically powered heat pumps, in many varieties, are destined to play an increasingly important role in heating and cooling buildings around the world. They convert vast amounts of otherwise unusable low temperature heat into heat at sufficient temperatures to warm buildings or heat domestic water.

 

Electrically powered heat pumps dovetail nicely with large-scale renewable sources of electrical energy such as utility scale photovoltaic systems, large wind turbine farms, or CHP systems operating on bio-gases produced from agricultural waste.

 

LOOKING BACK

The “classic” heat pump for residential heating and cooling debuted in North America during the 1970s. Big name HVAC companies developed markets for these early generation “air-to-air” heat pumps, which consisted of two major subassemblies: an outdoor condenser unit, and an indoor air handler. A refrigeration line set connected these subassemblies.

 

Although air-to-air heat pumps have been around for several decades, early generation products were not able to maintain reasonably efficient operation at low outdoor air temperatures. It was not uncommon to turn off those early generation heat pumps when outdoor temperatures dropped to or below 20F°. However, advances in refrigeration technology, including inverter drive variable speed compressors, and a process known as enhanced vapor injection (EVI) now allow air-source heat pumps to operate at very low outdoor temperatures, typically down to -13F° (-25C°).

 

Some of the largest global providers of heating and cooling hardware, now offer “cold climate” versions of air-to-air heat pumps. The most common configuration is called a ductless mini-split heat pump system. A single outdoor unit connects to multiple indoor wall mounted cassettes, each supplied by its own refrigeration line set. Each indoor unit can operate independently, allowing for zoning.

 

Although cold climate ductless mini-split heat pumps have carved out a nice market share over the last decade, they rely on forced air delivery of heating and cooling. As such they do not offer the potential to combine their high thermal efficiencies with the comfort provided by a well-designed hydronic distribution system. Furthermore, most ductless mini-split heat pump systems only provide space heating and cooling and lack the ability to supply ancillary loads such as domestic water heating, or pool/spa heating.

 

WATER RATHER THAN AIR

This is where a different heat pump configuration becomes attractive. An air-to-water heat pump uses the same concept as an air-to-air heat pump for extracting low temperature heat from outdoor air. The difference is that it delivers heat at very useful temperatures into a stream of water (rather than air) passing through its condenser.

 

Some air-to-water heat pumps are capable of producing leaving water temperatures of 130F°+, even with relatively cold outdoor air. This sets the table for use of hydronic heat emitters such as radiant floors, radiant walls and radiant ceiling panels, panel radiators, fan-coil convectors, and even contemporary low temperature fin-tube baseboard. It also allows the heat pump system to be configured to provide most, if not all of the energy needed for domestic water heating. With a suitable heat exchanger, heated water from an air-to-water heat pump can also be used to heat a swimming pool or maintain the water temperature in a spa.

 

WHAT IS IT?

Although most North American heating professionals are familiar with ductless mini-split heat pumps, as well as geothermal heat pumps, few of them are familiar with air-to-water heat pumps.

 

This is not the case in other markets such as Asia and Europe. According to the August 2015 issue of the Japanese publication JARN, the 2014 global market for air-to-water heat pumps totaled over 1.7 million units. The Chinese market alone represented almost one million units sold in 2014. The European market was 232,000 units, with France as the number one market, followed by Germany and the U.K. The same Asian-based firms that sell ductless mini-split heat pumps in North America produced many of these air-to-water heat pumps. Others came from European-based companies. The U.S. market was a tiny fraction of global sales.

 

Why such a difference? One likely reason is the small market for residential hydronics in the U.S. and Canada relative to Asian and European markets. Another is the very limited use of chilled water cooling in smaller North American buildings. Manufacturers also have to consider the cost of supporting a new product line in a market (such as North America) that requires a lot of “hand holding” before the installations become routine.

 

LOOKING AHEAD

This is not to suggest that a strong future market for air-to-water heat pumps cannot develop in North America. Several trends suggest just the opposite. Here are some key indicators:

 

1 | Growing interest in net zero energy (NZE) houses:  The typical NZE house has a very low loss thermal envelope and a sizable solar photovoltaic array on the roof. Net metering laws – where they exist – allow owners of photovoltaic systems to sell surplus electrical power back to the utility at full retail rate. Thus, surplus kilowatt hours produced on a sunny summer day could conceivably be “parked” on the electrical grid and reclaimed to run a heat pump on a cold winter night with no technical or economic penalty. That is a pretty sweet deal.

 

One common approach to heating and cooling NZE homes is to install two or three high wall cassettes, which is the indoor portion of a ductless mini-split heat pump system, in central areas and leave interior doors open for distribution of the heated or cooled air. Proponents of this approach stress that no interior comfort distribution system, such as ducting or piping, is needed. One online discussion of this topic states that if all interior doors are left open, interior air temperatures will stabilize at not less than 2F° below the air temperature where the indoor cassette is located. This discussion also points out that if bedroom doors are closed at night, which is a reasonable expectation, the bedroom temperature may drop 5F° below the temperature where the heat pump indoor unit is located.

 

Should these constraints be accepted without reservation?

I say not because there is a difference between just putting Btus back into a space to match the rate of heat loss and doing so in a way that provides excellent thermal comfort. There are also reasons builders put doors on interior rooms and it is not for achieving 5F° automatic temperature setbacks when they are closed. In my opinion, these constraints represent significant “negatives” for the ductless mini-split approach in a NZE house.

 

The alternative is to retain the thermal efficiency of the low ambient air-source heat pump, but switch to hydronics for the balance of the system.

 

Use a low temperature hydronic delivery system, such as a heated ceiling or floor, to allow good thermal performance from the heat pump along with proper distribution of heat throughout the building, whether the doors are open or closed.

 

Cooling can be handled using chilled water flowing through a central air handler, or multiple small air handlers, some of which look and operate almost identically to the high wall cassettes used with ductless mini split heat pumps.

 

All chilled water air handlers must be equipped with condensate drip pans and suitable drains. Cooling can also be done using a radiant panel in combination with a single small air handler. The radiant panel can handle most, if not all of the sensible cooling load. It must operate at chilled water temperatures that remain above the room’s dew point at all times. This can be handled with mixing controls. A single small air handler would also be required to provide moisture removal.

 

2 | In the U.S., the 30% federal tax credits on geothermal heat pump systems are scheduled to end as of December 31, 2016: This will remove a significant purchasing incentive, and force geothermal heat pump systems to compete against other types of heat pumps in an unsubsidized market.

 

3 | Air-to-water heat pumps are significantly less expensive to install compared to geothermal heat pumps: This is especially true if vertical boreholes are required for the earth loop. In my area, these holes cost about $3,000 per ton for drilling, pipe insertion, and grouting. Additional cost is incurred for connecting multiple vertical piping loops and for routing piping back to the location of the heat pump. Replacement of any affected paveme