There is a wide variety of electric heat pumps that are destined to play an increasingly important role in heating and cooling buildings around the world. They convert a large amount of unusable low-temperature heat into heat of sufficient temperature to heat buildings or heat domestic water. Electric heat pumps work well with large-scale renewable energy sources such as utility photovoltaic systems, large wind turbines, or combined heat and power plants powered by biogas from agricultural waste.
Unlike the past The “classic” residential heating and cooling heat pump first appeared in North America in the 1970s. Well-known HVAC companies developed a market for these first air-to-air heat pumps, which consisted of two main components: an outdoor condensing unit and an indoor air handling unit. Cooling line sets connect these components. Although air-to-air heat pumps have been around for decades, early products were unable to operate efficiently enough at low outside temperatures. It was not uncommon for those early heat pumps to turn off when the outside temperature dropped to 20 degrees Fahrenheit or lower. However, advances in refrigeration technology, including variable frequency drive compressors and a process called Enhanced Vapor Injection (EVI), now allow air source heat pumps to operate at very low outside temperatures, typically as low as -13F (-25C) . Some of the world’s largest suppliers of heating and cooling equipment now offer air-to-air heat pumps for “cold climates”. The most common configuration is called a ductless heat pump mini split. One outdoor unit is connected to multiple indoor wall units, each powered by its own set of refrigerant circuits. Each indoor unit can operate independently, allowing for zoning. While ductless mini-split heat pumps have gained a fair amount of market share over the past decade in cold climates, they rely on forced air for heating and cooling. Thus, they cannot combine high thermal efficiency with the comfort provided by a well designed hydraulic distribution system. In addition, most ductless mini-split heat pump systems only provide space heating and cooling and cannot provide auxiliary loads such as hot water or pool/spa heating.
Water instead of air This is where the different configurations of heat pumps become attractive. Air-to-water heat pumps use the same concept as air-to-air heat pumps to extract low temperature heat from outdoor air. The difference is that it transfers heat to the flow of water (rather than air) passing through its condenser at a very usable temperature. Some air-to-water heat pumps are capable of delivering 130F+ leaving water temperatures even when the outside air is relatively cold. This has provided the basis for the use of circulating heat radiators such as radiant floors, radiant walls and ceilings, panel radiators, fan coil units, and even modern cryogenic finned tube baseplates. It also allows the heat pump system to be configured to provide most, if not all, of the energy needed to heat domestic hot water. With the right heat exchanger, the hot water produced by an air-to-water heat pump can also be used to heat a pool or maintain a spa’s water temperature.
Which? While most heating professionals in North America are familiar with ductless mini-split heat pumps and ground source heat pumps, few are familiar with air-to-water heat pumps. This does not apply to other markets such as Asia and Europe. According to the August 2015 issue of Japan’s JARN, the global air-to-water heat pump market was over 1.7 million units in 2014. In the Chinese market alone, almost 1 million units were sold in 2014. The European market is 232,000 units, with France being the largest market followed by Germany and the UK. The United States manufactures many of these air-to-water heat pumps. The rest are from European companies. The US market accounts for only a small portion of global sales. Why such difference? One possible reason is that the market for residential circulation systems in the US and Canada is small compared to the markets in Asia and Europe. Another reason is the very limited use of water cooling in small buildings in North America. Manufacturers must also consider the cost of supporting a new product line in a market (eg, North America) that requires significant “hands-on” involvement before installation becomes routine.
Looking ahead This does not mean that North America cannot create a strong market for air-to-water heat pumps. Some trends suggest otherwise. Here are some key indicators: 1 | Growing Interest in Zero Energy (NZE) Homes: A typical NZE home has very low heat loss in the cladding and a significant solar PV array on the roof. Net metering—if it exists—allows PV owners to sell surplus power to utilities at full retail price. Thus, it is possible that excess kilowatt-hours generated on a sunny summer day can be “parked” in the grid and restored to run a heat pump on a cold winter night without technical or economic loss. This is a very nice deal. The common heating and cooling method of NZE homes is to install two or three high wall boxes in the central part, the inside of a ductless mini-split heat pump, and open the interior doors to distribute heated or cooled air. methods emphasize that indoor comfort distribution systems such as air ducts or ducts are not required. An online discussion on the subject states that if all interior doors are open, the indoor air temperature will stabilize at at least 2F below the air temperature at the location of the indoor box. This discussion also indicates that if the bedroom door is closed at night, which is a reasonable expectation, the temperature in the bedroom can be up to 5F cooler than where the heat pump indoor unit is located. Should these restrictions be accepted unconditionally? I say no because there is a difference between placing the Btus back into the room to match the rate of heat loss and doing it in a way that provides superior thermal comfort. There’s also a reason why builders install doors in interior spaces rather than to achieve an automatic 5F drop in temperature when closed. These limitations, in my opinion, represent a significant “negative effect” of the ductless mini-split approach in NZE homes. Alternatively, you can keep the thermal efficiency of a heat pump with low ambient air levels, but switch to a recirculation system to balance the system. Use low temperature circulating supply systems such as heated ceilings or floors to ensure good thermal performance of the heat pump and proper heat distribution throughout the building, whether the doors are open or closed. Cooling can be provided by chilled water flowing through a central air conditioner or several small air conditioners, some of which look and operate almost identical to the high-walled boxes used by ductless mini-split heat pumps. All chilled water air handling units must be equipped with a condensate pan and a suitable drain hole. It is also possible to use radiant panels in combination with one small air handling unit for cooling. Radiant panels can handle most, if not all, of the appreciable cooling load. It must operate at a chilled water temperature that is always above the room dew point. This can be solved with the blending control. A small air cleaner is also needed to remove moisture. 2 | In the US, the 30% federal tax credit program for geothermal heat pump systems ends December 31, 2016: this will remove an important purchase incentive and force geothermal heat pump systems to compete with other types of heat pumps in the unsubsidized market 3 | Air-to-water heat pumps are much cheaper to install than geothermal heat pumps: especially if the ground contour requires vertical drilling. Drilling, installing pipes, and filling these holes in my area cost about $3,000 a ton. There are additional costs associated with connecting several vertical pipe loops and laying the pipe back to the heat pump installation site. Replacement of any affected pavements or landscaping should also be factored into the cost of installing a ground source heat pump system. 4 | Diminishing returns are a factor: the difference in annual heating costs between heat pumps operating at seasonally averaged coefficient of performance (COP) can vary by 1.0 or less due to a better heat package and a lower heat load at home. Gradually declining operating costs of high efficiency heat pumps may not be able to absorb higher installation costs over the expected life of the system. Example: A residential building with a design heat loss of 25,000 Btu/h (at 0°F outdoor temperature and 70°F indoor temperature) and located in a climate of 7000°F•day has an estimated annual heating demand of approximately 39 million Btu/h year. . If this load were powered by a geothermal heat pump with an average seasonal efficiency of 3.3 (including the power required to operate the ground loop), the annual heating cost would be approximately $416. If the same 39 million Btu/year is provided by an air-to-water heat pump at low ambient temperatures with an average seasonal COP of 2.7, the annual heating cost would be about $508. The $92/year difference doesn’t offset the $7,000 to $9,000 (after taxes) installation costs that can easily add up over the life of the equipment. 5 | As the domestic heating load becomes smaller, the hot water load represents a growing percentage of total annual heat demand: by some estimates, domestic hot water load is 25-30% of total annual energy demand in a well-insulated modern home. Ductless mini-split heat pumps cannot provide domestic water heating, but properly configured air-to-water heat pumps can. Standard electric water heaters provide heat at an efficiency of 1.0, whereas heat pumps cannot provide hot water. If this energy is obtained by an air-to-water heat pump, it can be provided at an average COP of 2.5 per year. For a family of four that needs to heat 60 gallons of water per day from 50F to 120F, with an electricity price of $0.12 per kWh, the difference in annual household hot water costs between these scenarios is $270. This is about three times the annual savings associated with the ground source heat pump compared to the air-to-water heat pump in the previous example.
SUB-ZERO has developed an advanced vapor compression refrigeration system to further increase the heat output and coefficient of performance (COP) of air-to-water heat pumps operating at low outdoor temperatures. This method, called Enhanced Vapor Injection (EVI), uses another heat exchanger located between the condenser outlet and the thermostatic valve inlet, as shown in Figure 1.
This additional heat exchanger is called the “subcooler”. It lowers the temperature of the liquid refrigerant leaving the condenser, resulting in a lower temperature as the refrigerant passes through the main expansion valve into the evaporator. This is achieved by directing some of the liquid refrigerant through an electronically controlled thermal expansion valve, which causes the refrigerant to evaporate and extract more heat from the remaining liquid refrigerant flowing on the other side of the subcooler. The boiling refrigerant leaving the top of the subcooler is returned to special ports on the compressor and injected through the middle port between the spiral coils. The net effect of EVI is to increase the compression ratio of the compressor and cool the refrigerant entering the evaporator. Both effects reduce the heat capacity and efficiency drop at low ambient temperatures. Figures 2 and 3 show the heat output and efficiency of an EVI air-to-water heat pump currently available in North America.
Note that both heat capacity and efficiency are highly dependent on the outdoor temperature. This is always the case with air source heat pumps. However, the EVI-equipped air-to-water heat pump shown in these graphs maintains an efficiency of 2.55 at 120°F leaving water temperature and operating at 0°F outdoor temperature. With an outdoor temperature of 25°F and the same leaving water temperature of 120°F, the COP is about 2.8. In comparison, an air-to-water heat pump with a VFD compressor operating under the same conditions has an efficiency of about 2.3. This is a relative performance increase of approximately 22%, which means a 22% increase in heat output for the same power input. In addition, according to the manufacturer, a VFD heat pump cannot maintain a leaving water temperature of 120°F when the outdoor temperature is below about 22°F. At an outdoor temperature of 5°F, the last heat pump can only maintain a leaving water temperature of 104°F.
Less is more The graphs in Figures 2 and 3 show the benefits of combining an air-to-water heat pump with a low temperature heat distribution system. This is especially true for the CS. For example, at an outdoor temperature of 20°F, a distribution system that can supply a building’s heat load using 110°F water will allow the heat pump to achieve a COP of about 3.1, while a system requiring 130°F water will only allow about 2.5 COP. heat pump at the lowest possible water temperature. The best approach is to use an external reset control as the “logic” to maintain the temperature inside the buffer tank. Based on the current outdoor temperature, the outdoor temperature reset controller continuously calculates a “target” water temperature that matches the heat demand. It controls the heat pump to keep the expansion tank within a narrow temperature range centered around this target temperature. For example, if a buffer tank is supplying a system with radiant underfloor heating with a set supply temperature of 110°F at outdoor conditions at 0°F, the heat pump will attempt to maintain the midpoint of the buffer tank’s height between the low point of 107°F and 107°F. up to 113F. However, if the outside temperature is 35°F, the target supply water temperature will drop to approximately 90°F. Under these conditions, the outdoor reset controller will run the heat pump to keep the expansion tank temperature between 87 and 93F. These conditions and the overall operating range of external reset control are shown in Figure 4.
Putting it all together, in Fig. 5 shows a system designed for space heating and most of the hot water load using an air-to-water heat pump with a low ambient air level. The heat pump is connected to a relatively short line filled with antifreeze. In heating mode, the heated antifreeze solution passes through a brazed plate heat exchanger sized such that the water leaving the heat exchanger is no more than 5°F colder than the antifreeze solution leaving the heat pump. Since the heat pump circuit is an insulated closed circuit, it must be equipped with a safety valve, expansion tank, air separator, fill/discharge valve and a properly sized circulation pump. The heated water enters a short collector at the top left of the expansion tank. If one or more heat sources are active, some of this water is directed to the domestic distribution system. The rest goes to the buffer tank. The outdoor air discharge controller monitors the temperature of the sensor at the middle level of the tank and controls the heat pump independently of any room heat demand. The latter is required because the surge tank is also used to preheat domestic water using the components shown to the right of the tank. The buffer water temperature is usually lower than the flow temperature required for domestic hot water. Thus, the thermostatically controlled instantaneous water heater receives preheated water from the heat exchanger and provides the necessary temperature boost.
There are also many ways to use air-to-water heat pumps at low ambient temperatures with additional circulation systems. For example, the system shown in Fig. 5 may be expanded to include one or more chilled water cooling zones. It can also be adjusted for pool heating when space heating or cooling is not needed.
John Siegenthaler, graduate engineer, graduated in mechanical engineering from Rensselaer Polytechnic Institute and is a licensed professional engineer. www.hydronicpros.com
Hydron Energy and Modern Niagara Announce Partnership This partnership and funding will accelerate Hydron’s INTRUPTor commercialization strategy and support the production of Hydron’s own components in Canada. https://www.hpacmag.com/heating-plumbing-air-conditioning-general/hydron-energy-and-modern-niagara-announce-partnership/1004136347/
Next Supply opens a new warehouse in the Kitchener-Waterloo area. The new warehouse is expected to open in February 2023. https://www.hpacmag.com/heating-plumbing-air-conditioning-general/next-supply-to-open-new-warehouse-in-kitchener-waterloo-area/1004136351/
Today is the third and final (free) replay session of our Advanced Hydraulics Summit. John Siegenthaler will deliver the second part of his talk on zero-emission housing. To register, visit https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York. To register, visit https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York. Чтобы зарегистрироваться, посетите https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York. To register, visit https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York.要注册，请访问https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York。要注册，请访问https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York。 Чтобы зарегистрироваться, посетите https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York. To register, visit https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York.
The hockey arena uses VRF. A redevelopment project in Bridgetown, Nova Scotia provides an opportunity to save money. https://www.hpacmag.com/features/hockey-arena-embraces-vrf/
Join us on Tuesday, December 13th for the third and final replay session of the Modern Hydraulics Summit. John Siegenthaler will deliver the second part of his talk on zero-emission housing. To register, visit https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York. To register, visit https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York. Чтобы зарегистрироваться, посетите https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York. To register, visit https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York.要注册，请访问https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York。要注册，请访问https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York。 Чтобы зарегистрироваться, посетите https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York. To register, visit https://us02web.zoom.us/webinar/register/tZIlde-hqT0uGNyJRqFPkWDCB79I-oa-JOeN/success?user_id=QQT7xGmkTKyKdi0QEh45Rg&timezone_id=America%2FNew_York.