n cold climates[edit]
An air source heat pump designed specifically for very cold climates can extract useful heat from ambient air as cold as −30 °C (−22 °F). This is made possible by the use of variable-speed compressors and manufacturers include Mitsubishi and Fujitsu.[6] One Mitsubishi model provides heat at −35 °C, but the coefficient of performance (COP) drops to 0.9, indicating that resistance heating would be more efficient at that temperature. At −30 °C, the COP is 1.1, according to the manufacturer's data [7] (the manufacturer's marketing literature also claims a minimum COP of 1.4 and performance to −30 °C [8]). Although air source heat pumps are less efficient than well-installed ground source heat pumps in cold conditions, air source heat pumps have lower initial costs and may be the most economic or practical choice.[9]
A study by Natural Resources Canada found that cold climate air source heat pumps (CC-ASHPs) work in Canadian winters, based on testing in Ottawa (Ontario) in late December 2012 to early January 2013 using a ducted CC-ASHP. (The report does not explicitly state whether backup heat sources should be considered for temperatures below −30 °C. The record low for Ottawa is −36 °C.) The CC-ASHP provided 60% energy savings compared to natural gas (in energy units).[10] When considering energy efficiency in electricity generation however, more energy would be used with the CC-ASHP, relative to natural gas heating, in provinces or territories (Alberta, Nova Scotia, and the Northwest Territories) where coal-fired generation was the predominant method of electricity generation. (The energy savings in Saskatchewan were marginal. Other provinces use primarily hydroelectric and/or nuclear generation.) Despite the significant energy savings relative to gas in provinces not relying primarily on coal, the higher cost of electricity relative to natural gas (using 2012 retail prices in Ottawa, Ontario) made natural gas the less expensive energy source. (The report did not calculate the cost of operation in the province of Quebec, which has lower electricity rates, nor did it show the impact of time of use electricity rates.) The study found that in Ottawa a CC-ASHP cost 124% more to operate than the natural gas system. However, in areas where natural gas is not available to homeowners, 59% energy cost savings can be realized relative to heating with fuel oil. The report noted that about 1 million residences in Canada (8%) are still heated with fuel oil. The report shows 54% energy cost savings for CC-ASHPs relative to electric baseboard resistance heating. Based on these savings, the report showed a five-year payback for converting from either fuel oil or electric baseboard resistance heating to a CC-ASHP. (The report did not specify whether that calculation considered the possible need for an electrical service upgrade in the case of converting from fuel oil. Presumably no electrical service upgrade would be needed if converting from electric resistance heat.) The report did note greater fluctuations in room temperature with the heat pump due to its defrost cycles.[10]
Usage[edit]
Air source heat pumps are used to provide interior space heating and cooling even in colder climates, and can be used efficiently for water heating in milder climates. A major advantage of some ASHPs is that the same system may be used for heating in winter and cooling in summer. Though the cost of installation is generally high, it is less than the cost of a ground source heat pump, because a ground source heat pump requires excavation to install its ground loop. The advantage of a ground source heat pump is that it has access to the thermal storage capacity of the ground which allows it to produce more heat for less electricity in cold conditions.
ASHPs are sometimes paired with auxiliary or emergency heat systems to provide backup heat when outside temperatures are too low for the pump to work efficiently, or in the event the pump malfunctions. Since ASHPs have high capital costs, and efficiency drops as temperature decreases, it is generally[vague] not cost-effective to size a system for the coldest possible temperature scenario, even if an ASHP could meet the entire heat requirement at the coldest temperatures expected. Propane, natural gas, oil or pellet fuel furnaces can provide this supplementary heat.
All-electric backup systems have an electric furnace or electric resistance heat, or strip heat, which typically consists of rows of electric coils that heat up. A fan blows over the heated coils and circulates warm air throughout the home. This serves as an adequate heating source, but as temperatures go down, electricity costs rise. Electrical service outages pose the same threat as to central forced-air systems and pump-based boilers, but woodstoves and non-electric fireplace inserts can mitigate this risk. Some ASHPs can be coupled to solar panels as primary energy source, with a conventional electric grid as backup source.
Thermal storage solutions incorporating resistance heating can be used in conjunction with ASHPs. Storage may be more cost-effective if time of use electricity rates are available. Heat is stored in high density ceramic bricks contained within a thermally-insulated enclosure;[11] storage heaters are an example. ASHPs may also be paired with passive solar heating. Thermal mass (such as concrete or rocks) heated by passive solar heat can help stabilize indoor temperatures, absorbing heat during the day and releasing heat at night, when outdoor temperatures are colder and heat pump efficiency is lower.
The outdoor section on some units may 'frost up' when there is sufficient moisture in the air and outdoor temperature is between 0 °C and 5 °C (32 °F to 41 °F). This restricts air flow across the outdoor coil. These units employ a defrost cycle where the system switches temporarily to 'cooling' mode to move heat from the home to the outdoor coil to melt the ice. The defrost cycle reduces the efficiency of the heat pump significantly, although the newer (demand) systems are more intelligent and need to defrost less. As temperatures drop below freezing the tendency for frosting of the outdoor section decreases due to reduced humidity in the air.
It is difficult to retrofit conventional heating systems that use radiators/radiant panels, hot water baseboard heaters, or even smaller diameter ducting, with ASHP-sourced heat. The lower heat pump output temperatures would mean radiators would have to be increased in size or a low temperature underfloor heating system be installed instead. Alternatively, a high temperature heat pump can be installed and existing heat emitters can be retained
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