Why cold-climate heat pumps are becoming more common
It used to be, not so long ago, that heating by available reasonable cost heat pump alone was impossible throughout much of the US. What changed? Basically discovery and adoption of Enhanced Vapor Injection EVI. At first EVI was implemented in scroll compressors by injection of refrigerant into the scroll where it could, in effect perform as a 2 stage compressor. Mitsubshi pioneered a system where a low cost dual rotary compressor performed as 2 cascaded compressors with vapor injection between stages. The Mitsubshi approach essentially buried all of the EVI inside the sealed compressor. allowing a totally conventional refrigerant loop outside. Fujitsu soon folowed with their similar, even better performing products. The China clone makers couldn’t get the magical compressors. Enter Toshiba who developed a family of compressors with “hidden” EVI aimed at the clone makers of China. If not already, soon every brand will have a “hyper heat” family of models based on the different size compressors offered by Toshiba. All of which extend performance and allow rated heat output at 0c and 1/3 of minimum output at 47f. with a tiny cost penalty over conventional compressors. Some of the clones have better “tuned” hardware around the compressors and offer even wider performance range. Hidden EVI works by a single tube a “capillary” tube fed with some of the hot gas from the output of the compressor, wrapping around the vapor input to the compressor where the gas condenses and heat from the flow through the capillary is transferred to the gas going into the compressor. The same tube continues back around the compressor output where the liquid flowing through it is evaporated,cooling the hot gas coming out of the compressor and the flow through the tube becomes a vapor which is ” injected” between the two rotors. It makes for a very simple system requiring addition of only some tubing.