Tuesday Wall Stree Journal on page A3 has article
“Heat Pumps Get Trial Runs in Bitter Cold”.

sample quote

Standard Heat Pumps operate efficiently at Outdoor temperatures down to 35 degrees F,
but newer cold weather systems can operate down to 7 degrees
- and engineers are pushing heat pumps to go further.”

end quote

Long article then goes on to explain that these more expensive new heat pump systems have frequency changing inverter driven variable speed AND double stage compressors.

These newest air exchange heat pump systems cost in the $7500 range
but the US Federal government currently has a $2000 tax rebate for the most efficient systems.

Ground coupled air pumps cost about $11,000 more.

Rheem is mentioned as a manufacturer aggressively competing to be the “winner” of annual DOE testing,
but the picture in the article shows a Mitsubishi.