Neither is the one you are proposing since "big enough system to heat a house at cold temperatures" is just a question about what size of system to install.

The 1:1 threshold tells you about the temp that everyone needs to start thinking about alternative heat sources. If you system is undersized and/or your house is poorly insulated, that threshold may be higher, but those numbers are per installation and don't answer the broad question about if heat pumps can "work" in cold climates.

This kind of nitpicky debate gets so tiresome. Look, upthread comment implied pretty clearly that a particular heat pump as installed would "work" until zero net energy flow at "-20C". And I thought that was misleading, because that's not the threshold you use to size a heat pump as is doesn't reflect a steady state (at that temperature your home will be inexorably losing heat). And as I happen to have had personal experience in the recent past with measuring exactly this threshold on a newly installed heat pump, I thought people might be interested.

I'm deeply sorry if this offended you for some reason, but I still don't see how you're disagreeing with anything I wrote.

> Look, upthread comment implied pretty clearly that a particular heat pump as installed would "work" until zero net energy flow at "-20C"

I did not read it that way at all. To me they are clearly talking about the point at which your heat pump no longer provides a efficiency gain over a purely resistive heater.

I think where you went wrong is reading "gain" as "heat gain" rather than "efficiency gain". While the usage of "gain" is sort of ambiguous there, the rest of the comment thread makes it clear that the term 1:1 is not discussing the point at which your heat pump stops producing any heat but the point at which it becomes inefficient.

> that's not the threshold you use to size a heat pump as is doesn't reflect a steady state

I don't think anyone except you was talking about sizing the system, but about selecting the system type given the climate.

If you had come into this just saying "here's my relevant experience with sizing heat pumps", that would have been productive. However you strongly asserted that the OP was using the wrong metric, which they weren't.

> I'm deeply sorry if this offended you for some reason,

I'm not sure why you think I am offended.

I thought the comment was useful. I think for those that don't know the basis for the tech, they can read described temps as absolute values. I've seen people misunderstand thermoelectric heater/coolers (Peltier coolers) for similar reasons -- the tech can achieve a temperature differential rather than an absolute temperature as some people expect when comparing to traditional compressor-based fridges.

Relative and Absolute temps can generally mess people up too.

From the book Humble Pi by Matt Parker:

"In September 2016 the BBC news reported that both the US and China had signed up to the Paris Agreement on climate change, summarizing the agreement like this: "countries agreed to cut emissions enough to keep the global average rise in temperatures below 2°C (36°F)." The mistake here is not just that the BBC is still giving temperatures in Fahrenheit but that a change of 2°C is not the same as a change of 36°F, even though a temperature of 2°C is the same as 36°F. If you were outside on a day when the temperature was 2°C and you looked at a Fahrenheit thermometer, it would indeed read 36°F. But if the temperature then increased by 2°C, it would go up only by 3.6°F. The crazy thing is, the BBC initially got it correct. Thanks to the amazing website newssniffer.co.uk, which automatically tracks all changes in online news articles, we can see the chaos in the BBC newsroom as a series of numerical edits. To be fair, the article was part of the live coverage of breaking news and was designed to be regularly updated. The first version of the article that mentioned temperature gave the change as 2°C."

(https://books.google.ca/books?id=2IeVDwAAQBAJ&lpg=PT223&ots=...)

A modern heat pump designed for colder regions is reasonably efficient even at -25C, as in being able to output more than half of its maximum heat and still be far enough away from its 1:1 point to make it efficient (compared to resistive heating). So, still very useful as long as that particular output is sufficient to keep your home warm or at least just require a bit of additional heating, and that it's not that cold all the time. Obviously before investing in any type of heat pump one has to consider the nominal temperature range during winter in the area where you live. Even though I'm very happy with the performance of my comparably old heat pump there are definitely regions in my country where it would be less useful.