air source heat pumps, though they're improving won't do well in extreme cold; even if they can operate they'll still be running at much lower efficiency.
For temperatures significantly into negative territory a ground source heat pump would perform far better, where it can draw on a source of heat that will always be at least above freezing.
A hybrid system doesn't seem like a bad trade-off though..
I thought so too, but it seems reasonable considering we're 70% water, much of it flowing and actively taking stuff out and around. Plus cells are apparently way more dynamic than is typically conveyed in high school bio (I think there was a quantamagazine article a while ago). So it's perhaps surprising that a body's atoms aren't fully recycled even faster than that.
The Ca in bones is probably renewed very slowly. C H O N very fast, in particular water. Na, K and Cl are very soluble and also get renew very fast. Some rare elements like I are probably reused a lot and renewed very slowly. I'm not sure about P, my hand waving is confused. Assuming a burger per day, we ate our weight in meat every few years, but I'm not sure if all the Fe is absorved in and exchanged for the internal one.
Ironically the core application is written in rails, which may be part of the issue