https://www.siliconvalley.com/2024/12/19/larry-magid-tech-to-solve-home-heating-problem/

In a column published in July, I wrote about the Mitsubishi mini-split heating/air conditioning system that was just installed in my house. In addition to the required outdoor heat pump, there is a 9,000 BTU unit upstairs, which did a great job keeping our 208-square-foot bedroom cool this summer and has been doing an equally good job heating the bedroom this winter.  The 18,000 BTU unit in our much larger downstairs area was adequate during the summer but not quite strong enough to heat the entire area on colder winter days.

I wasn’t surprised by the problem downstairs.  I live in one of the thousands of 1950s-era Eichler houses in the Bay Area that was built with radiant heating under the floor. It worked great, but it takes hours for the radiant heat to kick in. It also used natural gas, which is not as environmentally friendly as electricity, so we disconnected the gas line and went all electric.