We lived in a house with this under-floor-air system until we moved last year. We inherited it from my mother-in-law a few years previously. She didn't really think of maintenance, and so turns out it was using the original gas heater system.

This was quickly condemned when we got it inspected - it had a crack in the heat exchanger and - these things were apparently only designed to last 25 years. Lasting 50 was pretty impressive/dangerous.

You can get replacement heaters for them, but it's pretty niche, needs to be imported from Europe, and because of the age normally involves dealing with asbestos insulation. It's also apparently virtually impossible to get the venting cleaned, as it's embedded in the flooring (under concrete for the group floor). (Edit: And this caused real problems for Asthma sufferers)

While it worked though it was pretty good, it heated rooms extremely quickly.