> ...but it turned out that our lot isn't good for a heat pump
> (small and lots of tall trees), so the floor system wouldn't
> pay for itself.
If you can borrow or build your own well drill, a vertical borehole loop field can be reasonably inexpensive. Also, lots of heat can be had from a small lot. Is your problem with tall trees the issue of drilling through the roots?
> ...a radiant floor system should have at least 8 meters of pipes
> per square meter of floor.
Interesting... Most of the hydronic systems in the US consider 6 inch spacing to be the minimum. Your specification works out to about 5 inch spacing.
If everything else is equal, your spec of 5 inch spacing should prove to be more efficient.
By the way,there was a free design tool that I used called "Hydronic Explorer 2" by a company called Slant Fin. (
Slant/Fin | High Efficiency Boilers, Baseboard and Radiant Heating) . You may still be able to find it. Easy to learn, very informative.
Cheers,
AC_Hacker