JoeSpareBedroom wrote:

A sun room with south-facing glass should not need a heater.

In an upstate NY winter, that's a generalization that's generally not true.

We might add "on an average day." NREL says 560 Btu/ft^2 of sun (290 diffuse)
falls on a south wall on an 29.1 F average day in Rochester with a 35.8 daily
max, so a low-mass room with R2 double-pane south glazing with 80% solar
transmission and good insulation and no holes in the ceiling might have
0.8x560 = 6h(T-32)1ft^2/R2, which makes T = 181 F. Warm enough? :-)

It should be a heat SOURCE for the attached house.

Nick