Frosting

[r.bartlett]

If the inside temp is 10 deg C the indoor coil frosts on cooling yet if on heating the outside temp is 10C then the outdoor coil doesn't tend to?

What's the difference -air volume perhaps?

[nike123]

Difference is TD of heat exchangers. TD, among others, depend on air/water flow.

[frank]

The frost is water vapour condensating on the cold coil, so it is not just about temperatures. You have to look at the humidity factor as well. DB/WB.