alasta:
Thanks Scott for the detailed analysis. It is really helpful.
I'm not keen on taking the 'cheap' route for the reasons noted by the plumber commenting above, so I agree with your final conclusion that it's not really a viable project until my existing appliances are approaching end of life. My home was built new five years ago so the appliances still have a decent amount of life left in them, but in another five to ten years time I will have a go at building a proper business case for the switchover.
Your point about heat pumps having cooling capability is notable. My place gets extremely hot in the middle of summer, so I would probably need to get some form of cooling if I ever get pets.
On cooling, my heat pump's purchase last year was primary (almost entirely), for summer cooling. The upstairs of our house has enough heat drift upstairs from the old lounge heatpump that not much more winter heating is needed.
I know that in Auckland, A well designed house should not need air conditioning, but my house is far from that, and ends up way to hot upstairs.
Ment I could get rid of my old hose out the window portable aircon, and I can work at home more, saving petrol. But I suspect still a net increase in energy use, but massive comfort gain.
Sadly is seems this issue has not been solved in modern homes, as regulations require heating, but not measures to prevent overheating.
www.nzherald.co.nz/nz/homeowners-hit-with-soaring-bills-as-they-struggle-to-cool-new-auckland-townhouses/
---------------------
Quite suppressed such a modern house in NZ has a combined gas hot water and space heating via radiators. Common overseas, but I have never seen one on NZ.
---------------------
The house being only 5 years old, would put me off also. Those appliances are far from end of life, and anything you touch on the interior (like ripping out the radiators) is going to have quite an impact on the Aesthetics.