What is the downside of oversizing a heatpump that will be used primarily for heating?

Question

I live in the extreme northern part of New York State.

I am looking to install a heat pump to reduce my demand for fuel oil in the fringe of the season.

By oversizing I mean erring to the next half ton up, when sized for the heating demand.

I intend to incorporate a domestic water heating heat exchanger.

I am aware that the bigger, the unit the more it costs.

I know that an oversized unit will not dehumidify as well in the cooling mode.

I know that generously sized unit will not be as efficient overall.

Will a slightly larger unit be able to provide more heat output in lower temperatures, marginally extending its useful season?

Answer 1

No.

Engineering is the purpose of designing for the lowest cost but still most effective solution.  Lowest cost being all things put together.

If you oversize your pump, not only is the intial cost higher, but so is the efficiency drop, more cost per output.  

If you are not worried about the extra cost as it saves in terms of your current system but is more than a more efficient system, then oversizing will provide a greater output but at a higher cost.