Keep it if you want a hybrid system; remove it if you are ready for a full-electric design that actually heats the house at design temperature. A cold-climate heat pump is meant to run well below freezing, with a published COP and capacity at 5°F. That does not automatically make the furnace worthless. At many Midwestern and Northeastern EIA averages, the furnace still has the lower unit cost of delivered heat at an 8.1 HSPF2 — and a kept furnace is also a backup that does not add a 10 kW strip kit to the panel. The dual-fuel page and the dual-fuel-versus-full-electric page are the operating-cost and replacement versions of this question; this page is the “I already have a decent furnace” version.
Reasons to keep the furnace: a commissioned changeover can follow your rates and the unit’s COP curve; you already paid for the flue and the gas service; the panel may not have room for a large strip kit; and a long outage is easier to ride through with a gas furnace on a small generator (blower and controls only) than with a heat pump plus strips. Reasons to let it go: you want off the gas bill, including the fixed customer charge; the furnace is near the end of its life; the cold-climate unit is sized to design load; or the indoor space is needed for a new air handler layout.
Do not keep a failing furnace as a paper backup you will never service. If you do keep it, have the changeover set on purpose — not left at a factory mid-30s default without a rate check. Use the calculator and, if strips are the alternative, the panel calculator.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.