1) You got a FIRE truck coming.
2) Wire is too small.
Problem is NOT circuit breaker.
3) Both 240Volt appliances on same breaker is a hazard, especially if space available in 200 amp service panel for more breakers.
4) Usually each 240V appliance has separate breaker.
5) Circuit breaker trips because of High heat on the wire.
If wire gets hot, it will start a fire inside the wall.
Electric code requires wire-size match breaker-size.
6) If you put larger amp breaker on same wire that is getting hot, then wire will get hotter.
Result: FIRE truck middle of night with folks jumping out window in glass-cut bare run across yard.
7) Each appliance has volts and watt rating on label.
EVERY appliance has label.
8) For example stove might be rated 8000 watts and 240Volt.
8000 watts divided by 240 volts = 33.3 amps
Stove would require 40 amp breaker and 8 gauge wire.
9) How to figure it out:http://waterheatertimer.org/Color-codewire.htmlhttp://waterheatertimer.org/How-to-install-a-subpanel.html
10) Do same calculation for new furnace.
12000 watt furnace divided by 240 volts = 50 amps.
Use 60 amp breaker and 6 gauge wire.
11) Summary: fixya wants you to be safe, and buy stuff showing in the ads.
Use a bigger breaker ONLY when bigger wire is also installed.
Call furnace installer, the installer should know to check safe wiring before installing.
If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7