You also need to remember the ignitor and the blower fan will NOT be
operating at the same time under anything resembling normal
application. Also, startup current of the eductor motor and ignition
will not happen simultaneously.So, what really matters is normal
running current of the eductor fan motor and startup current of the
blower.
If the blower is a brushless DC motor it will also be soft-start and
will not exhibit a large starting current spike.
May be true on traditional-designed furnaces but on new stuff, the
ignitor can run with the blower. Mine does it all the frequently.
It's a Goodman propane furnace, with a heat pump and a variable
speed blower. When it decides to de-ice the heat pump, the blower
will continue to run while firing up the propane. On these things,
the best way to know the current accurately is to monitor it over
time.