Wow, you'd recoup the cost of a new high efficiency furnace in no time vs. that ancient thing. Its probably barely 60% efficient if its a normal draft-furnace. You're sending 40%+ of your bill out the exhaust every month.
I have a 72,000 btu rated 92% efficiency furnace and it heats approx. 2,800sq feet even in the -5 to -10F temps we've been having no problem....I'd say when its 5F out it runs about 25 mins of every hour over 2-3 cycles. Modern furnaces are MUCH better than ones that old. Don't fix it, dump it!
125,000btu would be WAY overkill. Or at least it should be....how big is the house? Usually a furnace that size would be in a house well over 3,000sq ft!
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