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!

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)