Home built in 1975 with DAY & NIGHT furnace model #100U-19A, Input: 100,000 BTU/hr, Bonnet Capacity: 80,000 BTU/hr. Since original construction, 2 rooms added for total square footage at 2,650. Old furnace has rust spots. Expect holes to form soon. Must replace furnace now.

You can get all kinds of recommendations. Carrier, Trane, Lennox and others will make some people happy and others curse. Talk to your neighbors, Google the manufacturers and check the reliability reports. Then, get a number of specific quotes and go from there.