We had an ancient gas furnace with a big belly, that could never get the house above 68 even if we ran it full blast (and if you set the
thermostat to 68 it WOULD run full blast, non-stop) . It finally *cracked*. Giant flames, CO alarm, gas company tagged it as unsafe. No option but to put in a new furnace immediately. You don’t go through a Boston winter without a heat system. Happily, it died during an October cold spell so I had a little time to shop and scrounge together the money (*)
I put in a very-high-efficiency Carrier and the house gets as warm as we like for about half the money, it’s whisper quiet, and it has a variable speed fan that runs constantly and moves the heat around so we don’t have warm and cold spots, plus an air cleaner and a humidifier. With good humidity and air circulation the place feels warmer at a lower thermostat setting. The price difference between furnaces was not great.
(*) embarrassed now to say it, but scrounging the money at the time meant using one of those checks that come in the credit card ads, because I did not have $6k in an emergency fund. So I should add the interest cost to the total cost of replacing the furnace, but I did get it paid off reasonably quickly
That $6k also included a lot of plumbing and a new chimney liner , since our 1952 pipes weren’t up to current code, and legally in this state it had to be brought up to code when any work was done.