My very expensive American Standard Gold S9X2 high-efficiency gas furnace works great in 30 degree weather and up, but won’t keep my house warm when it gets in the 20s and single digits outdoors. In fact, when it recently dropped to zero, we woke up to 58degrees in here. I own a 95 yo house with 1,400 square feet (1,700 Square feet of heated space including the lower level rec room.). It took 5 hours to get up to 73 degrees inside. It’s a 60K BTU furnace. I have 40 windows in my home with new storm windows. Everything I’ve seen online says to multiply heated square footage by 50 in zone 4/5 (northern suburbs of Chicago) which means they should have installed a 70 or 80K BTU furnace. The HVAC company that sold and installed my furnace refuses to do anything except they did change the thermostat which didn’t help. I fear what will happen when we experience a normal winter (it’s been unusually mild this winter here in the upper Midwest.). Any suggestions would be much appreciated! Thank you!