I have a Trane Model XR 90 High Efficiency, 100,000 BTU’s to heat a two-floor space with about 1900 square feet. When the outside temp. is around 20 degrees F and I set the thermostat to Hold at 63 degrees during the nighttime, when I get up the thermostat is reading 60 degrees. It cannot keep the temp. at the pre-set 63 degrees even though it runs most of the night. It takes the sun shining in my windows several hours later before it brings the house temperature to normal setting of 67. Can someone give me some ideas what's wrong? Thanks.

It sounds like the furnace is struggling to sustain heating cycles through the night. Try this: increase your night setback temp to 67 for a couple nights, to see if the indoor temp is maintained or if it decreases during the night. Observe the furnace during the morning recovery - if burner cycles are short/interrupted/frequent, a problem exists.