OK, this POS doublewide has straight electric hot air heat, A/C, but no
heat pump. (Coleman Evcon) Years ago when I first moved here I closed
off the ducts to almost 1/3 of the house (two bedrooms and a bath) that
are only used when I have guests. At the same time I set the fan motor
down one notch for each mode so there wouldn't be too much pressure and
I'd loose heat & A/C out of duct leaks.

When the thermostat called for heat, the fan would start immediately,
blowing cold air before it had a chance to get warm. It would run at
what I'll call medium (not high because I stepped it down a notch).
When the thermostat opened, the fan would slow down to low, and it would
run a little while until the heater cooled down a bit then it would stop.

The A/C would also run at medium, not on high. I liked it that way
because it wasn't enough to freeze the evaporator, but it did dehumidify
the air much better than on high.

So my thermostat got screwed up (worse than it was already) from me
trying to adjust it so 70 degrees is really 70 degrees. It was worse,
setting it at 75 degrees yielded a temp of about 70 degrees.

Today I installed a new 4 wire thermostat to replace the old 4 wire
thermostat (child's play). I first made sure the proper color wires
went to the proper terminals on the old one. Yes, that was correct. I
removed it and installed the new thermostat with the wires to the proper
terminals. (Yes I adjusted the heat anticipator to .3) Now when it
calls for heat, the fan doesn't run until the heating strips are getting
hot, so that blast of cold air isn't nearly as cold or as long as
before! Great! Wonderful!!! But how did this happen?

My only guess is that a temp sensor in the heater wasn't working and it
just so happened to start working now, using the heat for the first time
this season. I can't think of anything in the thermostat that could
have "fixed" this problem.