Central Heat & AC thermostat... weirdness

[Tony]

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.

[Tony Hwang]

Hi, This is a normal behavior of 'stat. Yhink your old 'stat was bad.

[RBM]

The fan should be controlled by a thermal device in the plenum so as not to blow cold air. It appears to me, that the old stat sent power out on the green terminal whenever it called for heat. This would cause the blower to run immediately, but when the stat was satisfied, it killed power to the heating elements and the "g" fan circuit, allowing the thermal device to allow it to run in cool down mode, then go off. Don't you wish you changed the stat 30 years ago

[Stormin Mormon]

Oh, I can think of a difference. It's childs play, after all, and it's so, so obvious.

[Lp1331 1p1331]

RBM pretty much hit the nail on the head. Old stat was designed to bring the blower relay on as soon as a call for heat was made, with the blower on a/c speed. Meanwhile, one of the switches on the heat sequncers would close to bring on the blower on heating speed, but it was out of the circuit while the relay was energized. When the stat was satisfied, the relay and power to the sequencers cut off, so the blower then dropped to heating speed until the sequencers cut off.The sequencers bring the strips and fan on and off with time delay (in sequence--hence the term sequencer) Larry

[Tony]

I guess it was just a coincidence that it worked OK with the new t-stat. Today it went back to it's old ways, blowing cold air on startup with no delay to warm up before the fan kicked on. It's really difficult to read the schematic, it's all wrinkled. I have to look it up online and hope to find one. I'm now guessing it's either a thermal switch, or something called a "sequencer". The circuits on the old t-stat is the same as the new one.