VisionPro/heat won't come on reliably when in circulate mode

I'm using the VisionPRO TH8110U and have an older (1998) Tempstar furnace.

I'm using the fan circulate mode. The heat doesn't always come on when in circulate mode - sometimes the fan will be running continuously, the temperature will be several degrees below the setting (so heat should be on).

I'm guessing if the fan is running because of "circulate" and the temperature drops lows enough to turn the heat on, the furnace (for some reason) doesn't turn the heat. So the fan just keeps running. Is there a setting that can be changed on the thermostat for this? The circulate mode is why I picked this thermostat. I have tried the fan in the "on" position, and heat comes on when needed. It's something odd about circulate mode. I had tried a Lux thermostat earlier that had the same problem, but I just assumed that it was a quality issue with Lux.

Greetings Mike welcome to the Forum.

I'll tell ya right up front, I'm not familiar with either of the units you're using there, but I got a fair concept of control wiring, so.....

It may be one thing, and that's a Temp Switch, which is downstream from your Furnace. It shuts the unit down if the air goes overtemprature (like over 180°), and the second part of the Stat holds the Blowers off, till the air comes up to a reasonable temp to be useful.

Usually it's a box on the outside of the Duct, which has two sliders in it, and you might find the "air rise" slider, and drop the temp down on it a few degrees.
My best shot !

Have a good Day !
S.W.

Thanks -- I looked around the outside ducts and didn't see a switch. I'll keep looking, though, thanks for the tip.

What's odd is the furnace doesn't come on - so it's not like it's taking too long for the fan to start. The fan just keeps running, but only if it's in circulate mode and only occasionally.

I noticed there is a control setting on the thermostat for "thermostat controls fan -- electric" versus "furnace controls fan -- gas/electric". I thought I might switch to the electric setting, even though it's a gas furnace, and see if that helps. I'm assuming (and I don't know much about this stuff) that the only difference is that the furnace won't preheat the air for 10 secs or so before the fan turns on if the electric setting is used. Does that seem reasonable/safe to you?

Thanks!

Mike,

I don't see why you couldn't try the alternate settings. With you right there, you could intervene if it tried to get out of hand.

There's got to be some Switch somewhere down line from the Furnace, I just can't visualize the warm-up biased on time. Too many variables to not have and actual temp available before the Fan is permitted to start.

Have a good Day !
S.W.