Furnace/A/c fan running at same time

[rrmccorm]

Turned the furnace on for the first time today. The furnace works like it should (hot air coming from furnace vents) but the fan on the roof a/c also on, blowing air from the ceiling ducts (ducted air). It is a dometic unit and thermostat. Anybody know what this might be. I,m thinking the wall thermostat might be bad. The a/c works as should.
Thanks

[fonzie]

Set your thermostat to "auto" and all will be fine. Here is manual

[Mr. Dan]

Originally Posted by rrmccorm

Turned the furnace on for the first time today. The furnace works like it should (hot air coming from furnace vents) but the fan on the roof a/c also on, blowing air from the ceiling ducts (ducted air). It is a dometic unit and thermostat. Anybody know what this might be. I,m thinking the wall thermostat might be bad. The a/c works as should.
Thanks

Common problem -- gets reported 4-8 times per month here.

Cycle thermostat MODE through HEAT, COOL, FAN and stop there. Use + or - button to cycle Fan speed from HI to LO to AU (for auto).

[rrmccorm]

Thanks. Done deal!

[wcrafter]

Happens more than a few times a year...

[clk623]

I'm having same issue after replacing thermostat with a Honeywell. Will a Honeywell not work with A/C and furnace fans ? Honeywell doesn't have same settings as the Dometic.

[rockfordroo]

Originally Posted by clk623

I'm having same issue after replacing thermostat with a Honeywell. Will a Honeywell not work with A/C and furnace fans ? Honeywell doesn't have same settings as the Dometic.

Dometics work differently than other units; they are negatively switched, not positively switched, if I understand them correctly. (In other words, the "switch" that turns on the AC is "after" the AC, not before it, in current flow, or put it another way: Positive switching is +12VDC from battery to switch, to AC, to ground, versus Negative switching is +12VDC from battery to AC, to switch, to ground.) Causes problems with standard t-stats.

If you had an analog Dometic t-stat, your best bet is to replace it with a plug-and-play digital Dometic.

Maybe someone who's figured out how to do it with a standard t-stat will chime in.