Two Thermostats For Forced Hot Air Furnace ? And, Quest On Split A/C System

Hello:
Tring to understand the wiring in a 30 yr old, tri-level (split), house recently moved into. Kinda rough, without any wiring diagrams, schematics, etc. Old owner gone.
Would like to ask the following two questions, please:
a. House uses a forced hot air gas furnace for heating. Not at all familiar with forced hot air, as I've always had ahot water heating system, with individually controlled zones.
Was surprised to see a thermostat in the downstairs level, and one in the upstairs level that apparently control the hot air furnace. Would think that they would "fight" each other
I thought with hot air furnaces, you would "always" just have a single thermostat, and balance out the heat flow by adjusting the heat outlets in the individual rooms, etc.
(there are No electrically controlled gates/flappers in the ducts to make it into a two zone system)
How common is it to have two thermostats controlling a hot air furnace, I guess in parallel ? Good idea ?
b. A split air conditioning system was added about 20 years ago. Condenser outside, and an evaporator section in the attic, or at least this is what I think is there. Attic really tough to explore due to low height.
Haven't tried the A/C out yet.
But, did investigate a noise, and discovered it was coming from a blower in the attic, most likely part of the A/C system up there.
Found a 110 V switch in the attic that turns it on/off. Thought that this would be for the serviceman to turn power off, not for the homeowner's everyday use, of course.
But I cannot find, anywhere, a thermostat that controls this blower. Just the attic 110 V switch.
Was wondering if anyone might have any ideas on this.
What might this blower be used for ?
Would they use the blower is a reverse direction, likely, to work as an exhast fan ? If so, where could it possibly be exhasting to, assuming it's primary purpose is to blow cool air from the evaporator into the ceiling of the rooms below it.
Why no thermostat or other control for in the house proper ?
Any thoughts ?
Thanks, Bob
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I had two thermostats on my hot-air furnace. ( in the days before fancy thermostats )
One thermostat was set to the day time temp One thermostat was set to night time temp.
A toggle-switch connected either thermostat to the furnace. ( day or night temps. ) It was easier than turning the thermostat down every night then turning it back up in the morning....

<rj>
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Sounds like a big-time diy'er had the house <g>. I have the furnace scenario in my own home. The thermostats don't "fight" each other, though. The reasoning is, so the temp -never- is too cold upstairs or downstairs. If up/down gets too hot/cold we just adjust registers for more/less air flow. It didn't take more than a week or so to get a good balance between the two so the temps were reasonable both up and down. It gets worse though; we also have two furnaces; one for the main house and one where the double thermostat is. To get both furnaces running together took adding a Dutch door to the doorway adjoining the two areas; after that, no problem since it effectively separates the areas each furnace has to heat. Reason for two furnaces is the very long heat runs it would take to try to use one and get it to balance in any way.
The AC I dunno though. Maybe it's under powered btu-wise & so they just ran it all or nothing? We eventually got sick of the high cooling bills & chucked our central air, opting instead for a few window units so we could get cooling only when/where we wanted it. One big one in the LR and small 5k ones in the bedrooms. Drastically cut the cooling bills that way.
Lots luck!
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Robert, it's possible that one T-Stat is for heating and one for cooling. For some reason that is what I have in my split-level. On the cooling T-stat you have to turn it down to turn on the A/C as opposed to turning up the heating T-stat for heat. Now it's possible that both the T-stats are wired in parrallel for heat. This is OK. The t-stats will not "fight" each other. It's just a matter of which room will get colder first to turn on the heat. However you might have a problem that some of your rooms will have a temperature differences. Anotherwords your upsatirs rooms might be a lot warmer than your downsatirs, which would be wasting heat. As you mentioned, the right way to get uniformed heat is to have one T-stat and adjust the registers. But even doing that can sometimes not have perfect results. I have that problem in my lower level den as I learned this past winter. I was thinking of adding supplemental heat like an electric baseboard or a vented wall furnace.
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