Furnace running at odd times

My furnace turns on even when the house is warm! I supposedly have on demand hot water so I thought that unless it was cold or something was using the hot water the furnace shouldn't run. What am I missing? Sorry for my stupidity on the subject....I've always heated with electric or a monitor before.
As always.....thanks for the help. I'd be lost without this group!!!!
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Furnace of boiler? Furnaces heat air, boilers heat water.
In a system that both heats the house and hot water, it maintains a reservoir of hot water ready for use. If that water cools down too far, the boiler burner will start and bring it back to temperature. There is a differential setting for that also.
You want to be sure you don't have water circulating from the natural tendency of heat rising in the pipes also.
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Sounds like you have a boiler not a furnace. If you had a furnace you would have a water heater of some sort besides. The way the boiler works is you set the min and the max. If your water temp reaches the min, the boiler kicks on and heats the water back up to the max. Thus always hot water on demand. Also if the "thermostat" calls for heat at any time, as soon as the temp falls below max, the boiler will kick on. I know there are experts out there. Correct me if I am wrong.

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