I just moved into a small apartment that has oil tank / water baseboard heaters. At first the furnace would not come on at all, after hearing the "click" in the thermostat. Maintenence was sent 3 times before finally replacing the "relay switch", (that's what i was told anyway). Now that it's warmer weather and I don't need heat, I turned the thermostat all the way down to 50 degrees and I still hear the furnace run periodically but the registers don't actually get hot. My questions are, what's wrong with the furnace, if anything ? How much electric does the furnace use by coming on when the thermostat is off ?
This is one possibility, from my distant past... In a house with hot-water heating that I lived in as a teen, the boiler and hot-water heating were controlled in a different way:
Whenever the water in the boiler dropped below a certain temperature, the burner came on and heated the water in the boiler jacket. This would go on as long as the system was turned on, winter, summer, whatever. None of this water circulated to the radiators, until...
When the house thermostat called for heat, the circulating pump came on and pumped the hot water from the boiler around to the radiators. Of course, if the pump returned cold water from the radiators to the boiler jacket, the burner would eventually sense this and come on to warm the circulating water.
If your domestic hot water tank does not have a separate burner, and isn't electric, then that's why the furnace is turning on: to heat up your hot water tank.