I have an old oil furnace and I am looking for some clarification as to how it is supposed to work. It heats water filled radiators through out the house.
Currently it is connected to a thermostat by three wires. When it is needed the thermostat does heat the house but lets say it is 80 degrees outside, the radiators will not get hot but the furnace will still come on periodically for a short period of time. Is this how this system is supposed to run or might there be something wrong with the thermostat / wiring.
Note: The thermostat does manage the alert the furnace when it needs to but the air temperature is a bit different than the reading on the thermostat. ex thermostat says 65 and the actual temperature is 72.
Best Answer
There are so many variations within and between countries that it is hard to answer without knowing more about your system
Where I live, a very common system is Honeywell S-Plan and variations of this
L = hot/live
CH = Central Heating to radiators (pumped hot water).
HW = Hot Water to faucets/taps for sinks/basins/baths etc.
In S-Plan, electric power only reaches the furnace (AKA boiler) after passing through a series of devices:
Some furnaces/boilers have an independent power supply and control the pump so that they can overrun to circulate water to remove residual heat from the furnace/boiler.
None of the domestic systems I've had in my homes have ever needed to start the furnace/boiler unless the thermostat is calling for heat. I changed my room thermostat to a model with an indicator light that comes on when it is calling for heat.
The system described above is a high-voltage system, I believe many US systems use a completely different low-voltage control arrangement.