ShawnTRD wrote:See that's not how my current fuel oil boiler works. When the thermostat calls for heat the fuel oil boiler kicks on and comes up to heat.
You're not really saving much, typically boilers are most efficient the longer they fire but that is the only place you might see savings. The issue is if you keep a low idle temperature you need to expend more fuel to bring it back up to temperature. Specific considerations with coal is getting a really large fire to bring it up to temperature during low demand is not ideal, (e,g. someone takes a shower but there is no demand for heat). If you have a giant fire and the water is already up to operational temperature it can't absorb as much and a lot of that heat is going to go out of the chimney.
Let's put this into another context, suppose this was a hot water heater. People often turn them off during the day thinking they save something. If the hotwater heater is turned off at 8AM and the temperature drops to 90 by 4PM and you turn it back on you're going to use just about the same amount of fuel bringing it back up to operating temperature had you just maintained it at 140 throughout the day. The only time you start saving any significant money when you turn the hot water heater off is when it reaches the ambient temperature in the room and is no longer losing heat.
You can apply the same thing with coal, that idle fuel consumption is not getting wasted becsue it's being used to maintain the temperature. You don't need to expend a great deal of fuel to bring it back up to operating temperature.
One last thing to note, it's these wild swings in temperatures that usually cause outfires during the Spring and Fall. Perhaps the boiler comes on the morning becsue of a demand for heat, everyone is taking showers etc. It's all fired up and all demand for heat stops. Since you have a large fire the coal that is there is consumed rapidly, it won't run on it's own for hours and the timer is not running enough over those many hours to keep it lit. You would actually be creating this situation keeping it at a low temperature during idle and bringing it up to operational temperature on demand .