Yanche wrote:If you are looking to design a low operating cost system, size you coal boiler so it does NOT meet the heating requirement on the coldest design day of the year. Then meet the additional heat requirement with a second boiler, oil or gas fired. That way most of the time your coal boiler is operating at near peak efficiency. The coldest day of the year is a rare event and most of the time you will never need the second boiler. Having a second boiler is a wonderful backup for when the other one needs service.
You will need to pay attention when plumbing and wiring controls for this scenario, the average install does not have the capability to operate two boilers in tandem.
In general, your solid fuel appliance can have a lower BTU rating than the gas/oil unit you have in place due to the fact that it's output is continuous, the gas/oil units start and stop to meet load demand. My Burnham oil boiler has a gross input of 160,000BTUs and a net output of 139,000BTUs. The old hand fired Steel King is about 90,000 BTUs and heats the house better than the oil burner.
You should be thoroughly confused by now, does this help?