Got into an argument with my dad about boilers used to heat a building. He has a large building and it has 2 boilers used to heat the building. They have roughly the same ratio of height to diameter, just one has twice the volume. Pretty much i was staying two is better than one. He's saying turn off the big boiler and turn on the small boiler when its over 40 degrees outside and turn off the small boiler and turn on the big boiler when its less than 40 degrees. Natural gas is getting expensive, he's doing this to cut costs, but i personally think its increasing them.
Running a gas fueled boiler requires combustion and exhaust of (hot) combustion products up the flue pipe (which are unavailable to heat the building). Running two boilers would nearly double the loss of flue gases. Therefore the real question is whether one or two boilers are required to heat the building for a given outside temperature. Likely a small boiler will do the job above 40F and the large boiler may be needed below 40F. Usually a large 'machine' is more efficient than a smaller one if loaded to capacity. Power companies operate their most efficient plants for base loads and add less efficient plants for peak requirements. Nuclear plants are very expensive and must be operated near full capacity to be most efficient and economical. Now power plants often hold in reserve aircraft-type gas turbines for peak loads because they can be started and shut down much quicker than coal-burning plants. Your problem is similar to the Big problem they have every day. In summary, whichever boiler is operated nearest full capacity (at a given temperature) will be more economical.
Like most other mechanical and electrical devices, boilers have an optimal operating point, a specific heat output that minimizes fuel consumption per BTU. When a boiler is operated significantly above or below this optimal point, the fuel cost per unit of energy output increases. It is better to match the load as closely as possible with the best operating point, so I'd say your Dad is right. However, since the energy required to bring a boiler up to temperature is lost, the more frequently the boilers are switched, the more fuel is wasted. If the boilers were automatically switched every time the temperature passed 40°, I'd say beware of those days when the temperature hovers there. An efficient approach would be to include a deadband in each boiler's range so that the larger boiler wouldn't come on line unless the temperature reached say 30° after falling through 40° and the small boiler wouldn't come on line unless the temperature reached 50° after rising past 40°. This way you won't have boiler flip-flop on temperate days.
