We have observed that with same gas flow, some times Boiler's steam generation get reduced drastically. It is a clear indication of lower BFG calorific value.
You don't provide enough information to give a clear picture, but I suspect you're using some sort of process gas (for example, coke oven gas shows this kind of variation) and the calorific value is varying as a function of what is happening in that process. You need to add a higher heat content gas (for example methane) and measure the mixture for a steady heat content. Several companies make reasonably priced instruments for continuously monitoring heat content.
Here's one that could be used (though it's designed for air/fuel ratio, you just have to use it sort of "backwards")
B F Gas Calorific value
Re: B F Gas Calorific value
