This number will be lower than when the burner is not firing. Combustion report, if the boiler is underfired, there should be too much oxygen in the stack gases. If the gas pressure is too low, the burners will overheat and soot up the boilers. For instance, if they have propane gas and the boiler has natural gas orifice, the manifold pressure will be too low and the boiler will soot. The boiler will subsequently soot up.
Propane is heavier than air.
When a propane burner shuts off, the gas remaining in the manifold pipe continues to burn at the top of the orifice, below the burner and venturi tube. It takes about 15-45 seconds to burn the remaining propane. This will soot up the boiler quickly. The solution is to eliminate the short cycling. Open a door to the boiler room.