Incoming Gas Pressure
The incoming gas pressure must be stable in both in a static condition and when under flow. Unstable or insufficient incoming gas pressure leads to mis-firing and even failure of connected appliances. It causes a dirty flame to connected appliances, leading to soot buildup and a host of other problems.
A big key to the proper flow test of incoming gas pressure involves testing while all connected appliances are running. This often-overlooked aspect of the flow test often results in misdiagnosis of a given problem with not only boilers, but all types of gas appliances.