Three boilers are connected to a common header, each carrying 100 PSI. What would the pressure gauge on the header read?

In a system where three boilers are connected to a common header and each boiler operates at the same pressure of 100 PSI, the pressure gauge on the header will reflect the pressure in the header itself, which is determined by the individual pressures of the connected boilers. Since all boilers are functioning at the same pressure, the total pressure exerted on the common header does not increase. Therefore, the pressure gauge on the header will simply read the same pressure as that of the individual boilers, which is 100 PSI.

It's important to note that if the boilers were operating at different pressures or if there were a significant difference in flow rates or restrictions leading to pressure drop, the reading could potentially differ. However, given that all three boilers are identical and share the same operating pressure, the pressure on the header remains 100 PSI.