I am struggling to figure out what theory explains a situation I observed. A steam boiler had a pressure control which would turn off the boiler if it sensed 2 psi. The boiler would frequently switch off due to the pressure control sensing pressures exceeding 2 psi. When the pressure control was removed a blockage was found in the curled tube that lead into the pressure control. After the blockage was removed the steam boiler stopped sensing pressures in excess of 2 psi. What explains the effect of the blockage on the pressure sensor.
Things I have thought of:
Ideal gas law: $PV = nRT$, I think this would have no affect. Even though the volume of the system is decreased slightly due to the a narrower tube it is such a small decrease in volume relative to the whole system.
Bernoulli's Principle: If we measure pressure at a blockage the pressure should be lower than the overall pressure of the system because fluids flow faster and at lower pressure pressure through narrower tubes. We aren't measuring pressure at the blockage though so I assume that this does not apply here.
Poiseuille Equation: Change in Pressure = Flow Rate $×$ Resistance. I think the equation is a little more complicated with a gas, but the general idea should still apply. If we narrow the tube then the resistance will increase and we should see an increase in pressure at the control relatively to pressure in the rest of the system. I am confused by this idea because it seems to say the opposite of what Bernoulli's principle says, but seems like it is the most likely explanation here.
The pressure control is broken.
There is another principle at work that I don't know about.
Heating System Diagram: