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how underfired is too underfired and how to be sure piping is the cause
Bought a house two months back and it came with a Weil-Mclain HE-5 series 3 propane boiler with an a.g.a input of 133k BTU/hr. Even when the tank is full, on a day with a temperature above freezing, the inlet pressure to the boiler's gas valve sits at 9.1 inWC when operating. The manufacturer's stated range for inlet pressure is a minimum of 11 and 13 max. The manifold pressure sits at 8.6 inWC, and should be 10. I don't think that the white-rogers 36c74 is designed to lower the pressure reduction any less than the 0.5 it already is, and I obviously do not wish to crank on the adjument screw to find out.
The 120 gallon tank's regulator is a non-adjustable Fisher Controls R232-BBF, which I don't believe is the issue (although I have not attempted to check the line pressure at the tank end while under load).
My money is on the piping as the cause, but I assume there's some math involved to know for certain. The boiler is the only appliance on the tank, and the piping consists of 6 feet of 21mm outer-diameter black pipe at the boiler end (am assuming SCH 40 1/2"), and 20 feet of 15mm outer-diameter copper tubing back to the tank (am assuming 1/2"), making many sharp bends along the way.
My assumption is that this boiler has been operating this way since it was installed (back in the 1980's?). The air being non-adjustable, manifold pressure would seem to be fairly important. But just how off is off enough to be worried about? A combustion test at the outside vent gave these readings: