Rooftop Unit Low Manifold Pressure on a 2 psi Gas Header
A strip mall has five rooftop packaged units served by a 2 psi steel gas line from the meter to a roof header. Each unit has its own appliance regulator and a nameplate manifold pressure of 3.5 in. w.c. Unit 3, the farthest from the meter, has repeated ignition lockouts and soot on the heat exchanger. A technician measures 10.5 in. w.c. at the unit inlet, 2.6 in. w.c. at the manifold, and 3.4 to 3.6 in. w.c. at the other four units. The branch to unit 3 is 1/2 in. pipe, 40 ft long, taken from the 2 psi header, and the regulator vent terminates under the unit's weather hood. The technician proposes increasing the manifold pressure adjustment to 4.5 in. w.c. to compensate. The building owner wants the unit repaired without replacing the entire header.
1. Diagnose the likely cause of the low manifold pressure at unit 3 and evaluate the technician's proposal.
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At unit 3, inlet pressure is 10.5 in. w.c., so gas is reaching the unit; the problem is downstream of the inlet. A manifold of 2.6 in. w.c. against a 3.5 in. w.c. nameplate means the regulator is not delivering its rated capacity or is misapplied. Causes include an undersized regulator, a clogged orifice or screen, an obstructed vent (the vent under the weather hood is suspect), the wrong spring, or a regulator installed too close to an elbow. The 1/2 in. branch at 2 psi may be adequate, but it must be checked by sizing calculation and pressure-drop measurement; the other units prove the header pressure is acceptable. Raising the manifold adjustment to 4.5 in. w.c. is not a repair: it can overfire the unit, damage the heat exchanger, produce CO, and void the listing. The regulator should be sized for the unit input and adjusted to the nameplate, not beyond.
2. What tests and corrections should be performed, and what code and safety requirements apply?
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Tests should include inlet and manifold pressure with a calibrated manometer, regulator lockup and vent check, orifice and spring verification, a leak test on any opened piping, and combustion analysis before and after. The regulator vent must terminate outdoors and be protected from blockage. Replace the regulator if it cannot hold 3.5 in. w.c.; clean or replace the branch strainer; and verify the unit gas valve. After adjustment, confirm steady manifold pressure at high and low fire and CO within manufacturer limits. Code and safety requirements include using listed appliances and regulators, maintaining clearances and vent termination, providing a sediment trap and shutoff, pressure-testing the piping, and obtaining permit and inspection where required. Tag the unit out until corrected.
3. How would you verify the gas piping is adequate without replacing the entire header?
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Use the nameplate input and the longest run from the meter. At 2 psi, use the appropriate gas sizing table for the pipe material and schedule, and include fitting allowances. Measure pressure drop from the header to unit 3 under flow; if the drop exceeds the allowable, replace the 1/2 in. branch with a larger size. If the piping is adequate, the regulator is the problem. Do not compensate by raising pressure. Document the calculations and test results. The judgement point is to separate a piping capacity issue from a regulator performance issue using measured pressures, not to adjust the appliance beyond its rating.