Lockout After 20 Minutes on a Single-Pipe Lift
A two-story house has a 275 gal basement oil tank with the outlet at the bottom. The burner is on a platform 6 ft above the tank outlet. The oil line is 25 ft of 3/8 in. OD copper tubing with a single-pipe connection, a new 10 micron spin-on filter at the tank, and no return line. The burner is rated 0.85 gph at 100 psi. It starts and runs for 18 to 22 minutes, then the flame goes out and the primary control locks out. A reset restores operation after about 10 minutes, and the cycle repeats. A vacuum gauge installed at the pump inlet reads 12 in. Hg while running and climbs to 18 in. Hg just before lockout. The tank was recently filled, and the oil is clear. The pump is a standard two-stage pump configured for single-pipe service. The homeowner reports no problem last winter.
1. Interpret the vacuum readings. What do they indicate about lift, line friction, and restriction?
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Six feet of lift costs roughly 4.5 in. Hg, about 0.75 in. Hg per foot of oil lift. A clean 25 ft, 3/8 in. OD line at 0.85 gph plus a filter should add only a few in. Hg, so a normal reading would be well below 12 in. Hg. The reading of 12 in. Hg and the climb to 18 in. Hg indicate a restriction that is getting worse as the pump pulls: a plugged filter, a partially closed valve, a kinked line, or sludge at the tank outlet. It is not explained by lift alone. Because the vacuum rises just before lockout, the pump is losing oil supply, the flame fails, and the primary control locks out.
2. Is the 3/8 in. line adequate for 0.85 gph at this lift? Explain using line-sizing principles rather than a table.
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Line sizing is not just a matter of nominal pipe size. The 3/8 in. OD line may handle 0.85 gph over 25 ft in a gravity or two-pipe arrangement, but a 6 ft lift on a single-pipe system is a demanding application. The pump must pull oil up the lift and overcome line and filter friction; if the vacuum exceeds the pump rating, dissolved air can come out of the oil and the pump can cavitate. The measured 12 to 18 in. Hg is already high, so the immediate problem is restriction, not necessarily the line diameter. However, the single-pipe lift is a reliability weakness. A two-pipe return would let the pump return excess oil and help purge air.
3. Recommend a repair plan that addresses both the immediate lockout and long-term reliability.
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Repair plan: isolate the tank, replace the 10 micron filter, clean the tank outlet and shutoff valve, and blow through the line to check for sludge or kinks. Install a return line and convert the pump to two-pipe service per its instructions; add a vacuum gauge and a good filter housing. Verify pump pressure, nozzle flow, and combustion after the repair. If the tank outlet is at the bottom, consider a top-draw anti-sludge outlet or at least a sludge trap. Do not simply raise pump pressure to overcome the vacuum; that masks the restriction and can damage the pump.