One-Pipe Oil Line With Below-Burner Tank and High Vacuum
An oil-fired furnace in a detached garage is fed from a 275-gallon tank in the basement. The tank outlet is 6 ft below the burner, and a single 3/8-in. OD copper line runs 25 ft with four elbows to a single-stage pump set at 100 psig. The system is one-pipe; there is no return line. The furnace starts and runs normally for about 20 minutes, then the flame becomes erratic, the pump gets noisy, and the burner occasionally locks out. A vacuum gauge installed at the pump inlet reads 8 in. Hg while running. The oil filter is new, the tank vent is clear, and the tank is half full. Nozzle is 0.75 GPH 80°B; smoke is 1, CO2 11.5%, draft −0.03 in. w.c. The homeowner says the problem is worse in cold weather and after the furnace has run a while. No oil leaks are visible, but the flare fittings were recently reassembled.
1. Explain why this one-pipe arrangement is unsuitable, using the lift and vacuum readings.
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A one-pipe system depends on oil reaching the pump without a return path. With the tank outlet 6 ft below the burner, the pump must lift oil. A single-stage pump in this arrangement is being asked to pull a vacuum through 25 ft of 3/8-in. OD copper with four elbows. The 8 in. Hg vacuum is equivalent to roughly 10–11 ft of oil column, well above the static 6-ft lift; the extra is friction and restriction. High vacuum causes cavitation and erratic oil delivery.
The recently reassembled flare fittings are also a likely air-leak path. Air in the line creates a compressible pocket that makes the flame pulsate and can cause lockout after the burner has run long enough to pull the pocket through. The 20-minute delay fits a gradually increasing vacuum or accumulating air.
2. What are the acceptable corrections? Include line sizing and pump options.
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Best correction: convert to a two-pipe system by adding a return line to the tank and removing the pump bypass plug as required. A two-pipe system returns oil and purges air, and it is the standard fix for a below-burner tank. Alternative: install a two-stage pump rated for one-pipe lift; two-stage pumps can handle greater lift and are less sensitive to vacuum.
In either case, use 1/2-in. OD copper for the 25-ft run to reduce friction (3/8-in. is marginal at this length and lift). Remake the flare fittings with new nuts and verify no air leaks. Purge the line, set pump pressure to 100 psig, and confirm vacuum stabilizes below about 5 in. Hg. Final checks: steady flame, smoke 0, no pump noise, and no lockout over a 30-minute run.