Why Does My Furnace Lose Its Prime?

Leaking Lines

• Oil fuel lines are typically made of copper, steel, brass or iron. When they come into contact with concrete basement floors or if they’re buried underground, they can corrode and leak. Lines can also break when the house shifts or if they are stepped on. Look for oil stains or signs of dripping along the path of the fuel line. Use protective sleeves to reduce the chances of damaging the fuel line. The non-metallic tube will protect the delivery line from damage and corrosion.

High Routing

• Another way that oil pumps lose their prime is if the burner is served by a single line that runs from an underground tank or if the fuel line runs up high beneath the ceiling and then back down to the furnace. If this happens, not only could your system lose its prime and become air-locked, you could wind up with frozen pipes and other damage to your home. This problem is more likely to happen when the tank is low on oil. Oil service technicians recommend using two lines instead of one. The second line will give you an alternate route if the main line clogs, and you’ll avoid losing prime.

Foot Valve

• Foot valves are installed in some systems that only handle No. 2 fuel oil. The foot valve keeps oil from draining out of the suction lines; but if the foot valve is leaking, this can cause the pump to lose its prime. The location of the foot valve at the bottom of the tank makes it hard to reach if there’s a problem. Installing a check valve just above the tank in a horizontal run is a good solution.

Puff Back

• Puff back is a potentially dangerous situation that can result when a leak in the oil line draws air into the burner. If an air bubble remains in the oil burner nozzle, the pressure in the nozzle will drop from 100 pounds per square inch (psi) to just a few pounds as soon as the burner shuts down. The drop in pressure will make the air bubble grow larger, and oil will dribble out of the oil burner nozzle into the furnace combustion chamber. When the burner starts up again, the unburned oil explodes, causing a puff back that may blow soot throughout your home, along with poisonous carbon monoxide gas.