An oil furnace requires a primed fuel pump to keep the fuel flowing to the burner. If the fuel pump in your furnace loses that prime, it means that the burner isn't receiving oil the way that it should be. There are a few common reasons for a furnace fuel pump to lose prime, and you'll need to identify the cause in order to correct it before you prime the pump.
Causes of Lost Prime
No Fuel – If your furnace was working fine before suddenly stopping without warning or sound, it may be because you're out of oil. Most oil tanks have a gauge that allow you to monitor the oil level, but sometimes those gauges can wear out and be inaccurate. Check the level of the oil inside the tank to see if it's out, and call for a fuel delivery if it is.
Loose Nozzle – When the fuel nozzle is loose, it can allow air into the fuel line. That may interrupt the flow of fuel into the pump. You can tell that the nozzle is loose if you're smelling fuel around it. You might be able to tighten it yourself, but some of them require a special wrench. If you don't have a wrench that fits, you'll need to call a technician. Otherwise, let the furnace cool before you remove the cover to get to the nozzle, then adjust it carefully until it just stops turning.
Leaky Line – Sometimes, wear and tear on the oil lines can lead to leaks. When that happens, you'll be able to visibly see the oil drips around those leaky spots. Tighten all of the pipe fittings and replace any fittings that are leaking with a flared fitting, because those are air and liquid tight. If you have a punctured line, replace it with a new line.
Clogged Filter – As you use your furnace, the oil filters capture particles and debris that may be trapped in the oil. This can lead to clogs in the filter as these particles build up. When this happens, it interferes with the flow of oil into the furnace. If the pump isn't getting enough oil, it can disrupt the prime. Check the filters regularly and replace them any time they are visibly dirty.
Steps for Priming the Furnace Pump
Once you have identified and corrected the problem, you'll have to prime the pump in order to start the furnace again. Turn off the power to the furnace, then check the thermostat to be sure that the temperature setting is warmer than the current indoor temperature. That will help ensure that the furnace starts when the pump is working.
Look for the bleeder valve on either the side or front of the pump. You'll know it when you see it, because it's a hex-shaped nut. Put a bucket or similar container under the valve to catch the fuel once it starts to come out of the bleeder valve, then open the valve with a wrench.
You're going to have some air sputtering from the valve when you open it. Push "Reset" on the furnace to get the system started. This is necessary for the pump to start drawing fuel. Leave that valve open until you see a solid stream of fuel with no air pockets. Then, close the valve. The furnace should engage. If it doesn't, repeat the process. It can take a couple of times priming the pump before the system stays running.
If you aren't able to identify the source of the issue or your repeated priming attempts don't engage the furnace, reach out to a local furnace repair technician. He or she can troubleshoot and repair the problem, restoring your furnace to its normal function.
A few years ago, I could tell that we were having serious furnace problems. In addition to dealing with a house that was constantly too cold or too warm, we were also plagued by a noisy, smelly furnace that seemed to have trouble on a daily basis. Unfortunately, I didn't know enough about furnaces at the time to spot the problems quickly. One day, the entire system died, and it was beyond repair. After having that experience, I learned a lot about HVAC systems, so that I could troubleshoot future systems. This website is all about teaching you what you need to know so that you don't end up in the same situation.