There is one basic type of oil fuel boiler, and it is very similar in principal to a gas-fired boiler. They have been around for nearly as long as coal-fired boilers, and although more efficient than ever, they still run essentially the same way. As a result, troubleshooting an oil boiler may be a more familiar process than you would expect. However, even if you have not worked with boilers in the past, you can still do some basic troubleshooting before calling in a professional. Troubleshooting options for the non-professional are limited. However, you can take a few steps to isolate and identify the problem, even if you find yourself unable to resolve it alone.
Isolate the problem. The main problem with an oil boiler is that it is not heating. This can be deduced fairly easily because your water will be cold. There are two reasons that an oil-fired boiler is not heating. Either there is a malfunction in the oil valve or petcock, as it is called, or the thermacouple is no longer functioning properly. If the boiler is causing other problems, such as smoking or emitting funny noises or odours, you should not experiment with troubleshooting. Call in a professional as soon as possible to prevent a possible health or fire hazard.
Turn off the boiler. This can be done in the control box, which will be attached to the unit itself. Flip the switch from "On" to "Off."
Replace the thermacouple. A thermacouple is a mechanical oil valve that works by expansion and contraction. When it no longer functions properly, it will not open up the oil valve. The thermacouple is attached to the control box, which turns the unit off on the boiler itself. Lifting off the front panel on the boiler (it's held on by snaps and gravity, so it literally unsnaps, then lifts off), you will see the control box dead ahead. It is the only fixture inside with wires attached. The thermacouple will look like a thin copper tube that is attached underneath the box, and runs underneath the boiler. It will be attached beneath the box with a nut that needs to be unscrewed, generally with a 1/2 inch wrench, although that size may vary. Unscrew it by turning the wrench in a counter clockwise motion. Pull out the nut, and the end of the copper tube will come with it. Follow the copper tube underneath the boiler, and at the end will be an igniter. The igniter receives temperature information from the control box. When the boiler needs to fire up again, the control box sends a signal through the copper tube to the igniter, which then lights the boiler. The igniter end will be held there with a snap clip attached to a small steel plate that can be lifted off and removed. Once you have removed the old thermacouple, snap the igniter end to the new one, and rethread the new thermacouple onto the copper tube. Screw it back into place.
Test your repair job. Re-light the pilot light. The oil boiler will take a good 12 hours to start heating substantial amounts of water. Test your hot water after allowing most of the day for the boiler to work. If you run out of hot water very quickly, or cannot get any at all, then you have isolated the problem (the petcock valve). If a thermacouple is replaced, and the oil heater is still not heating the water, the main oil petcock valve is not functioning. This needs to be replaced, but a qualified service person should be called at this point. However, you will have saved yourself some cost by isolating the problem before bringing in a professional.
Always work with the oil boiler off, when it is not heating water.