One of the common issues with hot oil heating systems is when the burner will not light. There could be several reasons and I will highlight these below.

The Obvious (Duh!) Issues

This is basically asking, “Is it plugged in”? Is there power to the control panel and flame safeguard device? Are all gas valves open, including any valves in the supply line that are not visible from the appliance? Is the actual feedline temperature higher than the setpoint of the temperature controller? If so, the burner will not start because there is no “call for heat”. If there has been work on the gas line and gas was temporarily turned off, you may just need to reset the low gas pressure switch. This switch breaks on low or no gas pressure and typically has a reset switch on it. The high gas pressure switch is normally closed and breaks on high gas pressure. This is usually not the culprit, but it doesn’t hurt to push the reset button on that one too. Is the pump running?


If your heater and burner are located indoors, chances are this room is well ventilated in the warmer months; however, we get lots of calls about burner issues around October when the weather turns colder. The cause of this problem is that these heater rooms and the main manufacturing areas are being closed off or limited to outside ventilation. Sometimes makeup air is insufficient to replace the air that is exhausted. This creates a negative pressure in these spaces that can cause the low air pressure switch not to engage when the burner blower starts. In the safety circuit of most flame safeguard units there is a circuit that is looking for proof that the blower is running. This circuit is looking for the auxiliary contact on the blower’s motor starter and that the low air flow switch is made. A negative pressure will sometimes keep this switch from engaging.

A quick test to see if this is the problem involves putting a jumper across the air flow switch when the blower starts.

Disclaimer: Troubleshooting a combustion system should only be done by qualified personnel who understand the risks involved and who are competent working on live circuitry. Since this circuit is looking for two events (aux. contact and pressure switch), this in not risk critical. Furthermore, this is normally done in close proximity to the burner, so it should be obvious that the blower is running.

If the burner starts with this jumper in place, a slight adjustment to the air flow pressure switch should solve this problem. If this switch is set for 0.5” SP, for example, an adjustment to 0.2” SP may fix the problem. Just make sure the switch is not engaged when the blower is completely stopped. And remember to remove the jumper after the correction has been made.

Housekeeping Issues

Some hot oil heating systems are in dusty areas. Eventually dust will accumulate inside the burner and begin to restrict normal air flow through the inlet louvers and burner internals. A simple visual check can identify this and a good cleaning may be in order. I am not a big fan of air filters on burners. If these are not maintained on a regular basis, the burners could start to burn rich and cause more problems. A better solution would be to duct outside air to the burner’s air inlet.

Mechanical Issues

Knowing the fault codes of your flame safeguard system will allow you to pinpoint the problem. For example, if the burner goes through its pre-purge cycle, returns to the low fire start position but will not light, it’s probably an issue with the pilot. The fault code on the flame safeguard unit should indicate this. This could be caused by a cracked ignition electrode (spark plug), a faulty ignition transformer, a bad pilot solenoid valve or lack of a gap at the business end of the ignition electrode.

Flame safeguard systems will prevent light off if the pre-ignition interlock is not made. This is the proof of closure switch on one of the main gas safety shut-off valves to make sure it is fully closed before a burner start cycle. Then other interlocks are checked during the burner start cycle including seeing the pilot and main flames. Any cause for lockout will be displayed on the flame safeguard unit to let you know where to look for troubleshooting.

Still having problems? Give us a call at (770) 425-5556. We can help.