Propane furnace regulators keep the LP gas flowing into the appliance at a steady rate and at the correct pressure. The regulator is made up of an on/off valve, a safety shut-off, and a thermocouple.
The on-off gas valve allows the gas to flow into the appliance burners when the thermostat kicks the furnace on. It delivers an electrical charge which pops the valve open. These are only found on automatic operating furnaces. That same valve is responsible for pressure regulation.
The safety shut-off cuts the flow of gas if the pilot light goes out. The thermocouple is a sensor that is attached to the burner. When it senses the loss of heat, it sends a signal to the safety valve. The thermocouple does not allow the gas valve to open until the pilot light is manually relit.
As propane furnace regulators age, the springs inside sometimes wear out, causing the gas to be delivered at too high a pressure. This can also happen if the regulator is out of adjustment. The flame burns too high and too hot and may warp the heat exchanger. A malfunctioning regulator can also cause low gas pressure, which can be unsafe.
The high pressure problem can usually be fixed by an adjustment or replacement of the regulator. The low pressure issue may need further investigation. If the regulator is adjusted and another problem is later found and corrected, too much gas may be released into the burner.