Troubleshooting tips for Spark Plug type heaters.

Spark Plug Type Heater Principle                                

   In a perfect operational Spark Plug type heater:

  1. Plug the unit in. Occasionally, activation of  the reset button on the Safety Control will be required.
  2. When the motor starts to turn, the fan blows air through the combustion chamber, the fuel begins to flow, because the air pump (rotor) is directly connected to the motor shaft. Air is delivered to the nozzle adapter by the air pump at a precise pressure. At the nozzle adapter, the passing air creates a venturi effect, sucks fuel from the tank and thereby delivers fuel to the nozzle at a precise fuel/air rate for atomization. IMPORTANT: An incorrect air pressure rate at the nozzle will create an incorrect rate of fuel delivery at the nozzle. Instructions for setting the pump pressure are at .
  3. The Ignition Device (transformer – old name) immediately produces a spark across the spark plug.
  4. The photocell starts looking for a flame in the chamber.
  5. IF the COMPLETE system is performing to factory specs then there is combustion.
  6. The spark plug produces a spark continuously.
  7. The photocell takes over and monitors the flame. If the photocell loses sight of the flame, it signals the Safety Control to shut the heater down by tripping the reset.
  8. If the system is performing properly, the heater should be operational until out of fuel or the power is disconnected.


“My heater runs for 30 seconds and shuts down” is the most common question I am asked. Listed below are some of the more obvious troubleshooting tips:

  • If you brought the heater to our shop we would immediately check the pump pressure. Always-check the pump pressure FIRST. The pumped air eventually sucks the fuel from the tank. Low pump pressure translates directly to poor fuel delivery. Loss of only ˝ psi can cause as much as 30% reduction in fuel delivery (starvation).
  • By performing a pressure check much valuable time is saved in the troubleshooting process. And, it’s easy. For instructions visit .
  • If there is zero psi, then the problem is almost certain to be a broken rotor. If there is “some” pressure then adjust the pressure to specs.
  • If unable to reach specs, then look for an air leak. If you don’t want to check the air pressure- then take the heater to someone else to fix, because you will never make a heater mechanic! (No offense intended.) Correct pressure is critical. If you don’t have a low pressure (0-15 psi) gauge order HA1181 or HA1182 or find one locally. Our gauges are complete. We have accumulated all of the fittings so you don’t have to go to 3 different stores shopping for the correct components.
  • If there is zero psi, then the problem is almost certain to be a broken rotor. If there is “some” pressure, then adjust the pressure to specs. If unable to reach specs, then look for an air leak, using soapy water. Stay clear of electrical components (shock hazard) and the fan blade (knuckle hazard).
  • Next, shine a flashlight into the fuel tank. Look for water/debris/contamination. Any of these conditions will produce a flame-out condition causing the photocell to shut the heater down. Drain & replace the fuel, if necessary.
  • Check to see if the sparkplug is sparking. No spark? Check the ignition device (aka transformer, spark box). Note: the ignition device MUST have a good ground.
  • Check plug gap. It should be 60-80 thousandths, about the thickness of a dime.
  • If the sparkplug is worn on the tip replace it.
  • If all of the above are ok, REMOVE the spray nozzle, check for obstruction. Use compressed air (wear eye protection) from the front of the nozzle to clear.
  • Nozzles don’t last forever. The hole becomes worn, it then produces a spray instead of a fog which results in ignition failure. Replace it.
  • The Nozzle is worn. DESA recommends annual replacement. If the pressure is correct and the heater fires off & runs with the top cover off, but goes out when the lid is replaced, then it’s a 90% good bet that the nozzle is worn. Replace it.
  • The air pump (rotor) pumps air to the nozzle. In turn, the air sucks fuel from the tank then the air & fuel mix at the nozzle and creates atomization. IMPORTANT: if the fuel is sprayed instead of fogged then incomplete combustion will occur and the photocell will shut the heater down. If there is an air leak ANYWHERE between the pump and the nozzle THE PUMP WILL NOT BE ABLE TO ACHIEVE FACTORY SPECIFIED PUMP PRESSURE and you will NEVER be able to get the heater to run properly! Find the leak with soapy water and fix the leak, and then set the pressure. If the fuel filter is restricted or the fuel line is cracked there will be poor fuel delivery.
  • If all of the above checks out Ok, then the photocell is either dirty, poorly aimed, or defective. Test by temporarily bypassing.


Please understand that my recommendation for trouble shooting comes strictly from the description that has been given to me over the internet. If your heater was in our shop, I may have a different evaluation of the problem.

Setting the pump pressure

Illustration by DESA International

Use 0-15 psi gauge

Part number HA1181

Low pressure gauge & fittings inserted in the end

cover in order to set the pump to factory specs.


Locate the black plastic end cover that is mounted on the intake end of the motor/pump  assembly. Some end covers are round, some are rectangular. The end cover will have 2 protrusions, 1 long and one short, and a receptacle that houses the foam air filter.


The long protrusion houses the pump adjuster (relief valve) which consists of a ball, spring, & adjustment screw. The ball is inserted first, then the spring, then the screw  w/hole. The purpose of the adjuster is to allow excess air to escape. If the pump is operating properly the further the screw is run in the tighter the spring & ball, the higher the pressure. Therefore, if unable to reach or exceed the pressure required for your heater, then there is a problem “before” the air reaches the gauge, either the rotor is defective or there is an air leak (the 2 most likely spots to check with soapy water are the cork gasket that surrounds the lint filter or possibly a crack in the black plastic cover itself).


The short protrusion normally has a solid plug inserted. Remove the plug- insert the gauge.  

Turn the adjusting screw until the required pressure is reached. 

Q. What if the pressure can’t be reached?

A. Using soapy water check for very, lather type,  fine bubbles around the lint filter assembly point and check for a crack(s) in the black plastic end cover. Stop the leaks and the pressure will come up provided the rotor & vanes are intact, have the proper gap, and are functional. And, provided the motor is turning the correct revolutions. can replace every part on a heater,--- but if the air pump pressure is not correct....the heater will not run!!

Suggested reading- SPARK PLUG HEATER FAQ sheet