HSI Principle:                                         

Here’s the principle in a perfect operational Hot Surface Ignitor (HSI) heater:

  1. Plug the unit in.
  2. When power is applied, the ignitor (glowbar) glows. After a delay, 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 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.
  3. The photocell starts looking for a flame in the chamber
  4. When the atomized fuel contacts the glowing ignitor, there is combustion.
  5. Then, after 5-7 seconds the ignitor no longer glows and the flame continues to burn on it’s own and it should burn until the fuel supply is gone or power is disconnected.
  6. The photocell takes over and monitors the flame for color. Too rich/lean and the flame color changes, or any momentary flame-out and a signal is sent to the computer control board and there will be a shutdown by the electronics.


Now, these heaters are computerized and the computer wants to find a system that is operating at 100%.


IMPORTANT: At less than 100% the system shuts down for safety and environmental reasons. Simply stated- the computer is looking for a reason to shut the unit down because that is its job.


I am going to list some of the reasons that will cause a shutdown in the order of incidence, in our shop anyway.

  • Water in the fuel – people love to buy parts… but this is a free fix, just drain the tank. Shine a flashlight in the tank & look for water droplets & trash.
  1. The pump pressure not set to specs. There is not a fudge factor here. Instructions for setting the pump pressure are at http://reddyparts.com/pressure.htm .
  •  The air pumps on the old style heaters could be adjusted to suit the operator’s nose. If the heater smells just adjust the pressure a little until it smells good. Not so with the new heaters- you adjust to suit the computer, not you.
  • Air leak. If there is a crack in the plastic end cover, air line, fuel line, or nozzle adapter. These conditions create a lean mixture--poor flame color
  • Defective or worn nozzle. Desa warns about annual replacement in their manuals. Depending on annual “hours used” this may or may not be your problem.
  • Defective control board and/or photocell. Eliminate all of the above reasons for your heater’s failure before blaming the electronics.
  • Incorrect fuel- burn kerosene or diesel.


Troubleshooting the HSI (hot surface ignitor or glowbar) type heater

The first symptom that there is a problem is that the heater will run for 5 seconds and shut down. Or possibly, the heater will run fine with the top cover removed but when the cover is replaced the heater shuts down. Generally, this is a sign that the electronics are functioning properly.  Look for and eliminate all of the SIMPLE problems first.


The control board/photocell are on board to detect even the slightest flame-out condition or off-color situation. The system is looking for a “perfect” flame and if the photocell doesn’t see a perfect flame the computer will shut the heater down. Here are some things to check:

  • If you brought the heater to our shop we would immediately check the fuel. If it were suspected that any other fuel than KEROSENE or DIESEL we would drain the tank and replace with one of the 2.  The photocell/control board safety system does not like NAPTHA, PAINT THINNER,  FUEL ADDITIVES, etc and will shut the heater down if detected- off color flame.
  • Next, we would immediately check the air pump pressure. Checking the air pump pressure should always be the first step in troubleshooting. The pumped air eventually sucks the fuel from the tank. Low pump pressure translates directly to poor fuel delivery. Loss of only ½ 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 www.reddyparts.com/pressure.htm .
  • FACT: the slightest air leak  will cause a loss of pressure. This will cause a flame discoloration. The photocell will detect it the off-color and will shut the unit down. That’s its job.
  • If there is zero psi, then the problem is almost certain to be a broken rotor. But, if there is “some” pressure then the rotor is probably functioning properly, so adjust the pressure to specs. If unable to reach specs, then look for an air leak. Finally, 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 gauge order HA1181 or HA1182 or get one locally, if available. 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. This is not a sales pitch! If you don’t get ours, get one somewhere.
  • If the pump pressure is not correct, then set it to the specified pressure using a low pressure gauge (0-15 psi). NOTE: The computerized heaters are extremely pressure sensitive.  You will be wasting your time to attempt to adjust the pressure without a gauge.
  • For air leaks at the pump or between the pump and nozzle. Check with soapy water. Stay clear of electrical components (shock hazard) and the fan blade (knuckle hazard).
  • Hairline crack in the black plastic end cover.
  • Restricted lint filter.
  • Restricted nozzle. Blow out with compressed air.
  • Broken or cracked nozzle adapter.
  • Nozzle 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.
  • Fuel filter restricted. DESA recommends annual replacement.
  • Fuel pickup hose cracked or punctured. The fuel is picked up by suction from the air pressure.
  • Water in the fuel. You did look in the tank using a flashlight, didn’t you? Don’t assume the fuel is free of trash or water- even if you drained & replaced the fuel.
  • Contaminated fuel.
  • And any other reason that will affect the flame.
  • A defective photocell.
  • A defective ignition control board.
  • Thermostat models- defective temp control.
  • For problems in the electronics: The photocell & control board can be tested using an HA1170 diagnostic tester. Most people find that it is more economical to simply replace the photocell and ignition control board.



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- the first step to troubleshooting



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, and 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 should have 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. Of course, the motor must be turning the correct revolutions.


Fact...you can replace every part on a heater,--- but if the air pump pressure is not correct....the heater will not run!!

Suggested reading- HSI HEATER FAQ sheet