Have a model 55F Desa Heater - It ignites perfectly but will not stay lit when the ignition button is released. Have held it for as long as 2 minutes but still will not stay lit. Tried Switching 20 lb Propane Tanks To Full Tank -Same Thing. Every Now & again It DOES Stay Lit. Had It On This Afternoon, & It Finally Stayed Lit For About 30 Minutes - Then It Went Out. Tried To Re-light It & Same Problem - Works Fine Until Releasing The Ignition Button - Would Not Stay Lit. Any Suggestions ?

Chances are the thermocouple needs replacing.

If it's a direct spark ignition type heater maybe that's how it's designed to.work .When called for heat pilot open allowing gas then ignitor spark and ignite the pilot then gas valve open and allow gas through burner then unit fires off .
Once temp is met pilot valve closes and pilot out .

Sounds like the igniter needs to be replaced. It is normal for a heater to run five or so minutes that means the thermostat is working. then restart when the temp calls for heat but if the igniter does not work it will not light up again.

It probably is the thermocouple bot DO NOT TOUCH IT unless you are Gas Safe Registered

is the sensor for the thermocouple located in the right spot in the flame,remove it and if you have access to a propane torch,heat the end till its red hot and leave it connected to the gas valve,then start the unit and this will tell you if the sensor is properly positioned in the pilot flame on the unit itself,if not you could have a defective new thermocouple or another safety device keeping it from staying lit or the gas valve is faulty

You will first need to turn the control knob to pilot. Then depress the control knob button and hold it. Then press the push button piezo igniter several times until pilot lights or open pilot inspection cover and manually light pilot with match/lighter. You may need to hold the control knob for at least a minute. Then release the control knob and if the pilot stays lit then turn the control knob to on. If it does not light then repeat the process again.

WITH THE HEATER OFF, clean the pilot flame unit with a can of compressed air. Blow directly into flame hole then try it again. if it continues to "not" stay lit then you need a new thermocouple.

the heater makes use of a millivolt generator which generates a small dc voltage (500-750mv) that is applied to the pilot valve of your main gas valve. when you light your pilot, the flame bathes the generator with a flame. after a delay you release the pilot button. if the generator is working, the pilot valve will stay lit. if the generator is not working the flame will go out as soon as you release the pilot button. change the generator.

I have a Williams top vent gravity wall furnace (model 5009622) and I had the same problem. I had to take the cover off the furnace -- screw on top and two at the bottom (open the access door and there is one screw on each side) to get to the reset button for the vent safety shutoff system. On my furnace it's on the left side toward the top of the unit. I pressed the little red reset button and it's been working ever since. If your furnace is properly installed and doesn't have bird's nest in the chimney, it should work fine. Good luck!

two items come into play here - 1) the hot surface cutout and 2) the flame detect Cd cell. Check alignment of the Cd cell or just replace it. The hot surface cutout has two terminals that plug into the two tabs on the millivolt generator.