Re: heater will not stay lit on auto procom ml 300 blue...
If you have an ML300HGA, it is a manual-control heater, which means you turn it on and you turn it off. No thermostat. It does not have an AUTO setting.
Likewise, the thermostat-controlled models do not have a Manual mode. If you have the ML300TBA, it has a thermostat and will turn itself on/off in order to maintain a room temperature.
On manual models, the numbers (if any) on the dial are heat output selections. Set the control to the highest heat setting and that's what it does until you turn it off.
On automatic thermostat models, the numbers on the dial are temperature selections. Set the control to the highest heat setting and the heater will continue running until it is 85-90 degrees, then shut off. Set it around 2 and it will maintain a temp of around 70 degrees.
All Procom heaters are either automatic (thermostat-controlled) or Manual. None are both.
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The problem is more than likely in the flame rectification circuit. The little rod that senses the flame probably has barely perceptible corrosion on either its surface or on the physical wiring connections between it and the ignition control module. A little touch with the sandpaper at these connections should take care of it. I've had extreme cases where the actual furnace grounding conductor was creating the issue.
So we are talking about the same won't stay lit I will describe both ways. One is when you lift your fingers off the knob it goes out that instant which is probably the thermocouple needs replaced,look to make sure the tip of the thermocouple is in the flame of the pilot light first. The secound version of it is it goes out in a few minutes up to a few hours.The second version is most common.There is a part called a Thermocouple that is mounted next to the pilot light, it is around 3/16 of a inch diameter 1 to 2 inches long and should be mounted so the tip is in the flame of the pilot light. This is what tells the gas valve the pilot light is burning. If any dust ,cat hair ,dog fur what have you gets around the base of the hole the gas from this pilot light comes out of it will bend the gas, which move the flame so it does not hit the thermocouple like it should, this is below the flame so what ever is in there diverting the flame dose not burn up.A draft from a fan or door could do it also.I have used a procom ventless heater as my only heat for 7 years and have to blow it out twice a year. I bought a new thermocouple at the same time I bought the heater to have in the closet in case it went bad some night, it is still hanging there .Because it sucks air in from the bottom past the flame and out the top everything in the air gets burnt into a fine ash dust. So if you use compressed air to blow out your unit you might want to hold a wet towel up in front of it hoping a lot of that white powder dust will stick to it instead of flying around the room . My flame always looks so much better ,less orange and more blue after this .
sounds like your thermacouplar is going out and needs replaced. Sometimes these will act like they are working fine , but when the pilot stays on for awhile and later goes out it is a sure sign that it is bad