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MHU 75 Model Mr. Heater

I have just installed a Mr. Heater furnace in my garage. After I completed the installation I plugged the furnace in, set the thermostat and the furnace fired up and was running fine. I went into the house for about 30 minutes and when I came back out the blower was blowing cold air. When I checked the control board the LED was blinking 4 times which, according to the book means the high limit or roll out switch is open. The only information I can find in the book says that this normally closed switch opens on a temperature rise and to check for adequate combustion air before manually resetting switch. The switch is supposed to be located on the burner box top behind the ignition control board. I see what I believe is the switch they are talking about but I see no way to reset it. Can anyone help?

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  • rich2154 Dec 26, 2007

    I just want to thank hotuna for helping with my problem with my Mr. Heater. The information you gave me led directly to having heat in my garage on Christmas day. The roll out switch is defective and by bypassing it I was able to get the furnace to work. I will be ordering a new a new roll out switch this morning. Thanks again for your helpful response.

  • corradoking Dec 30, 2008

    I have same problem, tried bypassing and heater just runs the cold fan on startup that is suppose to engage after the flame is present. The heater makes no attempt to call for heat.

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The roll out switch is a small disc shaped part about the size of a nickle. There may be more than one. anyway there is a button shaped like a small peg in the center if it, push it in and it should stay in. You may need to re examine the limit switch, because this is what it sounds like. they are rated for a set temperature, so your application may need a higher temp. A simple way to check parts like these is to by-pass them by taking the wires off the part and connecting them together. If this solves the problem, that it. My only concern is that this is a new installation, and maybe its not venting adequately or its burning up all the usable air in the room. That's why most of these parts are on the equipment.

Posted on Dec 24, 2007

  • rligus1 Feb 25, 2014

    I bought a L4064B Universal Combination Fan and Limit controller from our local Grainger store to replace the original controller on an old Tappan model SGUF140D48E7, s/n 77 19739 140,000 gas furnace. I'm having trouble finding the furnace high limit setting, and fan on/off settings. The range of the old high limit was 170-210 F, and the fan range was 80-120 F. Can you tell me what the correct high limit and fan on/off settings should be? The manufacturers plate indicates temperature rise is 70-100 F.

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