Question about Mr. Heater MHU45LP

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I have a mhu45ng mr heater. When i turn it on nothing happens. It has 120v power and 24v power. The light on the circut board does not come on at all. The gas is on the thermastat is on and the power is turned on.

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Check the fuse, I believe it is a 3 Amp , there is also a safety switch, when you remove the panel it is on the top you can push it in and the light should light after a few seconds. Hope this helped Tim

Posted on Oct 16, 2010

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If you have cycled the power to the unit already, the fault code should have cleared. If the fault code cleared and then came back, sounds like either a issue the blower motor being shot, or a relay in between the ignition controller and or the ignition control is bad, not making the heater do anything.

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Thermostat will not come on, therefore unit will not come on


Your thermostat is probably wired directly into the furnace to supply the 24V to power the stat. You have to make sure you're getting 120V into the primary side of the transformer, and 24V out of the secondary side. If you've got voltage in and nothing out replace the transformer with one of the same voltage, and same or higher VA rating. If you don't have 120V in then start checking backwards starting with power in/out of the door switch, then back to the unit disconnect switch, and back to the breaker if necessary. If you've got 24V out of the secondary go check to see you've got 24V at the thermostat between the R and C terminals, if you do the thermostat is bad. As with any electrical wiring make sure you've got good connections on all wiring terminations. Good luck, if you need an explanation of how to check the power in and out of the transformer let me know!

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1 Answer

Green light is off on the circuit board. 120volts going to unit is ok. Couldn't find a fuse on circuit board?


There is most likely, a diode in line before the transformer. Also, check the switch and thermostat. To see if the thermostat works, connect an ohm meter to it, warm it up with a hair dryer, and than cool it down with a bag of ice or some canned air.

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1 Answer

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1 Answer

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If nothing will come on check the breaker to the indoor section, check that the service switch by the in door section is on and the blower door cover is tightly closed. If all these junctions are transmitting power then it may be a burned out transformer. If you can check for 120v on the primary coil of the transformer and 24v on the secondary that would tell you where the problem is. If theres no voltage on either side then backtrack from the transformer on the 120v side till you find 120v. If theres 120v but no 24v your transformer secondary is burned out. If theres both then the problem is downstream from the transformer: circuit board 3amp fuse, loose wiring.

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