Mr.Heater electrical issue

[Paintphreak]

Hello, I've got a Mr heater MHU80 unit installed and ran for a few months earlier this year no problem, however I switched thermostat units from a simple 2 wire to a digital 4wire to run the fan during the summer. It ran fine for a night or so, then popped the 3amp fuse. I replaced it, but it keeps popping. Not sure whats wrong? The unit has the green ground hooked up thru the junction box on the back to the main electrical wiring for 110v, but was wondering if it may need a separate chassis ground? I tried going back to the 2wire thermostat, but same result, just keeps popping the 3amp. Now with the new fuse, and no matter the thermostat, it will blow the 3amp fuse as soon as I plug the unit in! Any help appreciated. Thx

 

[Bert_]

How did you connect the new thermostat. I don't think unit heaters usually have a fan terminal.

 

[sarman47]

Here is what the manual has:

Link: https://www.mrheater.com/mwdownloads/download/link/id/30/

Sorry for the Caps, I copied and paste.

OPTIONAL THERMOSTAT INSTALLATION
IT IS RECOMMENDED TO USE 18AWG WIRE WHEN INSTALLING THE
THERMOSTAT. CONNECT ANALOG THERMOSTAT WIRING TO TERMINALS ‘R’ AND ‘W’ AS ILLUSTRATED ON THE SCHEMATIC DIAGRAM OR CONNECT A DIGITAL/WiFi THERMOSTAT TO ‘R’ ‘W’ ‘C’ AS ILLUSTRATED. NOTE: THERMOSTAT TERMINAL CONNECTIONS ARE MOUNTED ON THE BACK PANEL OF THE HEATER.
EXTERNAL THERMOSTAT TERMINAL STRIP WANING: DO NOT CONNECT LINE POWER TO THE THERMOSTAT TERMINAL STRIP.

To use the blower for air circulation only, your thermostat must have a “fan only” or fan selection setting. In case your thermostat has this option. an additional wire should be run to the “G” terminal on the thermostat connection block. See wiring schematic on page 13.

 

[Paintphreak]

Yes , I have the instructions, and wired with 18awg, can't figure out why it would keep blowing the 3aml fuse on the control board ?

 

[The Cobbler]

unhook the stat wires from the unit & see if the fuse still blows?

 

[Paintphreak]

Yeh, just tried that, blew another fuse!? I'm not sure whats wrong unless something is wrong with the control board itself? Anybody experienced same thing? Lmkthx

 

[sarman47]

I came upon this info from an old post https://www.garagejournal.com/forum/showthread.php?t=223379.

Also check that the foil insulation is not touching the high open switch and sparking the fuse to blow. I have heard of this as well.

 

[Paintphreak]

It was the foil on the sensor! Smh.... Thanks for all the input and special thanks to sarman47! ����

 

[sarman47]

Glade to hear it is all fixed. Sometimes it can be the smallest things. Here is to a toasty garage this winter............

 

[The Cobbler]

nice, easy fix !