One of the outputs of a Herpstat 4 or 6 no longer works.

Situations covered:

Herpstat no longer powers on.


An output on a Herpstat 4 or Herpstat 6 no longer provides power even though the LCD shows 100% power.  (If the LCD shows 0% power you have a software setting wrong.)


This is typically due to a safety fuse popping.  The safety fuse protects the circuit board against damage due to voltage spikes from the power company, short circuits from one of the heating devices, or plugging in a heating device in that draws power beyond what the output on the thermostat is rated for.


The solution is to replace the safety fuse.  These units include a spare safety fuse in their built in fuse drawer:

Herpstat EZ Series

Herpstat 1 Basic and Herpstat 1

Herpstat 2

All other models require a optional purchase of a spare safety fuse.  Replacement fuses can be purchased here.

You can also find these at many local home improvement stores.  Instructions for replacing the fuse are located at the back of the user manual.  Click here to select your models user manual.


After replacing the fuse you can test that output by plugging a standard incandescent table lamp into it and see if it receives power.  If that test goes well you can try your heating equipment after closely inspecting it for possible short circuits.  If the fuse pops again you will know the heating equipment is faulty and needs replacement assuming its rated wattage is below the rating of the thermostat's outlet.  


A good tool for checking wattage draw of heating equipment is a kill-a-watt meter.  These are easy to use devices that you plug into the wall and then plug your heating device directly into (no thermostat in between) and it will tell you how many watts it draws.  Then check your spec on the Herpstat to see if that wattage is under the max per output wattage.  Kill-a-watt meters can be purchased at most home improvement stores.

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