Quote Originally Posted by jglass38
Seems like overkill anyway to back up a proportional with a proportional. The odds that a Helix or a Herpstat are going to fail are not that great to begin with and the odds of it failing "on" is even less. Use an on/off like a Ranco or Johnsons as a backup. For some interesting reading, Dion once explained it to me like this when asked what the odds are that a Herpstat would fail "on":

99.9% chance it will fail off, .1% chance it will fail on. The reason is:



Anything goes wrong with the probe and it will shut itself off. It has a 5 amp fuse inside which means if it was overloaded the fuse would fail protecting the unit and of course powering off. The only component that can cause it to fail in the ON situation is the Triac that controls the AC going to the heat device. When these fail they have a chance at failing in the on position. Many other thermostats use triacs rated at the max output. That is why you hear of so many problems with other companies because the worst enemy to electronics is heat and triacs generate their own heat and must be heat sinked. As the heat goes up an electronic component becomes less efficient so a triac rated at say 6amps may only take 6 amps before it heats up and could fail at less than the rated amperage. I designed the Herpstats for the long run. Instead of using a 50 cent 6amp triac all Herpstats use high quality 15 amp triacs that cost about a buck more. The difference is even when the heat goes up my triacs are well within range and since the fuse limits the amount of amperage to 5 amps my triacs are not stressed. To put it in easier terms the triacs I am using are rated for 1800 watts (15 amps x 120Vac) and the Herpstat is only pushing 500 watts or the Herpstat II at 700 watts. Sounds silly to save a buck on such an important component huh? These are the reasons I have never had a customer with this failure.



Could it happen? I suppose a lighting strike or bad brown out condition or line voltage problem could cause it. But we have taken every measure to prevent THAT specific failure. :^)



For your reference all proportional thermostats use a triac. Most non-proportional thermostats like Ranco and Johnson Controls use a relay. Relays can stick and cause the same situation. I always tell people that are really worried to buy a relay thermostat such as a Ranco and set it for 4-5 degrees above their desired temp (or whatever they feel safe at) and then plug the Herpstat or other proportional thermostat to the output of the relay thermostat and set it to the correct temp. Then if there is ever a failure in the on position the relay thermostat will continue to regulate the temps until the problem is resolved.
Great Explanation Jamie!