They don't (herpstat back up) it needs to be a second back up on/off type. I use a zillas but am currently testing three hydrofarms at work and once that is done I expect that I'll be able to buy them cheap. The secondary needs to be rated the combined wattage of heater and primary t-stat. If you have an enclosure or two most on/offs will do the job.
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The wattage of the secondary need to be enough to handle all the power running through the primary and the primary itself, which one is rated higher is not important as long as both can handle the job. Hydrofarms are rated to 1000 watts I believe with out checking the specs.
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