Overheat protection: hot water cylinder / PV

Hello Adam.  I currently use DC to heat water using 50VDC on a conventional 1440 W, 120VAC element.  I am using the lower "trickle" heater, as you call it.  The element resistance is 10 ohms, which produces a current of 5 amps and a heating power of 250 watts.  Since we use little hot water, it works fine for us.  I use a timer to switch power on for 12 hours during the day, and shut off the power at night, since no hot water is being used during sleep time.  This minimizes any safety problem with overheating due to a stuck thermostat.  I did experience a problem initially, however, and wish to share this with you.  The 240vac/10a stat that you mention is the same as the one that I am using, and the contacts will burn from using DC current.  The life span is then very short.  I modified my electrical configuration to use a DC relay to switch the 50VDC to the heater element.  The thermostat then supplies only a small current to the relay coil and so far has operated without fault for several years time.  I am operating my household off the grid, and find this technique to be very efficient.  I wish you good fortune in your endeavors.  Jon C.

Adam, check out Aquastat controllers. They have a variety of high limit controls for most electrical situations and should be readily available at any plumbing supply.