Reading a few bad reviews about the relatively croocked thermostat of the hydor external heaters and found a few topics where people took those heaters apart to look inside (and posted pictures of it) and found a tiny crapy constructed electronic thermostat with a burned out relay on the PCB.
Most internal tube heater have much more internal space and use bimetallic thermostats, which are constructed much more solid, les accurate but also less likely to fail.
The inline hydor construction doesn't provide room for a bimetallic thermostat, so they need to use the smallest electronic thermostat available with rather tiny components. And it's this very tiny relay they use which if you have bad luck with the batch they constructed it with causes the trouble.
Actualy they use excactly the same thermostat as this one
http://www.banggood.com/DC-12V-50-to-110-Temperature-Control-Switch-Heat-Cool-Temp-p-933303.html
But than what they use is 6 times smaller, because it needs to fit internal in the heater.
This electronic thermostat it self runs on dc 12 volts so it needs a little dc 12 volt powersupply and it has a 10 amp relay when it's connected to 220 volts.. (10 x 220 =) 2200 watts switch capacity. So it's very well and safely constructed to switch any regular aquarium heater. I use this thermostat in my DIY build inline heater and also in my sump where it controles 2 x 100 watt internal heaters in the sump. And they work like a charme.
Use a thermostat like this as fail safe and make it switch the power socket where the heater is pluged in and hang the probe in the tank. You can calibrate this thermostat to the thermostat of the heater or with the thermometer in your tank. Turn the heaters internal thermostat up to 1 degree higher than the decired temp and set the external thermostat to the real decired temp. When the tank reaches this temp the power socket of the hydor is cut off from power.
So if the heaters thermostat ever fails and shortcuts you have a backup saving your tank.
What more do you want an extra €3,50 saving your day..
Ps. Well wht do you know?? Totaly forgot it was a UKAPS topic.
http://www.ukaps.org/forum/threads/hydor-external-heater-failure.31516/