Heater Suddenly Heating Too Much

Hi. I have a little Eheim  Jager Heaterstat 25w heater in my 5 (US) gallon tank. It's been holding the temp at between 76F and 78F just fine for a full month. Two nights ago at about 5:00am the thermometer alarm went off. I checked and the tank was at 81F and the heater was still heating. I unplugged it, and because it was 5:00am and I wasn't awake enough to think, I turned the thermostat down and plugged it back in again.

Yesterday morning when I got up I checked and the tank was at about 74F (the room temp was about 67F), so I turned the thermostat (while it was still plugged in and in the tank) until the light came on. Since then (most of yesterday and a lot of today) I've been trying to get the heater to heat to +/- 77 as it had been doing, but not having much luck. I have unplugged it now and am waiting for a while for it to totally cool off and then will turn it back on.

I am more concerned with the sudden spike to 81F than having to fiddle with getting it reset to where I want it, again. Had I not had the alarm on the tank, who knows how high it would have gone?  I have emailed Eheim for advice, but thought I'd check here to see if anyone else has had this problem?  I've used Eheim heaters for years and never had a problem with one before now.

Middle of the night, room temp hasn't gotten over 71F even in the daytime recently (it's not been hot up here, really), I think it was around 65-67F in the room when the heater had it up at 81F and the alarm was blaring. The light was still on when the alarm was sounding.

This is a replacement for one I brought over from the States. I wanted one that ran on local current. Thankfully I still have the US model and if I can't get this stable and the alarm goes off again I can always replace the new one with the old one. (It would be running off a transformer.)

The tank is now at 77F and I've FINALLY been able to get the heater light to turn off. Will see how that goes over the next several hours. The thermometer alerts me to low and high temps, so that's handy....

Apologies for resurrecting an old topic but I've read elsewhere that some folk use these external temperature controllers https://www.ebay.co.uk/itm/223436915756 in an effort to prevent a heater from over-heating if they don't auto-switch off, as some poor guy experienced with fatal consequences for his fish.

Hi Folks,

I can possibly help with this but I'm pushed for time right now.

I'll be back - hopefully, tomorrow.

JPC

Hi Folks,

I posted on this topic previously:

https://forums.thinkfish.co.uk/general-fishkeeping-advice/ways-to-prevent-(or-reduce-likelihood-of)-fatal-accidents/msg46445/#msg46445

In temperature-controlled aquarium heaters, they contain a heating element and a temperature controller, often called the thermostat. Temperature control can either be done by the somewhat dated mechanical bimetallic strip or an electronic sensor. Both are capable of failure. In my view, it is wrong to suggest that surge protection is the answer.

Note that some aquarium heaters do not incorporate any form of temperature control. Such heaters rely on an external controller such as those in my link above.

JPC