Any suggestions on how to fix this DIY ?

It's a Stina brand and the dial temp adjuster just gives scalding hot even when dialed to low- I suppose the contacts have eroded in some way.

Thanks in advance for any suggestions.

Mine gets too hot if the water pressure/flow is low. Check the supply line for obstructions. I live in a building that has the water tanks on the roof and during hot season, the water can be hot to begin with, especially in the afternoon.

Yup - if water flow is low most units can't regulate and it's HOT or nothing.

The heater triac could be shorted. They're cheap to replace. That means power to the heater on all the time except when switch in off position.

The heater triac could be shorted. They're cheap to replace. That means power to the heater on all the time except when switch in off position.

Thanks for responses.

Triac?

The heater triac could be shorted. They're cheap to replace. That means power to the heater on all the time except when switch in off position.

Thanks for responses.

Triac?

If need to ask you don't need to know.

It's a plastic thingy with 3 legs, probably about 1/2" square and 1/8" thick, it controls the heating output by chopping up the mains cycles.

And yes, the failure mode is often short-circuit (full on).

The OP said problem was "scalding hot". The only time these things get like that is with low water flow. With normal flow they won't, even at full on.

The OP said problem was "scalding hot". The only time these things get like that is with low water flow. With normal flow they won't, even at full on.

True

The OP said problem was "scalding hot". The only time these things get like that is with low water flow. With normal flow they won't, even at full on.

If the water flow is so low that it can get that hot the flow switch will activate and turn the water off to prevent this.

The OP said problem was "scalding hot". The only time these things get like that is with low water flow. With normal flow they won't, even at full on.

If the water flow is so low that it can get that hot the flow switch will activate and turn the water off to prevent this.

Maybe yours does mate but not mine - and apparently not the OP. My experience with these things is if the water flow is way too low the unit will shut off but I haven't seen or heard of any that shut off the water flow.

My experience is that if the supply water is even a little bit warm, it can get extremely hot very quickly...much more than when supply water is cool.

Even during hot season, if there has been a rainstorm at night, the water in the am can be quite chilly and the heater gets turned on...not unusual for the user to forget to turn it off. Later in the day, turn the water on for a shower and "Yikes!!"...a quick 2 step out of the way. Can take a while to cool it down.

Thanks for responses.

Triac?

If need to ask you don't need to know.

It's a plastic thingy with 3 legs, probably about 1/2" square and 1/8" thick, it controls the heating output by chopping up the mains cycles.

And yes, the failure mode is often short-circuit (full on).

You mean I should not be poking around in the machinery of an appliance that utilizes electricity and water? Agree.

I have an electrician I'd hire to do this, I just want to know as much about the problem to safeguard myself against undue charges. I'd like to buy the part myself for instance to avoid any unfair mark up.

Seems like the water flow, and blockage could be the problem, too.

Thank you all for responses.

The OP said problem was "scalding hot". The only time these things get like that is with low water flow. With normal flow they won't, even at full on.

Well too hot to stand under, not boil an egg.

My experience is that if the supply water is even a little bit warm, it can get extremely hot very quickly...much more than when supply water is cool.

Even during hot season, if there has been a rainstorm at night, the water in the am can be quite chilly and the heater gets turned on...not unusual for the user to forget to turn it off. Later in the day, turn the water on for a shower and "Yikes!!"...a quick 2 step out of the way. Can take a while to cool it down.

Don't think that's the problem. I do notice the water flow stays consistent despite the position of the flow adjusters.

It sounds like the shower unit might be a few years old? You should check for scale build-up inside the shower head. IE: They usually will unscrew from the hose fairly easy (don't lose the rubber grommet). Then check water flow with the head off. Also, remove the hose from the unit and check for clogged filter or the like. I really doubt it is an electrical problem.