Other water heaters using 2 thermostats only have ~5 degrees difference, for example 90°C and 95°C, so if the first thermostat fails then the second will fire at a temperature not far from the intended setting.

But with coffee machines the second thermostat has double the rating of the first. Why such huge difference?

And what does Tf on the second thermostat mean? Why is the first thermostat not lableled Tf?

 

Making some assumptions here: 125˚C is the normal shutoff point. Water has boiled. IF water boils out then there's nothing to stop the thermal runaway. Nothing except for the 260˚C TF (Thermal Fuse). Once the TF blows (assuming it IS a thermal fuse) it's blown forever. It doesn't reset. It's an anti-fire protection meant to keep the pot from melting down and possibly starting a fire.

My gas clothes dryer has a TF. I've worked on other dryers, gas and electric, that have blown TF's. Replacing the TF usually ends up fixing the problem. As long as the problem that blew it in the first place is not a chronic problem.

125C is pretty high for boiling water. Perhaps the bottom of the pot needs to reach that temperature in order to transfer its heat energy throughout the entire pot. Boilers are not my expertise.

 

Neither of those temperatures make much sense in regards to water -- 125°C is 257°F and 260°C is 500°F. Now consider that eutectic PbSn solder melts at 183°C (361°C).

Where is this thing mounted? I'm guessing it's mounted directly to the heating element?

Also, 260°C is NOT twice the temperature of 125°C. Celsius and Fahrenheit are both relative scales, so ratios have no meaning. The corresponding absolute temperatures are (to the nearest degree) 398 K and 533 K, so the second is about one-third higher than the first.

In the case of 90°C and 95°C, those both make sense with regards to water -- both are near, but below, the boiling point and so are likely sensing the temperature of water, not a heating element.

 

The reason the TF temperature is so close to the thermostat in the water heater is so the water temp. does not become scalding.

In the case of the coffee maker this is not so critical, so the TF is set to avoid a fire.

In the case of the coffee maker that range allows for an operator to intervein before the TF opens and avoids having to replace the unit.

Of course, all of that is just my opinion.

 

Where is this thing mounted? I'm guessing it's mounted directly to the heating element?

Yes, it's on the heating element.

For water dispensers the temperature is below boiling point because the hot vapor returns to the reservoir, which contains both the cold water and the mild water. The hot water tank has a small rubber tube at the top (traced in red in the attached photo) which does this.

One repair I had was a machine where the cold faucet was dispensing hot water due hot vapor continually dripping to the reservoir. These thermostats are cheap and inaccurate and the trigger point must have drifted towards the high side so the water keeps boiling.


In effect it became similar to a coffee machine where hot vapor is continually pushed up to the basket.