A friend of mine owns a RHEEM RTE18 electric water heater, and the thing suddenly stopped working.

He suspects that the following part is the culprit:

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Question, is that part a thermistor, or a simple fuse?

I can't lay my hands on the thing personally at the moment, but I'm first trying to figure out its components.

Here's a picture of the control board.

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I'll later try and see if the thing is using mosftets (it seems like it, there's 4 of them on the upper part of the board) to switch the heating elements. Then I'll check the what type of chips it's using, and do a little probing here and there, first with my MM, and then scope it.

 

Looks like a magnetic reed switch. Most instant hot water systems use either a Hall or magnetic reed switch to sense water flow to turn the heaters on. They will have a section were theres a magnet that moves with water flow. Check wheather the reed switch has continuity when a magnet is but by it. Then check wheather it closes circuit when put back where it was originaly mounted, with water flowing through it. MAKE sure this unit is isolated from the power supply when doing these tests.

 

Here this help you https://www.google.com/url?sa=t&sou...y1iyMqoJzjVpPfFjg&sig2=mo3vVEr-GKlriUmvAbrZxw
It's a magnetic float switch

 

Some troubleshooting hints are at http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Here's three manuals from Rheem ... https://www.manualowl.com/p/Rheem/RTE-18-Series/Manual/174137

 

I install rheem the link I posted is the troubleshooting guide

 

Yes, it definitely looks like one. Here's a picture I've just received.

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So you're telling me it's a magnetic reed switch? Where's the magnet supposed to be?

Here's another few pics.

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Also, the thing is using Triacs, not mosfets. And it seems to be an extremely simple analog control.

 

There SCR yours has 4 of them and the magnet is in the pipe the switch mounts on the the link I posted walk thew how to test yours too.

 

I did describe how to check this, but theres a posability that the magnet is jammed. If it is you will need to remove the inlet pipe & free it up.

 

Thanks all. I finally understand how the thing works. It has two back-to-back pairs of S6065J SCR's in charge of switching the heating elements, with each pair being driven by a MOC3063 optoisolator.

I told my friend to test the reed switch for continuity by allowing water to flow through the heater, and it did function perfectly, so the reed was not the problem. And yes, he did set it back to its original position. Then he told me that only one of the two heating elements suddenly started working. So I told him to remove the board's three chips (the two otpos and an LM324 op-amp) and to gently clean each of their pins using 400 grit sandpaper. And voilá, everything went back to normal, and the unit is once again fully operational.

My guess is that the chip's pins were slightly oxidized due to ambient humidity (the heater's in his basement), and were not making perfect contact in their sockets.

Problem solved... thanks all, especially @be80be for posting the Technical Service Bulletin, it's a great reference.