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PJHoffnet · 22-11-2019, 04:56

I have a 2010 Atwood Model EHM11-SM 110VAC water heater that supplies great engine assisted heated water, but no longer provides hot water on it's own.

As shown in the image, I have good voltage all the way to the element, so I'm fairly confident that the safety switch and thermostat are operating correctly. There is no voltage present on the downstream side of the heating element. The heating element reads 10 ohms of resistance (I can't find any info on what it should read).

I've sourced the termostat (figured I'd change it out with a new one while I had the take drained) and heating element, but haven't come across a safety switch (part # 92942).

So, I'm fairly confident I need a new heating element. Am I missing anything, or wrong on my conclusion? TIA.

**s/v Jedi** · 22-11-2019, 05:07

I’m afraid your conclusion must be wrong. When you measure 10 Ohms resistance over the heating element then it is fine. I bet there’s a safety switch somewhere....

Measure resistance over the hit and neutral wires that feed the water heater. Try to find where the break is.

billknny · 22-11-2019, 05:23

I’m curious, when you are checking the voltage at the neutral side of the heating element how did you do that and what did you expect to find?

A proper voltage here would always be very close to ground, higher only by voltage drop in the neutral wiring. So zero colts here is GOOD. Line voltage would indicate a broken neutral somewhere.

If you disconnect the neutral, then you should see line voltage on the neutral side of the element when it is on.

PJHoffnet · 22-11-2019, 13:13

Quote:

Originally Posted by s/v Jedi

I’m afraid your conclusion must be wrong. When you measure 10 Ohms resistance over the heating element then it is fine. I bet there’s a safety switch somewhere....

Measure resistance over the hit and neutral wires that feed the water heater. Try to find where the break is.

There is a safety switch ... it's the first switch in the series, called "High Temp. Limit Switch" in the attached diagram. That seems to be 'good' because there is voltage on both sides of the switch???

Quote:

Originally Posted by billknny

I’m curious, when you are checking the voltage at the neutral side of the heating element how did you do that and what did you expect to find?

A proper voltage here would always be very close to ground, higher only by voltage drop in the neutral wiring. So zero colts here is GOOD. Line voltage would indicate a broken neutral somewhere.

If you disconnect the neutral, then you should see line voltage on the neutral side of the element when it is on.

I checked the downstream (neutral) side of the element to ground and it showed 0VAC with both the hot and neutral wire attached to the element. I'll disconnect the neutral, and measure again to ground from the downstream side of the element.

===
I think I'll also jumper out both the High Temp Limit Switch and Thermostat and just let the current flow across the element to see if it will heat up then. If it does, I'll jumper out the switch and thermostat separately to see if it points to one of them being bad. As noted in the OP, I've already sourced the element and thermostat, still trying to find a High Temp Limit Switch.

Dsanduril · 22-11-2019, 13:47

Quote:

Originally Posted by PJHoffnet

... The heating element reads 10 ohms of resistance (I can't find any info on what it should read).
...

The EHM11-110 is rated 1400W@110/120V (depends on what spec you read).

1400/110 = 12.7A, 110/12.7 = 8.67 Ohm (or 110^2/1400, same thing)

1400/120 = 11.67A, 120/11.67 = 10.3 Ohm

Sounds like the heating element is measuring about right.

PJHoffnet · 22-11-2019, 20:51

Quote:

Originally Posted by Dsanduril

The EHM11-110 is rated 1400W@110/120V (depends on what spec you read).

1400/110 = 12.7A, 110/12.7 = 8.67 Ohm (or 110^2/1400, same thing)

1400/120 = 11.67A, 120/11.67 = 10.3 Ohm

Sounds like the heating element is measuring about right.

Thanks Dsanduril ... it's been a long time since I put the old P=IV and V=IR to use. Not sure why I wasn't smart enough to think about that.

I guess what I just can't figure out is if there is voltage to the hot/upstream side of the element why it's not heating the water. If the safety switch or thermostat weren't working, there would be 'trons getting to the element - right?

Dsanduril · 22-11-2019, 21:04

What is the voltage across the element (from the last green check to the red x)?

Should be pretty close to 110 (with slight allowance for wiring voltage drop).

It doesn't quite add up, resistance looks right and voltage seems like getting power. If you have a bad connection on the neutral side would expect voltage at the red x.

**GordMay** · 23-11-2019, 02:09

Testing for Ohmic Resistance (cold element):
Volts squared divided by watts = ohms

Cold resistance: measuring ohms across both screw, when no wires are attached to element and element is not heated.
Hot water in tank raises resistance on element.
Let element cool before doing ohm test, so you are measuring cold resistance.

PJHoffnet · 23-11-2019, 04:29

Quote:

Originally Posted by GordMay

Testing for Ohmic Resistance (cold element):
Volts squared divided by watts = ohms

Another indicator that the element is good.

Frankly · 23-11-2019, 06:29

Keep in mind that often making measurements with a high impedance voltmeter (typical modern VOM) on AC power circuits you can come up with some confusing results.

A high resistance in a series switch will limit any useful current flow yet when checked absent significant load current (ie voltmeter) you will see near full expected voltage.

Surest way is to power everything up (carefully only thing deadly as a heart attach is 120 VAC), voltage across the hit temp switch and thermo should be zero, voltage across element 120.

Frankly

sailone3032 · 23-11-2019, 09:26

If I had to guess, I woulf look at the overtemp protection. I work with heaters all the time and the overtemp is usually the issue with no heat. when you measure the element resistance it is best to disconnect both sides. Jumper out the over temp is good for a test but is is there for a good reason and should not be eliminated from the circuit. The other otrion is the thermocouple that measures the temperature could be broken. To test this you take the continuity across the temperature sensor and if it shows a open circuit replace the thermocouple which is an inexpensive item.

ranger58sb · 23-11-2019, 14:05

Quote:

Originally Posted by PJHoffnet

I have a 2010 Atwood Model EHM11-SM 110VAC water heater that supplies great engine assisted heated water, but no longer provides hot water on it's own.

I've sourced the termostat (figured I'd change it out with a new one while I had the take drained) and heating element, but haven't come across a safety switch (part # 92942).

So, I'm fairly confident I need a new heating element. Am I missing anything, or wrong on my conclusion? TIA.

We have the same Atwood water heater. And I've had to replace the thermostat/ECO assembly three times now over this boat's 17 years or so... and the heating element once, the first time I tried to troubleshoot. It turned out I probably didn't need to replace the element after all; the problem was likely only the thermostat/ECO assembly.

Check voltage at black and white, i.e., without the thermostat/ECO assembly in the mix. If you've got 110V there, I'd replace the thermostat/ECO assembly first, since its likely easier. If that fixes it, you're good. If not, replacing the element wasn't all that difficult if you can get to the cover screws.

Our original Atwood part number for the 110V assembly is 90037, or the more current number you might find at vendors after transition to Dometic is 92052. The parts are sold as a unit, with plate wires, ECO (top sensor) and thermostat (bottom sensor). Often available at RV dealers, or online.

-Chris

PJHoffnet · 24-11-2019, 16:29

Quote:

Originally Posted by ranger42c

The parts are sold as a unit, with plate wires, ECO (top sensor) and thermostat (bottom sensor). Often available at RV dealers, or online.

-Chris

Thanks Chris ... Jumpered around both the safety switch and thermostat and the heating element worked just fine - hot water in less than about five minutes.

Checked continuity across both the safety switch and thermostat (wires off, just the switch & thermostat themselves (separately of course)) - both of those were SAT. So, even though they seem to be ok, I'm just gonna replace the switch and thermostat and call it done.

I'm pretty sure my ECO and Thermostat are 'separate' and not on a single plate, but I'll take a look at it tomorrow when I'm on the boat.