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Old 04-13-2021, 11:46 PM   #1
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Purpose of 12 volt limit in Atwood water heater

I have an Atwood EHM6-SM water heater made in July 2006. This model is functionally same as a Seward F600 model with engine coolant assisted heating and only 120 volt AC heating element. No propane.

The 120v feed passes through a limit switch and the T-Stat then direct to the heating element. There is no control board anywhere in the system. There is also a 12 volt feed from the chassis battery to what appears to be limit or other safety device. The ground lead from this device drops below the floor and I "assume" it goes to a chassis ground, but have not traced this wire.

My question is what is the function of this 12 volt device at the top of the image. Sorry the imqges are all sideways. The unknown device is where the red and black wires attach from the wire loom. The wiring diagram does not show this device or these wires, only the 120 volt leads. The limit switch is lower left and T-Stat to the lower right in these images with the heating element to the extreme right.

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Old 04-14-2021, 12:00 AM   #2
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(MY Suspicion) Most all 120VAC DEVICES have a REMOTE SWITCH and this switch is 12vdc powered for safety, goes to a 30a/40a RELAY to turn ON/OFF?
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Old 04-14-2021, 03:12 AM   #3
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I cannot identify the device, can't see it.

But a black box with control wires going in as well as the 110 leg would be a simple relay so you don't run 110 volts to your switch. Keeps the control voltage at a safe battery/12 volt level.

Quick glance would indicate the 110 path is throug the relay and then through the two T-stats. One would be for the normal temp range of the water heater and the other would be the emergency shut off one if the temp control one went wild and stayed shut.

I recently did a great mod to my water heater. Both T-stats were changed as well as the heating element. The new heating element does not get it hotter but quicker. The new T-stats let the water get hotter though.

The water outlet is then connected directly to a mixer valve that regulates the output to whatever temp I deem safe. Now showering does not require chasing the shower valve up and down for comfort. The extra hot water being mixed with cold gives a longer shower time for sure and the heating element recovers faster.

My wife absolutely loves this mod. Me too.

Water at any of the faucets will not burn but are uncomfortable at the highest temp of just hot water valve turned on.
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Old 04-14-2021, 12:07 PM   #4
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Quote:
Originally Posted by THenne1713 View Post
(MY Suspicion) Most all 120VAC DEVICES have a REMOTE SWITCH and this switch is 12vdc powered for safety, goes to a 30a/40a RELAY to turn ON/OFF?
Nope. The water heater's on/off switch is line voltage (120), verified that. It goes from the 120v breaker - switch - heater.
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Old 04-14-2021, 12:13 PM   #5
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Quote:
Originally Posted by YC1 View Post
I cannot identify the device, can't see it.

But a black box with control wires going in as well as the 110 leg would be a simple relay so you don't run 110 volts to your switch. Keeps the control voltage at a safe battery/12 volt level.

Quick glance would indicate the 110 path is throug the relay and then through the two T-stats. One would be for the normal temp range of the water heater and the other would be the emergency shut off one if the temp control one went wild and stayed shut.

I recently did a great mod to my water heater. Both T-stats were changed as well as the heating element. The new heating element does not get it hotter but quicker. The new T-stats let the water get hotter though.

The water outlet is then connected directly to a mixer valve that regulates the output to whatever temp I deem safe. Now showering does not require chasing the shower valve up and down for comfort. The extra hot water being mixed with cold gives a longer shower time for sure and the heating element recovers faster.

My wife absolutely loves this mod. Me too.

Water at any of the faucets will not burn but are uncomfortable at the highest temp of just hot water valve turned on.
As in the above reply there is no relay in the 120v heater circuit. My "guess" right now is this is a heater sensor (limit switch) that closes a remote valve on the hot water line coming the engine that provides the engine assist heating in the tank. The black wire goes into a wire loom that goes forward and is difficult to trace among all the other wire looms in that route.

I've added a red circle around the mystery device. The red wire comes from a known chassis battery hot wire. The device looks pretty much like a limit switch without a reset button, same size.

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Anyone with experience with engine assisted water heating may have seen this setup. Perhaps more common on boats than RVs.

I agree with the idea of adding a mixer valve. We have one in our 5th wheeler and love it there. Presently water temp in this rig is 130F at the tap and so I would also need to replace the limit switch and T-Stat to get it up to a temperature where a mixer is worth it.
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Old 04-14-2021, 05:13 PM   #6
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The circled thing is a T-stat. You may have one for gas and and one for electric on some models. Actually two for each. Each one might have a high temp limit and one for the normal range of temps.
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Old 04-14-2021, 05:57 PM   #7
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Quote:
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The circled thing is a T-stat. You may have one for gas and and one for electric on some models. Actually two for each. Each one might have a high temp limit and one for the normal range of temps.
This model is engine assist and electric only. No propane.

The primary high limit and T-Stat are on the 120 volt side. The device in question only has 12 volts to it. After hours of web searching I have not found any reference to such a device.
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Old 04-15-2021, 05:21 AM   #8
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Even though yours is electric and engine heat only it could have come prepared for gas operation. Based on two set of t-stats it is possible.

It would be interesting to see the front of the tank where a burner might live if so equipped.

Maybe someone previous changed the tank?

Looking at the video of one it has only one set.
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Old 04-15-2021, 08:37 AM   #9
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Quote:
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Even though yours is electric and engine heat only it could have come prepared for gas operation. Based on two set of t-stats it is possible.

It would be interesting to see the front of the tank where a burner might live if so equipped.

Maybe someone previous changed the tank?

Looking at the video of one it has only one set.
This model does not include any gas options. The "front" of the tank is totally blank and smooth. Nothing to see except white metal panel. It looks like the one in the video, which I'd seen before, except has another T-Stat in the empty hole near the left edge of the bracket.

As I've said none of the diagrams indicate a second T-Stat. The ground side of the T-Stat goes forward towards the engine in a maze of wire looms.
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