Dometic WH-6GEA Water Heater Keeps Blowing Thermal Fuses

[shawncberg]

We have a 2021 Reflection 311BHS fifth-wheel that's just under a year old. Every other trip it seems to blow a thermal fuse. I keep spares on hand and it's always fine for a while after I replace one, but we're on our fourth fuse and it's not even a year old. I've also noticed on our vent cover that there is some brown soot, so I think it must not be burning clean. I found that a lot of these have air adjustments, but this model doesn't seem to have that as far as I can tell. I'm going to check it for bugs/debris, but I don't expect to find anything and also am unsure how to get it apart to properly check and clean.

Any ideas/advice?

[TedS]

It looks like the air shutter is missing. Between the gas jet and combustion tube.

[JimB33]

I have one of these in my 2022 2800BH. Blew the thermal fuse on the third night of the first trip out. Somewhat thankfully, this is a very common problem so it only took me a minute or two of Googling to find out that the thermal fuse was probably my issue. Since the fuse is inline with a male 1/4" spade terminal on one end and a female on the other, so I just took the fuse out completely plugged the end of the red wire directly into the thermostat. I left it hooked up this way during the day as long as we were at the site and I left the cover open and felt the wiring every once in a while to see if it was getting warm (never did). I unhooked the wire if we left the site and at night while we slept. When I got home, I bought a pack of 10 on Amazon and (knock on wood) have not had to replace it again. I did bend/route everything up as far as I could to get it as far away from the flame tube as possible, since the fuse blows at a relatively low temperature (90 C).

Every trailer I've had previously has always had a Suburban water heater and I never had an issue in 35 years with any of them. They don't have this type of fuse in their system.

I'm not very impressed with the Dometic/Furrion AC/Furnace/Water heater appliances in this unit - have had some type of annoying problems like this with all 3.

[shawncberg]

I have one of these in my 2022 2800BH. Blew the thermal fuse on the third night of the first trip out. Somewhat thankfully, this is a very common problem so it only took me a minute or two of Googling to find out that the thermal fuse was probably my issue. Since the fuse is inline with a male 1/4" spade terminal on one end and a female on the other, so I just took the fuse out completely plugged the end of the red wire directly into the thermostat. I left it hooked up this way during the day as long as we were at the site and I left the cover open and felt the wiring every once in a while to see if it was getting warm (never did). I unhooked the wire if we left the site and at night while we slept. When I got home, I bought a pack of 10 on Amazon and (knock on wood) have not had to replace it again. I did bend/route everything up as far as I could to get it as far away from the flame tube as possible, since the fuse blows at a relatively low temperature (90 C).

Every trailer I've had previously has always had a Suburban water heater and I never had an issue in 35 years with any of them. They don't have this type of fuse in their system.

I'm not very impressed with the Dometic/Furrion AC/Furnace/Water heater appliances in this unit - have had some type of annoying problems like this with all 3.

Yeah, if I only had to replace mine once I wouldn't be terribly concerned, but I'm on my fourth fuse in a year and we only use the rig every few months. I also moved it as far away from the flame tube as it would reach, but that hasn't seemed to help.

[Country Campers]

shawncberg

It looks like in your picture that the fuse is directly above the opening on the right side of the burn tube, if the lift side of the tube is covered on top try to move the fuse as far left as you can. Getting the fuse away from any heat rising from the burner tube will help. Also this model of water heater does not have an air shutter, so yours is not missing.

Brian

[howson]

It looks like the air shutter is missing. Between the gas jet and combustion tube.

That's the way the new shutters are configured, Ted. No adjustment available.

[howson]

shawncberg

The ECO ("energy cut-off", or as documented in the wiring diagram posted below for your unit, the "Thermal Cut-Off") is a safety device that opens when the thermostat fails to turn off the heat source. Both the ECO and thermostat are physically connected to the tank and measure temperature of the tank. Moving the ECO to another spot should not make any difference--the ECO will open if the tank temperature exceeds 180F.

For the ECO to open (exceed 180F) the thermostat must not be opening at 140F as designed. Therefore the most likely failure point is the thermostat does not always open at the right temp (replace it).

Bypassing the ECO safety device...that's an individual choice. Similar to using a paper clip to bypass a blown fuse. Everyone has their own tolerance for risk. (I might do it during troubleshooting but would not leave the unit on without a fuse--guaranteed I'd get sidetracked and would forget.)

For the soot comment, about the only check I know of is to verify the propane's 11" of WC while in operation. You're right about the burner tube--the manual even specifically addresses the issue.

[Melsman]

shawncberg

The ECO ("energy cut-off", or as documented in the wiring diagram posted below for your unit, the "Thermal Cut-Off") is a safety device that opens when the thermostat fails to turn off the heat source…

I think there is some confusion here. The E.C.O. (Emergency Cut Off) thermo switch is a separate safety device located in the upper central part of the schematic diagram howson provided. It is a separate device, physically collocated with the thermostat, but on a different circuit that only shuts off the gas if a temperature of 180 (80 C) is experienced by the water tank. The Thermal cut off fuse is a separate safety device wired in series with the thermostat that separates when temperatures above 90C are experienced in the water heater housing/control circuit area (not necessarily the tank) and appears, in the case of these water heaters, to be too sensitive for the mounted location (i.e. too close to the burner end of the burner tube) and thus blows practically with each use! ( Mine blew on first use as well.)

To solve this issue - given standard replacement parts, and realizing that the thermostat and thermal fuse are “polar agnostic” -I’m going to try moving the replacement fuse to the right thermostat terminal to move it further from the burner tube, but still close enough that a severe overheat situation will trigger it.

I’ll post my results.

Melsman

[Melsman]

As a follow-up… I connected the thermal fuse to the right-side thermostat terminal and oriented the fuse further to the right and slightly downward away from the burner tube. I tested the HWH and it fired up immediately, heated the water properly, and did not blow the replacement fuse.

So far, so good.

Melsman

[Scott'n'Wendy]

What is the purpose of the thermal cut-off? If it is a sacrificial device calibrated at 90*....it certainly has nothing to do with water temp or flame tube temp. Is it just for sensing ambient temperature for the heater control board? (electronics?) If that is the case, it is certainly mounted in a poor place.

[Jlawles2]

Scott'n'WendyI believe if you look at Howson's post, the thermal fuse and the temp cut off are in line. In the event the thermostat fails, the cut off SHOULD open and kill power to the system.

[Scott'n'Wendy]

Scott'n'WendyI believe if you look at Howson's post, the thermal fuse and the temp cut off are in line. In the event the thermostat fails, the cut off SHOULD open and kill power to the system.

Yes, but look at the placement and temp ratings. 90* for the cutoff is well below the thermostat temp of approximately 140*. The 90* thermal fuse is not anywhere near the water tank, it is in the open air by the electronics,..so it is not sensing water temp anyway. To me it seems it is there to monitor temperatures external to the tank. Which suggests the control board.
A schematic shows where it is electrically and shows what will happen if activated, but not it's design purpose.

[TedS]

My understanding is the thermal fuse is to sense external burner fire in the event of burner failure to contain its fire.

[howson]

Yes, but look at the placement and temp ratings. 90* for the cutoff is well below the thermostat temp of approximately 140*. The 90* thermal fuse is not anywhere near the water tank, it is in the open air by the electronics,..so it is not sensing water temp anyway. To me it seems it is there to monitor temperatures external to the tank. Which suggests the control board.
A schematic shows where it is electrically and shows what will happen if activated, but not it's design purpose.

The fuse is there to melt should flame "escape" from the intended path from valve to burner. A shot of my heater is below--note how the fuse is above the air shutter.

You may wonder why the thermal fuse and ECO are not in the opposite location on the schematic. At least I did--makes no sense to me that the ECO only protects the gas side. If the thermostat fails to open when on electric the element will continue to heat the water indefinitely. I thought the schematic was labeled wrong, but a close inspection of the wiring shows the schematic is accurate.

From a 2007 Atwood (now Dometic) training manual:

Current Atwood direct ignition water heaters are equipped with a thermal cut-off device. This device is located on the incoming power wire and is connected to the thermostat. The thermal cut-off is designed to permanently break circuit and shut down the water heater before excessive heat can cause damage due to obstructions in the main burner tube or flue tube caused by spiders or mud wasps. These obstructions can cause the main burner flame to burn outside the main burner tube. When the flame or the heat from the flame contacts the thermal cut-off, the circuit will open.

To really blow your mind, Suburban does it differently. There are separate, distinct thermostats and ECOs for both the electric and gas circuits (and no thermal fuse).

Howard

[Scott'n'Wendy]

Thanks for the feedback Melsman TedS howson
So if the thermal fuse opens, before replacing, I'm thinking it would make sense to temporarily jumper the thermal fuse and observe the water heater flame to see if it's burning correctly before fuse replacement?
It's been a good day..I haven't had breakfast yet and already learned something today. Think I'll pick up a spare for the parts bin soon.