What could cause the thermal switch on my A O Smith 40 gallon hot water heater to trip? The heater is 3 years old and it has tripped 2 times in the last 3 months. My plumber looked at it and every thing appears to be okay including the Thermal couple. The thermostat is set at a moderate level and the water doesn't get scalding hot. Thank You.

[#$%&]

some good advice given after someone else posted the same question on another forum.

The thermal switch has nothing to do with the water temperature. The temperature inside the combustion chamber operates it. Your heater's intake vents must be clogged with lint or dust.

Doug....The link does not work.

well I just googled "Thermal Switch" problem gas water heater and got a good link.
copied and pasted the do it yourself site and plbg.com didn't upload it. Not good business? ;0


BEVIEVE ME!! plenty of excellent plumbers here on this site to answer and solve this problem with the water heater.

quote; Not good business?

Do you recommend other plumbing companies to your customers? THAT would not be 'good business'.

haha sometimes I do! if the scope of the work fits their ops.

I don't think a clogged vent is the problem. When the heater was installed the old flue was removed and the vent going to the chimney was cleaned and recently my plumber put his hand near the flue when the burner was on and it was drafting good. Yesterday, I put my hand under the spigot with the hot water running and had to remove my hand real fast because it was too hot. I lowered the thermostat from A to a few notches below that and now I can hold my hand under the hot water without removing it so perhaps that's what tripped the thermal switch.

a 'thermal switch' is probably NOT the spill switch mounted on the draft hood

the t swx is probably the hi temp limit on the top tank stat

proven by lowering the temp setting ~ no longer tripping

cause: stratification of water due to usage pattern

don't worry -> be happy

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

bernbeau, the person said the plumber checked the thermocouple. so it has to be a gas water heater.
so it can't have a problem with the top t-stat.
he also talked about a clean chimney..
anyway, i would check the draft without the heater running. also check it at start-up.
just light a piece of newspaper on fire and hold it near the draft hood to see what happens to the flame and the smoke.

the OP said the plumber checked the "Thermal couple" which might be anything

Quote

1
Look for a rectangular control circuit panel on your water heater. Depending on which brand and model of water heater you own, the control circuit panel may be located on the back or side of the appliance.

2
Locate your water heater's thermal reset switch or button. The thermal reset is usually a red or white switch or button at the center of the control circuit panel.

3
Reset the thermal switch or button on your water heater. If your water heater has a thermal reset switch, flip it. If your water heater has a thermal reset button, press and hold it for up to 4 seconds to reset your appliance.

a thermal reset is a thermal reset ~ it is a high water temperature safety sensor regardless of its location

a flue spill switch, while it may act on temperature or CO, is a 'back-draft' safety reset (if no proper 'upflow' it would PROBABLY get hot) therefor........

pps. Therm-O-Coupling was once a trademarked name for a high temperature industrial switch assembly which eventually was used as a pilot light safety device

==============================================

"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638



Edited 4 times.

It probably has nothing to do with the "Vent", and absolutely nothing to do with the water temperature. IF the air inlets are clogged with lint the combusion chamber will overheat and trip the thermal switch.

quote; the t swx is probably the hi temp limit on the top tank stat

Only electric heaters have a "top tank stat", but they do NOT have flues and vents.

The "thermal switch" is part of the FVIR safety system and is usually resettable, although some are not.

LOL?number model of heater/number serial of heater.just asking/LOL



Edited 1 times.

Since I do not know of any "ops" that would be beyond my scope, but within theirs, I do not "refer" my customers to them. Although I have told some that if they want a cheaper plumber, I can give them a couple of names.

Hi, new poster here with a similar problem. Our heater is a Bradford White natural gas model, similar or the same as the A.O. Smith units. The problem is, burner shuts off after 10-15 minutes of heating. The unit looked new when we bought the house 6 years ago. It has always served us well. This problem started shortly after we returned from a 10-day vacation, however it was still heating when we got back, so this may be coincidence.

The initial symptom was the pilot light wouldn't start. I tested the thermalcouple, which worked to spec. I then replaced the thermostat/gas valve, which didn't fix the problem until we found the thermal safety switch had tripped. Pilot lit, then burner came on - but it goes out after 10-15 mins. We can reset the switch and it will work fine until it trips.

Checked the flue and it's clean. Passes smoke test, no evidence of backdrafting. Is it worth trying to replace the thermal switch? I can't think of what else might be tripping it. Any ideas as to why the burner would run hot? I looked for intake vents/screen to clean, but it appears the air just comes in the bottom, through large ports around the base of the unit.

Thanks for any help!

There is something that looks like a wire mesh underneath the burner (flammable vapor protection, FVIR?) and over time it clogs up with lint or dust and the switch trips. Believe it or not, I keep an old brush with a long handle to get it under there and clean it. It has worked several times over the years and saved me a lot of money for plumbers that come in and tell me they have to change the thermocoupler for $300.