Resistance of thermal switch in thermocouple - does it matter - hot water heater - generic replacement

Does the difference in resistance matter if you replace a 24-inch 30mv thermocouple that has a thermal switch versus one without the thermal switch?

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I bought a generic Honeywell 30mv 24-inch thermocuple (PN CQ100A1013) from Ace Hardware:

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That picture in the URL above is wrong because it's 24 inches:

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When I went to put it in the hot water heater, I belatedly realized the original thermocouple has an integral thermal switch.

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I know the length matters, from a resistance standpoint, but the hardware store owner had never heard of a thermocouple with at thermal switch before:

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The thermal switch is a safety feature that will shut down the pilot if the heat under the hot water heater gets too great:

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When I call Sears in San Jose at 408-274-2593, they confirm the part number is PN 9000056015 but that replacement part doesn't seem to have a thermal switch on it.

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Have you ever had to make the decision of whether to replace a thermocouple that had a built-in thermal switch with a generic thermocouple that had no thermal switch? Did the resistance match?

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Reply to
Danny D.
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I'd say the resistance doesn't matter because there probably is no material difference in resistance. But without the thermal fuse, whatever safety that it provides is gone. In other words, it should work, but without the additional feature. Does it? I guess the question comes down to is what you wind up with the same as my WH and most others that don't have this or does my WH and others have some other way of thermal cut off instead that yours doesn't have, thereby leaving yours unsafe? IDK

Reply to
trader_4

IDK either.

The book resistance of the Honeywell CQ100A1013 thermocouple is apparently

0.2 ohms but that's all I could find out by way of technical specs from either Sears or from Honeywell.

Sears confirmed the replacement part number is 9000056015, but when I look up *that* part number, none of the pictures show the thermal switch.

The fit of the Honeywell CQ100A1013 "universal" thermocouple isn't even close to the same as the original fit either as the mounting for the original is screwed on with a plate while the "universal" Honeywell thermocouple is a screw on nut which doesn't have a corresponding mating female.

Therefore, the "universal" thermocouple isn't even close to universal, although I can probably bend some metal to make it fit - I'm basically making my own parts out of raw materials.

When I called Honeywell technical support at 800-468-1502x1x1x9x1 to find out if they have a closer replacement than the Honeywell CQ100A1013 that I bought, they drove me nuts in so much as they don't even understand the simplest thing about the product they sell.

All they could tell me is that it has to go 1/2 inch into the flame.

When I asked for a thermocouople with a theroswitch, they told me to use the Q340A1074 but that doesn't seem to have a thermoswitch either.

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So I'm basically making my own parts from raw materials because the mounts aren't even close on *any* of these so-called replacement thermocouples.

Reply to
Danny D.

I concluded the "universal" mount stood zero chance of fitting.

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There is just no way a "universal" thermocouple is going to stay put there.

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Just look at this bracket for my original thermocouple.

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For now, I sandpapered the thermocouple and put it back in, but I think I'll either have to find an exact replacement or build my own bracket assembly.

I can't believe everyone runs into this fit problem. Do you?

Reply to
Danny D.

According to this A O Smith webpage, see page 18 at

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"the thermocouple is not a separate replaceable part."

"There is no routine service associated with the TCO"

"The TCO is an integral part of the thermocouple and not replaceable as a separate item. A pilot burner assembly must be reinstalled."

See picture and instructions on page 20

Not knowing your specific model & serial numbers, 2 possible parts are shown at

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P/N's 9006013005 or 9006014005

Reply to
Retired

Why do you think the resistance matters? Did you put a replacement one without the thermal fuse in and does it work? If it does, end of story on the resistance part.

The real issue I see is if this is just additional protection or is it protection in place of other protection that a typical WH would have? I would guess it's probably additional, but IDK and it's your WH.

Reply to
trader_4

No, because this is the first time I've seen or heard of this thermal fuse. But my last WH is 15 years old and IDK what's in new ones.

Reply to
trader_4

You left out the best part. One of those is $117, the other $144!

Doh!

What's a regular TC cost? $15? Looks like they changed a design from a cheap, common TC to this thing that costs 10X. How much additional protection it provides, is it really necessary and/or worth it, IDK. All I know is they were making WHs for how many decades with the old style and I didn't see any widespread carnage. I thought the biggest issue was either blocked flues generating CO or idiots using gasoline or similar solvents in close proximity.

Reply to
trader_4

On Sat, 20 Aug 2016 05:58:32 -0700 (PDT), trader_4 wrote in

+1
Reply to
CRNG

Thank you for locating that training information as it sure does seem like they designed the $10 thermocouple to not be replaced easily!

The existing thermocouple is attached at two points, one of which is screwed on at the S-shaped 180 degree C thermoswitch (which seems easy to remove) but the other of which is seemingly firmly attached to a plate that also has the pilot light tube just as firmly attached.

So it seems that the thermocouple can't be 'wiggled' out, although I haven't tried using real force yet.

It's only slowly dawning on me that the thermal cutoff switch (TCO) is making the thermocouple not "universal".

That manual was *fantastic*. Thank you for finding that!

I'm not sure how you knew I had an AO Smith hot water heater, but I do.

I called the residential service number listed in that manual at

800-527-1953 (x2 x2 x2 x1) and spoke to a very nice support person who explained that there are two filters that need to be cleaned just as described in that wonderful training manual.
  1. Cordorite (she called it chromatic) anti-flame filter
  2. LDO (she called it metal mesh) dust and anti-flame filter

The lady at AO Smith asked for my serial number, and given that, she told me what you said, which is that the thermocouple with Thermal Cutoff Switch is not replaceable.

She said I need the $88 MSRP pilot assembly PN 9003455005 which she said I should be able to get in San Jose at:

- Slinky Brothers 408-494-7948 (7-4:30 M-F)

- RV Cloud 408-378-7948 (they don't sell AO Smith anymore they said to try a place named "rubenstein on monterey highway who is not open on weekends)

- TW Smith 408-249-9880 (they're not open on weekends)

Reply to
Danny D.

I think resistance matters because we're talking millivolts here, but most people, from what I've read, seem to match resistance simply by matching length - so for me - that part is easy since it's a 24-inch long thermocouple.

I'm ok without the "additional protection" which, as I understand it, only comes into play when the bottom of the water heater is at a certain temperature high enough to blow the thermal fuse.

Since I can read a "180" on that thermal switch, for now I'll assume it triggers at 180 degrees although I don't know if that is C or F.

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Given 180C = 356F and that 180F = 82C, I guess it could be either way as neither temperature should happen in a house - but it's probably C.

Rest assured, I would love to put in the $10 24-inch 30mv Honeywell CQ100A1013 "universal" replacement thermocouple - but it just will never fit unless I entirely rebuild the bracket assembly.

Reply to
Danny D.

After calling the AO Smith residential support phone number at

800-527-1953, I found, after giving the nice lady my serial number, that the part number I need is the $88 MSRP pilot assembly P/N 9003455005.

Unlike cars, I have found out that MSRP is generally *lower* from the manufacturer than the parts suppliers in San Jose, so I expect it to cost more if sourced locally.

Even so, I called the dealers she suggested, only one of which was open on a weekend, who himself, no longer sells AO Smith parts. So I'll have to wait for the weekend to find price and availability at local sources.

Googling, the pilot assembly seems to be a whole bunch of stuff:

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Reply to
Danny D.

Thanks to Retired, the moment I talked to the nice lady at the AO Smith phone number in his training manual, I realized that the thermal fuse is a

180C thermal cut off which self resets after a few minutes.

But I don't think the TCO is my problem. I think my problem is the thermocouple but sandpapering the 2-inch thermocouple last night seems to have helped keep the pilot going so I am ok until Monday when I can call local suppliers for the $88 AO Smith pilot assembly part number 9003455005.

However, what I didn't know was that the nice AO Smith lady told me that I should be cleaning two filters on the water heater every six months, which are the flame arrester (non replacable) or filter screen (replacable) which she said is explained in a video here:

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She also referred me to a filter-cleaning PDF here:

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Reply to
Danny D.

You're welcome

FWIW, I did a Google Images "Search by Image" of one of your pics, and got a good hit on an A O Smith web page. (google is your friend ;-) )

Reply to
Retired

That manual was great. Nice color pictures. Nice explanation. And it had the all-important AO-Smith 800 number which is really good (and far better than Sears and Honeywell, both of which were horrid).

I'm amazed that your image search worked, but I appreciate you doing that as the sticker on the side of the AO Smith water heater had a sears number so I had called them first (and wished I had not) since they gave me the wrong part number (which I could tell was wrong from looking up what it looked like).

I called the AO Smith people back and got another nice person, who kindly is sending me, gratis, an AO Smith Filter cleaning kit PN 9006477005.

Googling, here is a picture of what I think they're sending me:

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That google search also found this 2015 AO Smith parts catalog:

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This seems to show two part numbers for the propane pilot assembly:

9003455005 & 100109243

I wonder if I should add the sacrificial anode to my list of parts to get?

Mine is under the hot water nipple, where the part number includes the hot water nipple where a minimum order is $50 at (800) 433-2545 AO Smith Sacrificial Anode 9009148005 $38 + $10 UPS Ground

Do you guys replace anodes at around 10 years?

Reply to
Danny D.

I'm not a plumber so I don't know if the resistance matters or not, but I do know that the thermocouple generates 30mv which is puny so any additional resistance to the 24 inches of wire could make a difference and I don't know what the resistance of the 180C thermal cutoff (TCO) is.

If the 180C thermal cutoff adds appreciable resistance, then the resistance of a 24-inch universal thermocouple will be different than the resistance of this 24-inch thermocouple with TCO such that it won't be 30mv anymore.

But putting a replacement thermocouple seems to be far more difficult than people make it out to be, simply because AO Smith designed this as a non-replaceable part. It can only be replaced as a "pilot assembly", part number 9003455005.

Reply to
Danny D.

trader_4 posted for all of us...

Thank government regulation.

Reply to
Tekkie®

Just to report back, the sandpapered thermocouple has been working, so I have time to order the correct parts, probably from AO Smith themselves as they have an order number for parts:

When I buy the pilot assembly, should I buy an anode?

I have hard water (calcium carbonate rich). How often do you replace your anodes?

Reply to
Danny D.

I don't. There are two competing theories on this. One is that replacing it will extend the life. The other is that the tank is still going to fail at about the same time from other effects anyway. If you want to replace it pull the old one part way out to see how much is left, do that every few years to keep an eye on it. I just went the lazy route. So far, about 15 years on a garden variety State.

Reply to
trader_4

Danny D. posted for all of us...

I don't have that type heater. You may want to see if you can get the anode out. Might be a problem. I think I would procure the anode locally as I think shipping may be expensive.

Reply to
Tekkie®

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