Water heater thermocuple - what could this problem be?

Factual Questions
1

Last week, the pilot light on my (natural gas) water heater went out. I re-lit it. The next day it was out again. For a few days, I was able to re-light it, but it kept going out. Then one evening I could not get it to stay lit.

I knew enough about pilot lights to figure that it had to be a problem with the “sensor” (as I called it then) that had the job of shutting the valve if the pilot light went out.

So I called a water heater repair company. Unfortunately, I couldn’t be there when he came, so my wife had to deal with him. He did replace the thermocouple (as I now know is the more correct term for it), but he warned my wife that the water heater is old (he claimed it was from like 1980) and so replacing the thermocouple probably wouldn’t be a long-term fix.

Sure enough, the pilot light keeps going out. For the last few days, I’ve been able to get it re-lit easily. But late last night, it just wouldn’t stay lit. I tried and tried, kept the pilot knob pushed down for over a minute, but still as soon as I let up on it the pilot went out.

This morning, however, it lit and stayed lit first time.

I’ve been doing some reading about thermocouples and how they work in a pilot light system, and I’m left with the question of what could possibly be causing the symptoms I’m seeing?

The end of the thermocouple that’s in the pilot flame generates a small amount of electricity. At the other end is a little electromagnet. when the pilot is on, the electricity flows, the electromagnet is powered, and it keeps the valve open.

So if the pilot goes out as soon as I release the knob, it must mean that the valve is closing. And that must mean that either 1) the thermocouple isn’t generating electricity (or isn’t generating enough to fully power the electromagnet), or 2) the valve is dirty or sticky or something, and so even a correctly fully powered electromagnet can’t keep it open.

But in either of those cases, I would expect that the pilot just wouldn’t stay on, period, not that it would fail to stay on in twenty tries last night, and then work fine in one try this morning. Unless maybe the valve stickiness is highly temperature-sensitive. I live in California, and so maybe last night it could have been 60 and this morning 65.

Anyone knowledgable that can shed the light (pardon the pun) on this? The repair guy said we should replace the entire water heater, but that seems just wrong to me when the problem must be in some small valve or electromagnet or something.

2

One other thing to think about, is have you done anything that might cause a backflow in the exhaust vent system that blows out the pilot? Like hooking up a bigger dryer to the exhaust vent or anything, or a backflow valve getting stuck?

3

Ok, that’s one I hadn’t thought of. But I have not done anything in the way of adding or changing an appliance, or touching the vent system.

Maybe the vent somehow got blocked at the other end? Blown leaves or nesting birds? That’s a possibility I could check.

4

Well it’s just something to think about, not very likely. I don’t think they are ever supposed to be vent connected with any other fan driven exhaust anyway, but lazy workmen sometimes do lazy things. Just something I ran into a time or two.

5

I assume you’re also 100% sure there isn’t a draft moving along you’re basement floor. It wouldn’t take much to blow it out.

6

Yeah, pretty sure. The water heater is in the garage, on a raised platform about two feet above the floor. The garage door is routinely kept closed. And there’s no appreciable wind outside. And when I light the pilot, I can see that the flame is steady, and then when I release the button it quickly shrinks back into the tube and goes out. Seems to me all the signs of the gas shutting off as soon as I let go of the button, rather than the flame being blown out by a draft.

7

If the water heater is over 12 years old, stop what you’re doing and replace the whole thing on your schedule. If the heater really is from 1980, you’ve gotten an exceptionally long life out of it and it’s probably just itching to spring a catastrophic leak at the most possibly inconvenient time.

8

It is possible that the new thermocouple is defective

9

I am a plumber/HVAC person.

There is no way to know for sure without seeing it, but my educated guess is that you need a new water heater.

Your research was both good and accurate. In practical terms, however, pilot solenoids (which is what that is) often fail in the way you’re experiencing. They often fail is spurts----they work, they work, they don’t work, and so on. The solenoid just gets weaker and weaker. And…sometimes it works and sometimes it doesn’t. (or works intermittently)

You could replace the whole gas valve (the valve actually contains 2 valves; the main valve and the pilot solenoid), but with the unit being 12 years old it’s probably good money chasing bad money.

It’s certainly possible that you have a bad thermocouple, but highly unlikely. it’s also possible that it’s being blown out. But if nothing has changed in and around the WH I’d say it’s not likely either.

My Guess: a failing gas valve.

My Solution: a new water heater.

10

One other thing…

For the thermocouple to generate electricity, (actually less than 1 volt) you need good impingement. The top 1/4 to 1/3 of the thermocouple must be solidly in the flame, and usually that means the top of the thermcouple will be cherry red.

Ofen a thermocouple is condemned when actually it wasn’t oriented correctly in the flame and the impingement wasn’t sufficient to generate heat/electricity.

11

One other, other thing.

A closed vent damper won’t cause the pilot to blow out. It will just kill you.

“Backdraft” dampers in flues are good intentioned [ostensibly energy saving] devices. But the energy savings are negligible, and the risk of them getting stuck is high.

If you replace the WH, no vent/backdraft damper.

And FYI, in my locale I would charge $200-300 to replace the valve depending on how hard it was. (given it’s age) A new WH would be $700.

The average lifespan of a WH is 8-12 years; although I see some at 15 years. At any rate, this WH is at the end of it’s expected life span.

I understand if money is tight, or you’re just tight. :wink: But I wouldn’t feel bad about replacing a 12 year old WH.

12

WAG, but won’t a brand new water heater be significantly more efficient and likely pay for itself (via savings in gas use vs. older heater) in its lifespan?

13

Probably not.

The efficiency of a standard WH will be similar to the existing unit. There are higher efficient units (easy to identify because they use a PVC flue) but they are significantly more expensive. And there are “on demand” tankless units that are great. But they will be 2-2 1/2 the times the cost of a garden variety WH.

So yea, you can get [significantly] higher efficiencies. But you’ll pay big money—and that spreads out the payback.

14

Thanks, I really appreciate the expert advice. It’s not that I can’t afford a new one, but that it seemed silly to replace the whole thing if the problem was just in some small part that could be replaced. According to my wife, the repair guy quoted $1500 for a new one; based on what you said, hopefully, that was including installation.

If I can stray slightly from GQ, what is your opinion about tankless water heaters? They sound like a good idea, but the gas-fueled variety seem to require special gas lines and venting. But the electric powered ones seem to be (at least currently) more of a niche; I can’t find any place that specifically says they install them. And neither sounds like something I would want to install myself, if only for safety reasons.

15

1500!?! Is your unit a 75 gallon, or power vented high efficiency unit?? I’m in SW Ohio, and the prices here are substantially less than Chicago, for example. But $1500 seems high, even for a place like Chicago.

Go to Lowe’s/Home Depot and check out the prices for water heaters. The Big Boxes have so much purchasing power that their prices are often lower than wholesale houses that sell only to contractors. What that means is that you can buy a WH as cheaply as I can. (in fact, many plumbing contractors get their WHs from Home Depot) A garden variety 40 gallon WH is around $275-350 or so. (of course more for 50, 75 gallon etc) With installation we charge $700-750. I’d like to get more, but competition won’t let me. (We make 4X the profit on a furnace installation) Maybe $1500 is the going rate in your area, but I’d shop that number, for sure. I’ve seen placards in Home Depot stating that they will install a WH for as low as $550!

As to the tankless, the 2 brands I’m familiar with are Rinnai and Bosch. They are both outstanding. They are not DIY units. They are difficult to install if you’re not experienced, particularly the flue. They are expensive, and even the [special] flue material is expensive. We charge around $1700-2200 for one, so compared to a garden variety 40 gallon unit for $750 you’re paying a big premium. Depending on how many people who live with you, and your hot water consumption, I would guess it will take 4-6 years on average to recoup that $1000 premium via lower gas bills. The unit should last 20 years.

If you’re socially concious (one valid motivation for many people), have the means, and plan to stay in the house for a while, a tankless unit is an excellent choice.

16

$1500 to install a normal 40 or 50 gallon standing pilot natural gas heater has to be some sort of “We think you’re panicking with water squirting out everywhere and want the thing done RIGHT NOW!” pricing. I hate to say that it might also be “The little lady knows nothing about water heaters so we can pad the bill royally with “code compliance” items like replacing the gas line all the way back to the meter.” It’s an ugly scam, but I’ve seen it done time and time again.

Even for the Bay Area, a region not known for bargains on anything, $1500 is wacky, and nearly double what Sears was going to charge me two years ago, and that was with a few compliance things like $85 for a plumbing permit from the county and a drip pan. Once I reminded them that the heater was in a garage and local code did not call for drip pans in garage installations, their quote magically dropped by $200. :dubious:

17

Thanks very much, the raindog and gotpasswords. I suspect that the $1500 included both items that gotpasswords suggested. But I might not have bothered to look any further without your input. So, this weekend I’ll be off to Home Depot and Sears for some shopping.

18

You’re welcome.

Let us know how it turns out.

19

When my boiler’s pilot light wouldn’t stay on a few years ago (I had much the same symptoms you do) it was because the pilot’s flame was only half the size it should have been. The boiler dude replaced the tiny gas line leading to the pilot and we’ve never had a problem since. He said that flame was so small it let the thermocouple get too cold.