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My just over 2 year old, professionally installed gas hot water tank frequently loses the pilot light. I've had the installer come out 4 different occasions to look, and every time, he's stumped at what the problem could be. He blames it on the pilot light blowing out because of wind gusts, or dust, or starvation or any number of things that are not tank related, and therefore not his responsibility.

As this was my childhood home that I just bought from my parents, I know the previous two hot water tanks lasted a total of 36 years, in the exact location, under the exact same conditions. The pilot light on those two went out exactly once, and that was when gas to the whole neighbourhood was shut off after someone was digging where they shouldn't have been.

Yet, this tank has shut off at least 8 times in under 30 months.

I've recently discovered that it is a limit switch that is causing the pilot light to go out. I can't prove it, but I don't think the tank is getting too hot; the water temp is kept quite low (slightly under 120 degrees), it seems very consistent temp when you use the hot water (aka it not's violently hot from time to time), there is excellent fresh air intake, a smoke test shows the flue is clear, etc.

It's just this little switch that pops:

http://www.johnwoodwaterheaters.com/en/parts_catalogue?part=315358-001&lookup=view

enter image description here

Once I press the the little tab on the top right side of the switch, there's a physical 'click' and then then the pilot light can be lit once more.

I've told the plumber about the switch, and he says that it's impossible that it could be the "high limit" because I'd have to replace the whole gas valve. When I show him that is this little switch, and I can reset it it, he tells me that the switch is not resettable.

So, my question is—besides needing a new plumber—do these little switches ever go faulty? Can this switch, which is 100% resettable just be a lemon? Is it as simple as ordering the part and replacing it by re and re-ing the two screws that hold it in?

The pilot light has gone out 3 times in the the last 3 days, including twice today (once over night when no one was using any hot water!) and each time I have done the following little routine:

  1. Attempt to relight the pilot, with no success. (The light never blinks, and the pilot will not stay lit)

  2. Remove the cover, reset that little switch.

  3. Light the pilot light, and it flashes the light after just a few seconds.

I figure that if I can easily replace the limit switch, I can eliminate it as the source of the issue. If the tank is actually overheating, the new limit switch will pop too, right?

If it helps, the exact model of hot water tank is here:

http://www.johnwoodwaterheaters.com/en/parts_catalogue?part=B4320&lookup=part_list

By the way, sorry for the novel. I've just been so frustrated about this tank, I wanted to make sure I got all the details out. :)

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That limit is part of the Flammable Vapor Ignition-Resistant (FVIR) feature, which is required on all modern water heaters (since July 2003). This particular limit, is designed to detect when there's a problem with inadequate combustion air.

Causes of inadequate combination air include, but are not limited to:

  • Explosion of combustible vapors that have seeped into the combustion chamber.
  • Inadequate venting.
  • Inadequate makeup air.
  • Buildup of lint, dust, dirt, or oil on the screen.

Make sure you're not storing anything that could cause flammable vapors to be near the heater (gasoline, etc.).

Make sure the exhaust vent is sized properly, and is not blocked or obstructed.

Make sure there's an adequate source of makeup air.

Make sure the lint, dust, and oil screens clean.

This is a fairly modern safety system, so it's no surprise that the old heaters didn't have the same problem.

  • Ah, thank you. So maybe the original plumber is not completely out to lunch then! There is definitely not anything flamable stored in the the mechanical closet; it's just the hot water tank and the furnace. (The tank is just over two years old, and the furnace is about 10 years old). The closet is fresh air vented, and unobstructed. The exhaust vent is clear (smoke test), and the dust in minimal. I'll go and vacuum everything and try and make the place look like an operating theatre next. :) Thaks again! – TunaMaxx Dec 14 '15 at 18:43
  • Since you say it's in a closet (a fact I missed, or maybe unmentioned), make sure the makeup air intake is large enough for both the furnace and water heater. It could be that when the furnace is on at the same time, the heater can't get enough air. – Tester101 Dec 14 '15 at 20:17
  • Or if both are tied into the same exhaust vent, that might cause problems too. – Tester101 Dec 14 '15 at 20:20
  • I may not have mentioned the closet actually! There is a (shared) 4" fresh air inlet into the closet, and well as about 36 square inches at the top and bottom of the bifold doors. They are separate exhaust vents, as the furnace is high efficiency (PVC?) white plastic pipe and the water tank has classic B vent. – TunaMaxx Dec 14 '15 at 22:15
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    Yes, so far anyways. Replacing the switch did not cure the issue, and it tripped again in about 6 hours. However, after a thorough clean / vacuum session, it has been fine for ~ 48 hours. The screen on the underside of the burner was very dusty and essentially clogged. Also, the ring of screen/mesh at the bottom of the tank was not installed correctly and wasn't filtering out dust and debris. I also cleaned it and reinstalled correctly so it was actually doing it's job. So far so good. Thank you again for your help! – TunaMaxx Dec 16 '15 at 20:57
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Provided that you have inspected the environment thoroughly and found that the air supply is solidly non-flammable and there is adequate airflow as outlined by @Tester101, the part is only CA$22.00 here (US$16.00) and there is a high probability that it could be defective, go ahead and replace it.

If the limiter is the problem, you have saved a lot of time trying to identify it. If it is not the problem, you have a spare on hand and you can rule out a major suspect.

  • Thank you. That was my thinking too, and I'd already ordered a switch just in case. Is it just a matter of undoing the two screws that hold it on, or is there something on the backside that needs to be held in place as well? – TunaMaxx Dec 14 '15 at 18:46

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