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Discussion Starter · #1 · (Edited)
The house I just moved into has a 75 gallon water heater. It heats up the water pretty fast, so if someone takes a shower and the next person takes a shower 20 minutes later, the second person will have plenty of nice hot water.

So far so good. The problem is that if there is a long interval during which no one uses hot water (12 hours or more), the water will be just warm enough with the hot on all the way and the cold off - and that's the best case scenario. The worst case was when we returned from a 10 day vacation and my wife took a shower. I hadn't turned the water heater down so I expected her to have hot water. But she came out yelling that it was COLD.

This doesn't seem right. Of course I could just turn up the temperature (which is already set at higher than the recommended setting), so that the temperature will still be acceptable after no-use intervals that, at the current setting, result in unacceptably cold water. But that seems inefficient. My guess is that there's a water temperature sensor, and either that is faulty or something that "listens" to it isn't hearing right.

Any ideas on what's going on here and how to fix it?

Edited to add: It's an AO Smith natural gas model with a pilot light, installed 6 or 7 years ago.
 

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Sometimes that sensor probe that goes inside the tank gets crusted with sediment affecting the sensor depending on the water quality.

If that's the case, there is no really way to fix it other than replacing the temp controller. They are pricey. at over 7 years old, it's probably out of warantee I'm guessing.

Are you on well or municipal?
 

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Discussion Starter · #4 ·
Municipal water - in fact the same municipality where I had a water heater in another house last 20 years!

I think this water heater has a 10 year warranty, although I'm not the original owner.
 

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Given that a term of absence made a small problem develop into a larger one, I will guess that the orifice is plugged on the burner.

That's just a guess but worth checking out.
 

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Connect a garden hose to the drain valve. Put the other end of the hose in your laundry sink. Open the drain valve and see if you get any debris out.
May help, may not.
 

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Discussion Starter · #7 ·
Given that a term of absence made a small problem develop into a larger one, I will guess that the orifice is plugged on the burner.

That's just a guess but worth checking out.
I'm guessing you mean that the gas can't leave the burner orifice and therefore I'm getting a partial flame, or maybe no flame. I don't understand why that would be a bigger problem when the temperature drops "naturally" (no water flowing into/out of the water heater) than when it drops due to someone turning on the hot water (causing a sudden cooling of the water temperature in the heater)?

Connect a garden hose to the drain valve. Put the other end of the hose in your laundry sink. Open the drain valve and see if you get any debris out.
May help, may not.
I'm guessing it won't. The heater was drained and refilled a few weeks ago.
 

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retired framer
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See if you can find date of manufacture on it and sometime the plumber writes the date of install right on the tank.
 

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After the long absence you get cold or cool water, does it recover? How long before it comes back to your original description?

Bud
 

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Generally any buildup of calcium or whatever on the tank or control element won't stop the element from turning the burner on. The buildup insulates the element so it takes longer to respond to changes (widens the differential) but eventually does respond. That sounds like the first part of the problem you described.

The further development sounds like a complete failure to get burner operation at all or at least at any usable level, which is likely caused by a different problem. I suspect a plugged burner orifice. It can happen due to dirt or rust in the pipes or a spider or fly making a home near the source of the methyl captan that they love so well.

The orifice may have been partially plugged for some time and contributing (or completely responsible) to the slow response you had with system recovery potential.

It's all conjecture at this point so a visual inspection of the burner flame would be more revealing. Even twisting the control knob back and forth to see how wide the range is from the off click to the on click would tell a lot about the condition of the valve internals.
 

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Discussion Starter · #11 ·
See if you can find date of manufacture on it and sometime the plumber writes the date of install right on the tank.
Installed 2013, according to what was written on the tank.

After the long absence you get cold or cool water, does it recover? How long before it comes back to your original description?

Bud
It recovers pretty fast. I haven't tested it systematically, but the next person to take a shower gets nice hot water even if it's just 20 or 30 minutes after the first person. I've noticed that when the water heater burner kicks on, it's rarely on for this long (assuming no hot water usage). And it does kick on while the first person is taking their cold shower.

When we first noticed this, I just turned up the temperature a notch, and now for the most part even the first person gets hot water. But if there's a longer than normal gap since the last hot water use (e.g., someone takes a shower late in the evening after we haven't been home all day, so it's been 16 or 17 hours since hot water was last used), the first user will notice that it's cold (and complain to Mr. Fixit, aka me).
 

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Discussion Starter · #12 ·
Generally any buildup of calcium or whatever on the tank or control element won't stop the element from turning the burner on. The buildup insulates the element so it takes longer to respond to changes (widens the differential) but eventually does respond. That sounds like the first part of the problem you described.

The further development sounds like a complete failure to get burner operation at all or at least at any usable level, which is likely caused by a different problem. I suspect a plugged burner orifice. It can happen due to dirt or rust in the pipes or a spider or fly making a home near the source of the methyl captan that they love so well.

The orifice may have been partially plugged for some time and contributing (or completely responsible) to the slow response you had with system recovery potential.

It's all conjecture at this point so a visual inspection of the burner flame would be more revealing. Even twisting the control knob back and forth to see how wide the range is from the off click to the on click would tell a lot about the condition of the valve internals.
Thanks - but I'm not sure which further development you're talking about? There's really only one problem - water heater doesn't turn on unless cold water is let into the tank.

I like your explanation of a sediment-insulated element. The question is, how to replace it? As far I can tell from Google, one has to replace the entire gas valve. Does that come with a new element that gets placed into the tank? Or does it come without a new element, so I'd end up with a new valve and an old element (and exactly the same problem)?
 

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By further development, I mean the complete failure of the burner to turn on and the tank to go cold from top to bottom. Not just warm but completely cold!

Now if you're telling me you were able to put demand on the water flow and it caused the burner to kick on and heat the entire tank back up to proper temperature, I will have to agree that you need a new valve since it requires "street cold" (from outside the house) water to overcome its differential in order to actuate.

The temperature sensor is part of the valve and not individually replaceable.
 
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So once it gets going it will keep going maybe too hot but when not in use it seems to shut itself off?

There is an overheat switch that will shut it off. But that should not reset itself.

https://mspplumbingheatingair.com/b...r-running-out-of-hot-water-faster-than-normal


A broken dip tube

Every water heater has a “dip tube” that’s responsible for pushing the incoming cold water down to the bottom of the tank to be heated.

After that cold water is heated, it rises to the top of the tank where it can be pulled out whenever you need hot water somewhere in the house.
But if that dip tube breaks or falls off, incoming cold water no longer gets pushed to the bottom of the tank. Instead, it mingles with the already heated water at the top of the tank, lowering the temperature of the water that is delivered to you. And the more cold water that enters the tank, the sooner you run out of hot water altogether.
Reasons a dip tube can go bad include:
  • Normal wear and tear. Some dip tubes are made of plastic that have a shorter lifespan than others.
  • It’s defective. A lot of water heater manufacturers sold water heaters with defective dip tubes from 1993–1997. These dip tubes were prone to cracking and breaking apart long before they should have.
 

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Discussion Starter · #16 ·
The water doesn't get too hot. It gets to about what I'd expect (given the temperature setting) when it does kick in.

If the dip tube were bad, then the problem should be that the hot water runs out quickly. That's not the problem at all - if it starts hot, the water stays nice and hot throughout a long shower (it's a 75 gallon tank, after all). Also, a broken dip tube doesn't explain why the water starts out cold after a long period of NOT using hot water. That's sort of the opposite problem one would expect from a broken dip tube.
 

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If you plan on doing it yourself, crack the gas valve loose a bit before draining the tank. They are extremely tight and the weight of the tank will be a great help in holding "backups" for you. It's no job that a competent DIY'er can't handle but you need the right tools and a bit of ability so be your own judge and proceed.

Good luck.
 
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retired framer
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Do you have a bath or shower that only has one spout but a 3 handle faucet.

Where you can leave the water on but turn the center handle to stop the water.
 

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Discussion Starter · #19 · (Edited)
Do you have a bath or shower that only has one spout but a 3 handle faucet.

Where you can leave the water on but turn the center handle to stop the water.
Yes, one tub has that, I think. We've never used it. The others are single handle, or one has hot and cold plus a valve for switching from tub to shower. We've observed the problem in both of these, as well as at sinks - i.e., it's not limited to one faucet.

Why?

PS: Here's an oddity that may be related, but probably isn't. After we came home from vacation, I noticed a little water in that tub we never used. Just a little puddle under the faucet of the supersize whirlpool tub. I've been mystified as to what could have caused it. Possibilities I've considered and rejected as implausible include:
-cat sitter took a bath
-cat took a bath
-cat sitter gave cat a bath

...but maybe there's some plumbing-related explanation.
 

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retired framer
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Yes, one tub has that, I think. We've never used it. The others are single handle, or one has hot and cold plus a valve for switching from tub to shower. We've observed the problem in both of these, as well as at sinks - i.e., it's not limited to one faucet.

Why?
I seen that one before the two valves can be left open and the water can go hot to cold or cold to hot what what ever pressure drop there is on one side.
The people were getting a warm toilet for the second person in the morning.
 
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