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We have a 30 gal water heater in our garage. The unit was turned off three+ years ago but the water was left inside.

  • Can mold, etc. grow inside it?
  • Do we need to replace it before using it again?
    • If not, do we have to sanitize it?
isherwood
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Insinkerat
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  • electric or gas heater? – Ecnerwal Jan 27 '23 at 14:37
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    How old is the tank? Plus 10 or 15 years the chance of it having a leak soon(next 5 years) probably puts replacement at the top choice. – crip659 Jan 27 '23 at 15:39
  • Ppl answering here really should google "hygiene flushing" and explain why that's a thing if water stagnation is irrelevant. – DonQuiKong Jan 27 '23 at 23:03
  • @DonQuiKong sounds like you should write your own answer ;) – mmathis Jan 28 '23 at 02:41
  • @mmathis I have no idea if you can use that water heater or not. But I know just enough about how much stuff can grow in perfectly potable water within days to know that some answers here are giving reasons for conclusions that do not support that conclusion. – DonQuiKong Jan 28 '23 at 08:45
  • This is why everyone who goes on vacation and fails to disinfect their water system upon returning to the house dies. Oh, wait, that's not actually a thing, you're just fear-mongering... – Ecnerwal Jan 28 '23 at 13:47
  • @Ecnerwal ok, let's see. Theres scientific evidence: https://www.nature.com/articles/s41396-018-0101-5 Then there is also common sense. Then there are public buildings spending a lot on automated flushing to avoid this problem. Further, water for dialysis for example is put through ultrafiltration and reverse osmosis and so on, surely just for fun. But yeah, "ppl don't regularly die from this" is surely an argument. – DonQuiKong Jan 29 '23 at 09:51

5 Answers5

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If the water supplied to your house is potable, there really isn't much in there to grow. That's the point of water supply treatment, followed by water distribution at sufficient pressure that leaks go out of the pipe, not into it.

While a good flush is reasonable, there probably isn't much to get excited about in there in the first place, (it had clean, potable water in it, from the supply to your house - not ditchwater) and if the temperature is set correctly you'll kill anything in there (i.e. sanitize) with heat, anyway.

Ecnerwal
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    If the food in your fridge is edible, there isn't really much to grow in there. We either have very different definitions of "not much" or you should really rethink this answer. Btw. Bacteria can produce toxins that are not "killed" by a little heat. – DonQuiKong Jan 27 '23 at 23:10
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    @DonQuiKong Edible food is very different than potable water. Even in normal use, water sits for days in pipes and warms up. Toxins will be flushed out with the water. – jpa Jan 28 '23 at 09:58
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Can mold, etc. grow inside and do we need to replace it before using it again? If not, do we have to sanitize it?

  1. Yes, mold, bacteria, and other organisms can grow inside.
  2. Perhaps you do need to replace it. The tank may also be rusted. I'd inspect it closely after cleaning it to be sure it's sound.
  3. Definitely sanitize it
jwh20
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    What does the mold/bacteria/et al feed on? – Hot Licks Jan 27 '23 at 14:09
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    Anything that might be in there. Are you assuming that the water supply has ZERO organic matter? Open up a used water heater and you will find a lot of stuff not the least of which is a "slime" that clings to the sides and top especially near the bottom where it's cooler. – jwh20 Jan 27 '23 at 18:06
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    @HotLicks the same stuff the bacteria feeds on in live water heaters that grow bacteria. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5437349/ yes, that is a thing. – Harper - Reinstate Monica Jan 27 '23 at 23:01
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    @jwh20 Just looking at older water pipes might give an idea(or nightmares). – crip659 Jan 28 '23 at 00:20
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If the water is relatively pure, there's nothing to feed mold or bacteria. Yes, a little bit will grow at first, but soon all of the crud in the water will be consumed.

Just to be on the safe side, I'd drain and flush the tank (with the heater off), then put about a gallon of Clorox in it while turning the water on and off in bursts to help mix it. Then turn the heater on and let it cook overnight.

Hot Licks
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    Of course, then drain and flush it before using any of the water from it. – keshlam Jan 27 '23 at 15:04
  • I'd think 1) run it until no more hot water, 2) let it fully reheat the water in the tank. Lather, rinse, repeat until no more bleach smell. Then run it some more, just to be sure. – FreeMan Jan 27 '23 at 15:05
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    1:29 is a somewhat absurdly high bleach ratio. "You want between 50-200 parts per million (ppm) of bleach in the recirculating water (Step 7) for disinfecting your water system. Do not mix bleach solution that is greater than 200 ppm. A bleach solution with greater than 200 ppm of bleach will reduce the disinfection effectiveness." quote from https://www.health.state.mn.us/communities/environment/water/wells/waterquality/disinfection.html – Ecnerwal Jan 27 '23 at 15:24
  • This is ill-researched nonsense. "nothing to feed on" wrong. "gallon of Clorox" wrong. Not least how do you get a gallon of Clorox out of the water heater entirely? You'll have to dilute it a lot to get from 29:1 to 20,000:1 which is what the MN health department is calling for as a minimum disinfect level. Your water will be bleach-contaminated for months. You'll all be blondes! – Harper - Reinstate Monica Jan 27 '23 at 23:02
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If the unit was still connected to the homes plumbing and that was intact, there would be only residual water that was chemically treated in the tank. Sure the chemicals would be depleted, but there would be no harmeful organisms in the water to begin with.

I would open the inlet water and leave it under pressure for a few hours. First turn off the breaker if not off already.

Inspect for leaks. If none then turn the inlet water off and drain the tank. ( open the pressure relief valve to let air in.) open the inlet water and flush the tank for a min. or so. (close the pressure relief) Then close the drain and let the tank fill.

Check the thermostat's and adjust to 150 deg.(this temp and above kills anerobic bacteria that can give the water a bad smell) Turn on the breaker and let the water heat. Open all the faucets and let the water run for a few minutes to flush everything. Don't forget the line to the refrigerator for ice / water.

You should be good.

RMDman
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  • You run hot water to your refrigerator supply? – Ecnerwal Jan 27 '23 at 15:27
  • @Ecnerwal, Yes, here in my area it is usual for the refrigerator water supply to be tapped from the hot water at the sink or directly at the HW tank. The idea is the anode rod attracts minerals and makes the water to the fridge less likely to develop deposits there. Many areas get their water from aquifers high in minerals. – RMDman Jan 28 '23 at 02:48
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Right off the bat, turn the water heater up to 60°C / 140°F. Held at that temperature for a day will kill most pathogens.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5437349/

In fact, keeping it at that temperature generally is a good idea for the same reasons, however 140 degree F water can scald, so it requires either anti-scald faucets (i.e. joystick style faucets which do that) or a thermostatic mixing valve feeding the hot water lines.

Don't run it unless it's 100% full.

You also might replace the anode. Water heaters don't corrode randomly; they have a sacrificial anode which is supposed to draw off corrosion onto it. If the anode is fully depleted then the water heater will corrode. You're supposed to change them from time to time. When people wring hands about water heater life, ask them about the anode - I bet you get a blank stare "what anode?" The condition of the anode may also tell you about the condition of the rest of the water heater.

Harper - Reinstate Monica
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