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My neighbor and good friend is a union steamfitter, and general handyman. He had to replace his electric hot water heater last year and he did it himself.

My wife and I have to replace ours now, and he says its a super simple job and he has offered to help me do it. I came across this article and many others that warn that these heaters can actually explode if installed improperly, and it has us a little concerned (to say the least!).

I understand that the knee jerk, default response in these situations is: if you feel uncomfortable doing this yourself, have a professional do it. And I get that, I really do. I'm just trying to weigh all our options here.

So I ask: under what conditions can electric hot water heaters explode? It sounds like (if I'm reading these articles correctly) that they explode when both the temperature is set too high and the expansion tank is malfunctioning. So if that's the case, then my followup question might be: OK, well, how hot is "too hot" and how can I test my expansion tank to confirm its functioning properly?!

However, if there are other instances where these types of heaters can explode, I'd like to know about them, and what the diagnostic/operating procedure is to prevent those situations from happening.

At the end of the day, if I don't feel 100% confident in the process, we'll have a pro do the install. But if its as simple as checking a few things or taking care to avoid certain specific pitfalls, I'd rather save the ~$2500 and just do the install with the help of my handy friend.

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    The Mythbusters did a whole episode on this. Apparently there's several features on modern water heaters specifically to prevent this sort of thing, and they had to disable all of them (including a few they wouldn't even mention on television to avoid giving people ideas) to reproduce the problem. Mind you there are many ways a water heater can fail that don't involve exploding or turning into a rocket that you still want to avoid (leaking, electrical, etc.), but it's probably not quite as dangerous as that article implies, unless you've got a very old unit. – Darrel Hoffman Dec 4 '19 at 22:22
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    Fifteen hundred dollars to swap a $1k HW tank? uh.... I'm available ;) – Mazura Dec 5 '19 at 1:28
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    In the unlikely event of the tank exploding, while there is clearly danger involved, we probably aren't talking a hiroshima size explosion here... The tank will split open and dump all it's water, and there will be some explosive force, but it will probably be more a matter of redecorating all rooms from water damage than killing everyone and flattening the house. Home insurance in most cases would cover it. The various videos online showing massive explosions usually have made sure the tank is half filled with air for entertainment purposes (compressed gas explosions are much worse) – Oliver Mattos Dec 5 '19 at 9:04
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    Are you really going to trust a website that says stuff like " all water heaters use is some form of combustion to heat water inside the holding tank." This is not true, my electric water heater has no combustion. They are just trying to scare you into using their services. Your neighbor is a steamfitter, you can trust him with your waterheater, and learn in the process. I learned by helping my dad when I was a teenager, now I've replaced one myself over 5 years ago with no problems. – Glen Yates Dec 5 '19 at 17:38
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    It's a "water heater", not a "hot water heater". If the water was hot then you wouldn't need to heat it. – Kat Dec 5 '19 at 21:58
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Yes, that's a scary article but keep in mind that both safety devices have to fail to have the tank explode. The pressure release valve and the high limit on the thermostat both have to malfunction. Both of them to my knowledge operate at around 200 degrees F. The maximum you can set most water heaters is 150 degrees F but most recommendations are to set them at no higher than 120 degrees F. Also you should check and test the pressure release valve periodically for leaks and operation.

Replacing the water heater is a relatively easy task. Just make sure it's completely full before turning on the breaker or you could burn out the elements. Also, avoid using those corrugated pipe. Sweat copper tubing all the way. Your steamfitter friend can instruct you on this. Good luck

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    Thanks @JACK (+1) -- it looks like I'm confusing the pressure release valve and the expansion tank. I assume that new electric water heaters come with (new) pressure release valves as well? – hotmeatballsoup Dec 4 '19 at 14:33
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    @hotmeatballsoup Yes they do but you have to connect them to your down pipe. Follow directions on how to do this because it can affect the operation of the valve. – JACK Dec 4 '19 at 14:44
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    I'd also like to note that the two safety devices that need to fail are not really affected by installation. You don't touch the thermostat or wiring to install the heater other than to set the temperature, and the pressure valve is pre-installed and not adjustable. – JPhi1618 Dec 4 '19 at 15:33
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    @JACK: My understanding is that the downpipe connection doesn't impact safety against explosion, just against mold or bacteria growth (if it goes too far down and ends up in standing water due to a leak) or spewing out extremely hot steam where it could scald someone (if it's too short). – R.. GitHub STOP HELPING ICE Dec 5 '19 at 0:59
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    It's not just a failure of a safety system, but a bypass or otherwise disabling of the safety system. As someone mentioned, Mythbusters did a show on this and mentioned that some people cap off the release valve, instead of letting it drip. An inexperienced/amateur installer might want to do this, but the neighbor likely knows not to do this. – computercarguy Dec 5 '19 at 20:43
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Consider the sources; it is plumbers giving you a scare story to get you to hire them. See also "aluminum wiring".

It will take a chain of 3 events at once:

  • failure of the thermostat, causing the heater to overheat the water, boiling it.
  • the pressure relief valve fails to operate, preventing the pressure from simply blowing out the relief valve; that's what it's for.
  • water is unable to backflow out to the street, which would also relieve the pressure. This would fail because someone fit a "check valve" to prevent backflow without also fitting a pressure relief valve on the house side.

If it's so unlikely, why does it happen at all? Because it turns into dominoes. First, the city requires retrofit of an anti-backflow check valve. The family either skips the pressure relief valve, or sites it poorly. Second, the house's normal pressure changes cause the pressure relief valve to spit water on their stuff. They angrily cap it off. Solved! Third, the hot water heater's pressure relief valve starts to spit (or is ancient and is silted up or rusted solid). Capping the other one off worked, so they cap this off too. Then, they are down to a single point of failure, the thermostat.

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    Honestly if someone caps the relief valves, they're fully responsible for the tank exploding – Nelson Dec 5 '19 at 3:19
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    @Nelson If you weren't familiar with plumbing, how would you know it was bad? – user253751 Dec 5 '19 at 10:43
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    @user253751 Why would you do something you are not familiar with without understanding what happens? Oh... humans, I see. – glglgl Dec 5 '19 at 10:48
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    @glglgl because water is coming out and {washing away the garden | making the floor mouldy | looking bad}, as Harper suggested in the answer. See also this other HNQ where there is water coming out somewhere it shouldn't be, so the asker is planning to block it up. That is a default common-sense reaction when water is coming out of somewhere it shouldn't be. (In the linked case there is no explosion risk, but the equipment might be able to get damaged) – user253751 Dec 5 '19 at 10:53
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    Also, you would have to assume the steel tank of the heater was the weak point in the system. Every flex hose to a faucet or washing machine and every pipe fitting along the way would have to hold tight long enough for a steel tank to over-pressure and explode. Heck, even the plastic innards of the toilet would be subject to the pressure since it would also pressurize the cold side of the heater just as much. – JPhi1618 Dec 5 '19 at 19:36
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An electric water heater explodes when the water in it starts boiling, producing pressurized steam that causes the heater to rupture. In order for this to happen, three things all need to fail; if any one of them works properly, no explosion will happen.

  • First, the thermostat needs to get stuck in the "on" position. If it's working normally, or if it sticks in the "off" position, the water in the heater will never get hot enough to start boiling.
  • Second, the high limit switch needs to fail. This switch is a second, non-resetting thermostat that cuts off all power to the water heater if it gets too hot. It's set at the factory to a temperature higher than the main thermostat can be set to, but well below the boiling point of water.
  • Third, the pressure relief valve needs to get stuck closed. This is a valve, usually on the side of the water heater, that will open to release steam or pressurized water. It's set to a pressure above normal water pressure, but well below the maximum pressure the heater can handle.

When installing a water heater, the most important thing is to make sure the pressure relief valve has a free-flowing connection to somewhere where the water can drain. The other safety mechanisms are configured at the factory and are pretty much foolproof, but this one can easily be disabled by improper installation.

  • I once saw where an electric water heater was plumbed to the incoming water supply (at high pressure) rather than a loft tank; also the thermostat was bypassed; some while after being switched on, it did not explode, but became a steam rocket, detached itself from its mounting and the water pipe, went through the ground floor and upper floor ceilings and the roof, flew over 2 houses, and landed 200 yards away. Meanwhile the severed pipe flooded the ground floor. I saw the TV news report. By a miracle nobody was hurt, but the damage was going to be 5 figures of UK pounds. – Michael Harvey Dec 7 '19 at 16:02
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Well, the answer is right there in the article you linked:

Always check your hot water heater’s pressure relief valve to ensure it is in good working condition. If you have any concerns that your water heater is not functioning accurately, have an inspector come take a look at it.

The best precaution you can take to avoid having this happen to you, is to set your water heater’s temperature no higher than your manufacturers suggested setting.

Note that it talks about the pressure relief valve (technically temperature & pressure relief valve), not an expansion tank. The pressure relief valve is usually mounted on the side of the water heater and looks similar to this (on a new water heater, the piping won't be there): pressure relief valve

To test this valve, remove the piping if you can't see where it terminates (you'll need to see if water is coming out the end). If you can see the end, you can leave the pipe on. Carefully lift the lever on the valve up. Do not lift it all the way as that will lock it in the open position. If you get water coming out, you're good. If not, or the valve is stuck, you'll need to replace the valve.

Regarding the temperature setting, if you stick to the safe range indicated on the thermostat (usually it will say something like danger of scalding for the higher ranges), you will be fine.

  • 1
    that discharge pipe looks a bit high. – JACK Dec 4 '19 at 14:47
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    The "Don't set your temperature too high" is now affected by scientific knowledge about Legionella and other bacteria, which says you need 140F to stop legionella from breeding in your tanked heater. Instructions have not been updated to reflect this, because installation instructions must be approved as part of a heater's UL listing, and manufacturers are unwilling to grind through the UL listing process again. – Harper - Reinstate Monica Dec 4 '19 at 19:20
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    @RussellMcMahon Unfortunately, there is no compromise temperature for scald/legionella. Hot enough to kill legionella is deep into the scalding range. The only Rx the North Americans have found is either a) tankless; or b) combination of scaldy 140F + thermostatic "joystick" faucets rigged to be unable to reach scalding temp. – Harper - Reinstate Monica Dec 5 '19 at 18:47
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    @Harper-ReinstateMonica the compromise in Europe is to raise the temperature once a week to kill legionella then turn it down again. Admittedly most homeowners are too lazy to do this but it is done in larger, managed buildings. – Level River St Dec 5 '19 at 23:50
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    @Harper-ReinstateMonica I'd rate 140 F/ 60C as "not nice" but bearable for brief exposure. Fall in a shower under that and you are in trouble. Dip your hand in or under it briefly and it hurts but does not mark ongoingly. | I rate 55 C as "just able to keep your hand in. Maybe". I use that mainly as an engineering temperature asessment test rather than as anything to do with tap water. || Do the US have tempering valves? ... Garglabets ... -> OK. You call them mixing valves. We mandate them on a centralised basis. – Russell McMahon Dec 6 '19 at 0:49

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