Water heater relief valve -- POW!

Oh, boy, do I feel dumb. But hopefully someone will learn something from my embarrassment that will help to keep them safe.
We have a gas-fired water heater here. It has been our practice for a while now to cut the thermostat on that to minima if we are leaving the house for more than a day at a time. Why pay and waste energy to heat and re-heat water which no one will use?
On returning, I turn the t-stat back up to a level which provides comfortably hot water. There isn't any degree rating on the rotary knob, so I have figured out empirically what the "hot enough" point is.
It never, ever would have occurred to me that I should install some sort of a physical guard to prevent knob travel beyond that point. And I could have absolutely bloody well sworn that I had shown my wife what the set point was supposed to be.
Apparently not. We came home today from holiday travels, and I went to wash my hands and said, "Ah, it's cold, the water thermostat should be adjusted back up."
She was in the garage and said, "I'll get it!"
I was roadburnt, and didn't pay any attention to the possible ramifications of having someone else do a job which I normally would do.
A few hours later (during which there was lots of hot water use for baths and whatnot) we were relaxing in the back of the house. Suddenly, we looked at one another. "Did you hear that?" "Yep. But no idea what it might be."
Reconnoitering the front room and kitchen, nothing. She looked into the garage and said, "Ack! The water heater has exploded!"
I ran out and saw nothing of the kind (fortunately). What I did see was a pressure relief pipe (which in this case discharges straight onto the concrete floor) spewing steaming water.
Glanced at the rotary knob on the water heater thermostat. It was ALL of the way over. Probably thirty angular degrees of travel past the "hot enough" point. If there had been someone with the wit to inscribe a burner setting from 1 to 10 on the rotary surround, this setting would have been 11. Utterly maxed out.
Touched the pressure relief pipe just below the PR valve. Nearly burned my finger on it.
Oh feathers. Cut the thermostat to zero. Turned on the utility sink hot tap. Incredibly scaldingly hot water came out and kept on coming out, in a huge cloud of steam. Sent the wife to run other taps on full.
I should have had the presence of mind to go valve off the cold water inflow, but there is no way to do that next to the heater -- that action requires a trip outside to shut off the master, and I had my hands full dealing with lots of suddenly very wet stuff which we had stored in the garage.
After a couple of minutes, the steam coming from the discharged water was greatly lessened, and the pressure relief pipe felt much cooler when touched, though still flowing visibly. I tripped the lever on the relief valve and that reset it. Flow stopped. Crisis over.
The unit has now cooled down to garage ambient temperature, and I am not going to turn it back up until we have carefully thought through the return-to-service process.
Okay, lessons learnt up to this point:
(a) Always, ALWAYS document on the device what the proper normal setting should be;
(b) Strongly consider mechanically excluding settings above "normal" -- even settings which would not have blown the relief would have been easily hot enough to scald at the tap;
(c) Take the time to arrange graceful failure modes even for seldom activated systems -- routing the pressure relief pipe to an exit hose had occurred to me before, but I had never made it an action item;
(d) Brief people on how to set controls if they're not yet so informed -- and people who find themselves unsure should take time to ask;
(e) Be sure to test safety systems on a regular schedule -- I shudder to think what would have happened if that relief valve had not tripped when it did. I have flipped the test lever in the past when I thought of it, but will be formally rigorous about it in future.
That's what I have learned. Here's what I am yet to learn, and about which hopefully someone will be able to inform me:
(f) Of what gotchas should I be aware when attempting to return the unit to service?
(g) What are the odds that the heater has been damaged by the overheat/overpressure event? The relief valve tag specifies release at 150psi/210F, which is pretty impressive. What's more, this unit has a build tag dated 1985! Assuming that it was installed at that time (it may be old stock) that is impressive longevity. My experience has been that modern water heaters seldom last more than a decade even without adverse events of this type. And while I would have liked to eke out another few years, if the heater is likely to have been compromised, I don't want to risk having it split wide open a few months later on.
(h) Even if the tank and heater assembly seem OK, should I think about replacing the relief valve itself? It was fine before this, with no drips or hisses. I do want to be quite sure that the relief will open when it needs to, though. Note that there is also an unknown in that a new-in-box valve might itself be junk -- the current valve at least has been demonstrated to work properly under field conditions.
Comments and feedback actively solicited. I'd prefer to see them here in the newsgroup rather than mailed to me: this in-box is utterly soaked with spam.
And before I close, let me take this opportunity to wish the top of the season to you all!
C
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It shouldn't have been damaged; hopefully you won't get the pressure/temperature up that high again. Personally I would not want a 20 year old heater in my house. I have a 25 year old heater in my cottage, but a leak won't do anything horrible there.

not, you are darn lucky it worked! Again, I would replace the whole tank.

they design them so that the highest setting isn't too high, but perhaps your thermostat is off also. Another good reason to get rid of it. Do you have a closed system without an expansion tank. If so, it was bound to happen eventually.
I have tested my system by letting it cool down, and then setting it to the highest temperature, while monitoring the pressure. It doesn't exceed the street pressure (90psi) so I know the relieve valve on my pressure reducer is working. Probably should test it again, or at least clean it like the instructions say.... (or bite the bullet and put in an expansion tank)

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Similiar story that happened to me. I was working in a 6 family apt building. One of the apt's had a bathtub faucet that was leaking hot water bad. I did not have the right parts with me. So I shut off the bathroom hot water feed in the apt and I told the tenant " I'll be back tomorrow with the parts." Well the next day I came back and I found water in the basement. Apparantly some tenant thought the water was not hot enough ( probably because of the bad leak) and took it upon himself to raise the T-stat. And because I stopped the leak the day before by shutting off the main bathroom feed, the hot water heater built up in temp and pressure and let out water in the relief valve. So I had to replace the relief valve as well.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I second the comment about not having a 20 year old gas water heater. That is way beyond the typical life expectancy of 10-13 years. If it were mine, I would replace it right now.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Probably the relief valve tripped on temperature, just what it was supposed to do. With the relief valve that old, yes I would replace it. The water heater should be fine. If it ever leaks, the water will only wet the garage floor. If it were in the house, or where the water could damage the structure, I would want an emergency pan under it piped to the outside.
DO NOT put a hose on the relief valve discharge pipe, that would restrict flow. It should be piped to the outside with an elbow turned down. The relief valve discharge discharge should be piped FULL SIZE, with no more than 25 feet of length and no more than 4 elbows in the length (Old code but a good idea). If the relief valve discharges on the floor, the pipe should terminate no more than 6" above the floor to reduce spalshing.
Stretch
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

You actually know nothing of the sort. If the pressure doesn't go above the street pressure, you know that there is no anti-backflow valve where the water comes into your house. If the relief valve was limiting the pressure, water would be coming out the relief valve outlet.
Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

house exceeds the street pressure. Then the relief valve allows backflow.
Actually I was confused about what I said, since I did this two years ago. The pressure never got above 85psi and the street is 90psi, so the relief valve never got tested. Sorry.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
chris snipped-for-privacy@hotmail.com wrote:
snip / / / /

snip / / / /

Be grateful for the relief valve. In the old days they would explode, sending the center through the ceiling like a missile and landing God knows where. Had a neighbors land on my front lawn once after the explosion woke me from an afternoon nap.
Frank
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Temperature/pressure relief valve (T&P) This valve is designed to prevent a tank from exploding if temperature or pressure exceeds safe limits, by opening and venting. Unfortunately, residential valves are somewhat prone to failure. They should be checked once a year by pulling up on the handle. Water should flow freely out and stop when you let go of the handle. If it does nothing, runs or drips, then the valve should be replaced. Banging on the handle with something hard, like pliers, sometimes will stop drips or even runs. If not, replace the valve. Hooking up the drain line with a union or flex connector makes T&P replacement MUCH easier.
People don't like to test their T&Ps. But then, we don't think it's so much fun to wake up in the hospital, or to patch a big hole in the roof, either. When water heaters explode, it's catastrophic. People are injured or die; buildings are severely damaged. Test your T&Ps! And one more thing: T&P drain lines should go down and out. Never up. If the valve opens, water will pool there and corrode it shut. Or freeze in the line in colder climes. We've seen lines plumbed uphill so many times we've lost count. But there SHOULD be a drain line, usually to within about six inches of the floor, or plumbed outside. That's code around here. It's to prevent you from being scalded if the valve should open while you're standing next to it.
Finally, if water is running out of your T&P line, look for the cause. It might just be a bad T&P. But it could also signal high-pressure problems or a dangerously defective control. Don't ignore it!"
this information and more at:
http://www.waterheaterrescue.com/pages/WHRpages/English/Longevity/hot-water-heater-safety.html
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
buffalobill wrote:

http://www.waterheaterrescue.com/pages/WHRpages/English/Longevity/hot-water-heater-safety.html
Got any facts on how many of these explode, how many people are injured, how many die?
More important, do you have any facts on how many TP valves just go bad and how many actually release pressure because of pressure or temperature build up?
TP valves are a good safety device. Thinking about them and testing them is a waste of time that could be better spent paying attention to your driving, looking both ways before crossing the street, getting the ladder steady before climbing, putting the under counter chemicals out of the kids/grandkids reach, etc. etc.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

<SNIP>
Next time just turn the breaker off......
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
He probably doesn't have a breaker.
wrote

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote

Did you notice he's talking about a GAS water heater ?
Happy modeming, Bill K.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Red Neckerson wrote:

Just for the sake of our eduction would you mind telling us where he would find a breaker that would affect the functioning of a "gas-fired water heater"? Unless the unit is a high efficiency type that uses blowers for exhaust and combustion air there is no breaker for a "gas-fired water heater".
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 26 Dec 2005 21:34:22 -0800, chris snipped-for-privacy@hotmail.com wrote:

See this explosion:
http://www.waterheaterblast.com/index.html
Oren "My doctor says I have a malformed public-duty gland and a natural deficiency in moral fiber, and that I am therefore excused from saving Universes."
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Generally, the one component you might replace is the "zinc"- the protective anode rod; likely way beyond time for that.
I'd prepare to replace heater in the next year or so- monitor it for any leaks or odd behavior. Pretty simple process, especially if you have all the fittings an tools you might need ready. Newer units have additional safety devices, and are capable of higher efficiency than yours.
You NEVER want a careless person twiddling around with something like that t-stat; sorry, but your wife qualifies. At the very least, someone could have gotten very badly burned- boy, are you lucky!
What I find much simpler is to leave the t-stat alone, and turn the control from "pilot-only" to "on" or vice-versa. But then, mine is not in garage, so there's no threat of freezing.
J
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
chris snipped-for-privacy@hotmail.com wrote:

I've thought about a mechanical stop that would prevent this.

Yes an actual explosion could easily leveled the garage!

Well it seemed like the pressure relief valve did it's job! Usually that overflow is routed to a drain if one is available. An interesting thing is to Google "water heater explosions" and Watts, who invented the T&P valve. I have seen shows about how steam locomotive boilers have exploded and also pictures of heating boilers and water heater explosions. Sometimes they'd find the exploded heater blocks away. Imagine a 75 gallon superheated steam rocket! Also one site recommended replacing that valve every three years! but then they were made them. So all in all you actually had a good holiday. Richard
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

What I would suggest is rather than twist the thermostat, switch the gas valve to pilot or off. Pilot if there is any danger of freezing or humidity is high, off otherwise.
If its constructed so that lighting the pilot is a royal pain, forget off and just turn to pilot.
--
Rich Greenberg Marietta, GA, USA richgr atsign panix.com + 1 770 321 6507
Eastern time. N6LRT I speak for myself & my dogs only. VM\'er since CP-67
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.