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