WH pressure relief valve & wall thickness

Personally I would drain the water heater - either open both low-point drains and the WH pressure relief valve (ironically), or take the anode out.  Then let it refill normally.

Several of us have commented on the interior walls when we've done mods, but you would have to search around to find that info.  I'm not sure anyone has had to put a hole through the exterior wall.  Or do you mean the interior of an exterior wall?

Do a search here for "factory tour" and you'll find the pics of the half-built pods.

I may not drain the system, just isolate the water heater like one does to winterize. Then remove the anode, drain, put it back and refill.

The problem is by opening the pressure relief valve the cushion of air that normally rests at the top, was removed.  That cushion is needed as the water temperature increases.  Water doesn't compress as the water heats up and camper water systems are too small to absorb the extra pressure.  Even homes must now have a separate air bladder installed to help with this even though those systems are much larger.

I'm truly amazed at the various thoughts.  My very first home was an 1860's farmhouse with a jet pump hooked up to a sand point.  The pump eventually went south and I replaced it with one of the bladder types and had to pump air into the bladder.  It was vaguely explained 'how much air pressure' and I ended up guessing at the pressure and living with it for the next fifteen years with nary a problem. 

So I thought this would be fairly simple.  I drained all the water from the fresh water tank (no city  water hook-up was ever involved).  I opened all the hot water valves thinking they are somehow involved in the system (after thinking more later I realized the water pump is in between so that was a moot effort).  O.K. Water is gone. I opened the pressure relief valve just a crack and it spit an almighty blast of spray all over the work area, me and anything else within three feet.  That was nuthin'. I then decided it would be better to loosen the anode and let all the water out the bottom end.  I used the 1-1/16" socket I'd found and ratchet the anode out....once it got close to the last few threads it started spitting and I ratcheted faster. "POW" the anode said as it shot across my driveway hitting a brick stair trim wall...along with that 'bullet' came all six gallons...maybe only 5-1/2 who can measure it; but it covered a Lot Of Driveway with white bits of crud and all that water pretty much instantly.....another gallon trickled out and there I stood in total amazement.  I had opened a 'bomb' without knowing it.  I hope anyone reading this will learn from my mistake...what was it?  I should have gotten a large rag or similar and covered the work area and released the pressure valve into a old bucket or waste basket....even the garbage can.  There is obviously quite a bit of static pressure left in these tiny water heaters.  Needless to say the mess of spray was everywhere and I broke out the garden hose and set it on "jet spray".  I then examined the slightly bent anode...straightened it...it was missing a bit of the porous material; but plenty was left for the rest of the season...and reinstalled it, closed the pressure relief valve and refilled about 18 gallons into the fresh water tank and went inside and turned on the pump and got everything up to snuff.

I welcome all your hilarious comments; but I'm sure there are a few out there that will definitely treat there WH with a bit of respect about this pressure issue.  Thanks for all your comments and we're heading for the Black Hills tomorrow night.

Yes, it's easy to think that it must have something to do with air at the top, but it's not actually. If you go look at water heaters at the hardware store of your choice.. if that was how they worked, some water heaters have the safety valve halfway down their body.. lot of air..

As stated, opening the pressurized system is.. entertaining. it's best to turn off pump or disconnect from city water, then open the sink faucets to relieve the static pressure before removing the anode.. or.. make sure to have someone videoing the pressurized opening..lol

Here is a link to what they do and how they work.. If the system is working properly, while there is a slight pressure rise based on the WH expanding the water in the tank, your plumbing system is designed to hold that pressure, it's when it malfunctions, and heats to high, that bad things can happen. MythBusters did a pretty good episode on it. I think they shot the core of a standard residential WH 400 feet into the air, after it went through a ceiling and roof structure.. Took well above 300psi in the system if I remember right, and they achieved that pressure by removing/bypassing all the safety devices..

Watts

I was forced to learn a whole lot more about plumbing in homes then most people need to know because of the reef tank, I have all kinds of "extra" pluming, pressure gauges etc. Then I got to learn all about RV plumbing by not using a pressure reducing valve when hooking up the 'stream in a campground that had "spotty" pressure.