Firstly, a Plumbing DisclaimerThis is just the documentation of one's experience. Consult a real plumber (I'm not one, nor am I your plumber) for your plumbing related needs.
Hot Water Stopped Working (but was easily fixed temporarily, again and again)
The hot water slowed down to a trickle throughout the house, but cold water ran fine at all times.
While this was happening, I tried to do a number of searches, and yet found few results, so here it goes.
I live in an area with "hard" water. In other words, it's well-water, municipally served (recently sans fluoride), that tastes awful, but isn't as bad for soap/lather as it is maligned to be. Other than tasting awful, it also scales things up, but a little acid (I suggest vinegar) can get rid of that with time and heat, except in your pipes.
Many people install water-softeners to "soften" the hard water with salts, resulting in a high-sodium water that you're not supposed to drink, but doesn't scale up your pipe.
The Symptomatic Hot Water Heater's Symptoms
Periodically and suddenly, the hot water would stop working throughout the house, cold had no problems. Somehow it was determined that gently clanging on the hot water tank's intake (while running a hot water tap) would fix the problem for weeks at a time. To give you an idea of how gently the clanging was, a glass jar was once used without breaking it (the jar or the water heater). Eventually after a year of this, the trick stopped working, despite all desire for the problem to just go away.
So, the theory was that calcium/sand scale had accumulated on the cold water intake of the hot water heater, clanging it would break it up enough to allow an expected amount of water to flow, but it wouldn't take much to obstruct flow again to a trickle. Clanging would allow expectedly hot water to flow for an expected period of time, so the tank's elements and internals were suspected to be okay. Testing the pressure-relief valve (which you supposedly should do yearly), provided more evidence that the problem wasn't downstream of the tank. Clanging likely prolonged the inevitable, because it didn't clear the buildup from the tank, just from the pipe until it collected at the bottom (or until some large piece of silt came through the water system and plugged whatever little opening there was).
Now Time to Permanently Fix the Hard Water Buildup
Turn Off the Electricity
To prepare for the inevitable pipe replacement (and it turned out to be quite a bit more), we turned off the circuit breaker to the tank (to prevent risk of shock, and to let the water to cool down). This tank was installed relatively recently enough to have its thermostat turned way down, but older ones could be set to scalding temperatures, so we didn't want to miss this step. As the "hot" water flowed at a trickle, a hot tap was turned on to flush hot water out and bring the water down to a safe temperature.
Empty Out the Tank
Then the water turnoff to the house was turned off. One's home may have other valves that make sense to turn off as well*. Then the tank was drained via the flush tap at the bottom of the tank. As we didn't have the proper tools, this was done by gravity (ie: not a pump), through a ton of garden hose to a ground drain. This will take a few hours, and gives you time to grab tools. We were to leave several hot water taps on while doing this to prevent a vacuum and prevent water from "falling down" into the tank because of someone later on inadvertently opens a tap. This took a few hours, and left time to go to the hardware store.
*Don't think that one can just turn off a valve for the hot water tank because cold water could come rushing back into the hot water tank if someone inadvertently turns on a hybrid hot/cold tap (cold will flow into the hot water tank).
A Small Detour: Could it Actually be the Pressure Relief Valve?
The fellow at the hardware store that began with "Home", but didn't end in "Depot", suggested that the problem could be a calcified/limed-up pressure relief valve on the top-mounted pressure relief valve units. We suspected this wasn't really the problem, but since this valve is threaded to the tank, it could be non-destructively taken off and inspected. I much prefer non-destructively solving problems before cutting through pipes. Unscrewing this (keeping in mind that there was no pressure in the system, which one should first verify by testing the valve at this point) showed it to look completely clean. Surprisingly so. It also allowed us to see into the tank with a flashlight to verify that the tank's innards, that part above the water level (it took a while to drain) is rather free of calcium. The elements looked quite clean as viewed from the inside, rather than having to remove them.
Back to the Cutting Pipe Part
It was time to destructively find the problem. Hacksawing into the cold-water about 12" up revealed surprisingly clean pipe, but staring down the tube with a flashlight revealed the obstruction, and a long screwdriver releaved that it wasn't budging. We were hoping we'd be able to cut the pipe, push out the problem (or dissolve it with weak acetic acid) and then couple it back together, but it didn't look like this was going to happen.
Another hacksaw cut was made low enough such that the intake threaded 90 degree adapter could be unscrewed (and inevitably replaced). Some water did flow out at this point, but not a whole lot. Several pounds of sand/silt/lime deposits were snaked out of the bottom of the water heater with entirely non-professional tools. (Don't use your sewage snake, this is a potable water source we're dealing with, we used a coat-hanger). It would be impossible to clean it all out with this tools, but I doubt it was necessary. It's probably a bad idea to scratch any coating that the inside of the hot water tank may have, so we carefully chose a coated hanger. This cleanout should buy us several years.
PEX, Plastic Pipe, SharkBite: No Sweating Required (Physiologically&Pipingly)Now for replacing the pipe destruction. Love it, or hate it, PEX is the way to go, even if one can sweat copper pipe or have a house with existing copper pipe. The SharkBite connectors negate the need for any overly special tools. They're expensive (some dollars per fitting), but since we only needed a couple fittings, it's the way to go. They can attach PEX-to-PEX, PEX-to-copper or copper-to-copper in seconds and removes as easily with a cheap tool, but somehow makes a strong leak-free seal. It's quite extraordinary and might make more plumbing jobs DIY.
The Home Hardware had the connector with the threaded end for the hot water heater and the Sharkbite on the other end. Perfect! I'd highly suggest getting a Sharkbite elbow fitting rather than doing a 90 degree with the PEX tubing. We didn't forget to replace the teflon tape on the threaded connectors that we re-placed. We taped an extra SharkBite undo key to the hot-water heater in case it's needed.
The Hot Water Tank Restart Process
After turning off the tank-flush valve, re-attaching the pressure-relief valve (teflon tape!), and leaving a hot water tap on, the household water valve was turned on, water began to rush into the tank and no leaks were found. Well, it turned out that the tank-flush valve wasn't completely shut, but a fraction of a turn with a screwdriver solved that. Once water started flowing out of that hot water tap, the tank must have been full and the tap turned off. The pressure in the system increases as this point, so we re-inspected for any leaks. Now the circuit breaker was turned on and the water started to heat up (over a good period of time of course). A few weeks later, and everything has still held up.