I've read a lot of water heater threads on this forum, and they seem to generate more questions than leaky faucets or even leaky toilets. Let me share some suggestions for installation and repairs.
I'm probably going to turn this into a blog post, so let me know what other info (or edits) you'd like to see.
17 years ago(!) I upgraded our rental property with a water conditioner and a high-efficiency gas water heater. Today's high-quality water heaters can provide at least as much service life. Our water heater was guaranteed for 12 years because we bought the expensive model with better materials (and a longer warranty) instead of the cheaper 6-year models. We were also told that the resin beads in the water conditioner's ion-exchange tank were only good for 10 years. My operating experience suggests that the design engineers were using lifespan factors of at least 2x. Considering the labor required to install both pieces of equipment, it's worth buying quality and paying extra for high efficiency.
Back then I had a lot less plumbing experience, and I made plenty of mistakes, yet I still managed to handle all of the soldering and connections with library books and a few trips to our local hardware store. Today I'd go straight to FamilyHandyman.com and search their articles & videos. I might still solder copper pipe because it's usually cheaper, but there are now plenty of other materials and no-solder fittings that make the process even easier. In other words the installation has been dumbed down to the point where a brave neophyte can handle the job. If you have the mechanical skills to replace a toilet or a faucet without spraying water (or sewage) across the room then you're ready to tackle a water heater. Just give yourself plenty of time for mistakes and extra trips to the hardware store.
I acknowledge that water heaters have electricity or gas and hot water and other stuff that can harm you or horribly disfigure you. I also agree that there are building codes and earthquake requirements and other scary authorities who will severely chastise you (or at least hold up the sale of your home) if you don't comply with all of these mandates and their safety precautions. If any of that would cause you to lose sleep (or body parts), then please stop reading this post and seek professional plumbing assistance.
Installing a whole-house water conditioner gave me the opportunity for several other submariner home improvements. First, I moved the house's water reducer indoors (by the garage door) at a brightly-lit and convenient height (e.g., sitting in a
recliner chair) for adjustment or replacement. Next, I put piping unions and valves on both sides of the water reducer so that I could easily replace it (30 years later) or easily shut off water to the house. Finally, we put a drain pan (and a drain pipe) under the new water heater. This is cheaper (and more effective) than a leak detector-- although you could certainly install one of those too.
The water conditioner consumes a $5 bag of salt (40 pounds) every month or two, but it's well worth the recurring expense to remove the calcium & magnesium from your water. (It's also a great time to add other conditioning equipment like pH adjustors or iron-removal chemicals-- especially for well water.) Back in 1997 you had to custom-order at $1000+ from companies like Ametek, but today you can buy a $500 Kenmore or GE model at Home Depot. In 17 years I've had only one leaky o-ring that cost 15 cents to replace. I've installed water conditioners in two houses and both times it's completely eliminated the mineral buildup on faucets, countertops, and toilets. I've saved hours of toilet scrubbing and scraping-- 60 seconds of swishing with a nylon brush is sufficient. We no longer use harsh solvents or abrasive materials to clean ceramic or metal or countertops, so even though we're buying salt for the water conditioner we're still ahead on expenses-- and way ahead on labor.
The water conditioner really pays off at the water heater. There's not enough minerals in conditioned water to bake out on the tank wall (or an electric heater element, or the gas heater core), and there's no sediment accumulating on the bottom of the tank. I used to religiously flush a gallon or two through the tank drain every three months, but I never got anything other than H2O. (And a worn-out tank drain valve.) Now I only do it every 3-5 years and I still get no sediment.
When I installed the water heater in 1997, I didn't know to upgrade the anode rod. As a result my first anode rod was six inches longer than the water heater's vertical clearance, but luckily I was able to move the heater's exhaust stack aside to replace the anode rod. Now when I install a water heater, one of the first things I do is remove the anode rod and apply thread tape so that I can more easily take it apart a few years later. If I have time then I'll buy a segmented anode rod (if it's available) that only requires a foot or two of vertical clearance. These two precautions (if you have the time) will save you hours of maintenance frustration (and four-letter plumber's vocabulary) down the road.
If your water heater is near your driveway then you could adapt the drain valve to handle a garden hose connection. A high-quality brass drain valve (instead of the plastic version) will last a lot longer-- and you can attach a garden hose to flush your car's fender wells with heated water.
I didn't appreciate another issue when I bought our high-efficiency water heater, but it uses a combination anode rod. That means the tank's hot-water outlet is connected to the anode rod (see the photos), so the tank needs a special Bradford-White combination anode rod. These are a pain to remove because you have to use the serrated jaws of a Ford wrench (or a pair of vicegrips) to unscrew them without gouging the threads or crimping the connector. I'm not sure whether combination rods are still used in today's water heaters, but I'm going to avoid them if I can. "Normal" anode rods can be removed with a 1- 1/16" socket on a 3/8" drive and perhaps an extension bar. Those aren't exactly standard household tools, either, but they're a lot easier to find in a hardware store.
A water heater's gas piping is almost as easy to connect as an electric water heater's wiring. The gas pipe to the water heater usually includes a local shutoff valve, the connectors are standard sizes of high quality, and it's very easy to check the joints for leaks with a soapy solution. On electric water heaters I usually add a timer box that allows shutting off the power-- especially if you have teenage residents.
When I installed our gas water heater in 1997 I planned to check the anode rod every five years. Unfortunately that didn't happen until 2006, when I was able to verify that the anode rod and its supporting wire core had been completely consumed by sacrificial corrosion. (The "rod" was only an inch long.) I checked the new anode rod in 2009 and it looked fine. It was still satisfactory in 2012 but I knew that I needed to keep an eye on it. The photo shows what I found last week. Note that it still had half of its original diameter in some places, but one-half of a diameter means that you only have one-quarter of the original volume remaining-- and the wire core was already exposed. Eight years seems to be the full lifespan of the second anode rod, so I don't feel so bad about neglecting the first one for nine years.
The water heater is 17 years old, well past its warranty, and abused on its first anode rod. However it still heats the water as fast as ever and it has no sediment. I'd originally planned to replace it this year (instead of just the anode rod) but the tenants show signs of staying for another four years. I hate doing plumbing in high-pressure situations ("Dude, can I take a shower yet?") so I'm going to take my chances. I'll shop for a new water heater now, but I'll try to hold off the installation until the tenants move out. Maybe I'll replace it the next time they're away from the house for a few days.
One final piece of advice: don't test the pressure-temperature relief on your water heater. I don't care what the little tag says-- that's brought to you by the National Association To Buy More Water Heater Relief Valves. Every time you touch that valve, you risk it not being able to reseat properly. (I've demonstrated this foolishness many times on million-dollar nuclear reactor relief valves-- your tax dollars at work.) Once a relief valve starts leaking then you have to either overhaul it (good luck!) or replace it (easily done but usually your fault). If your water heater ever has too much pressure or temperature, the relief valve will be the first thing to relieve it-- even if it was only tested at the factory 17 years ago.
The photos show a generic new anode rod alongside the old anode rod. (Yeah, I have a couple cannibalized spares in the garage that I might use someday.) The white plastic part (with the hole) is the outlet for the hot water, which leaves the heater through the threaded pipe at the top to the house's hot water piping.