Easy when your stuff is in the garage but mine is in the attic. Too much to screw with once a year or so.
I looked long and hard at this the past couple of years because I am at the 10 year mark. So, it is just a matter of time on when I will be replacing my 50 gal. If is was in the garage I would just do it myself as I have done at other homes but the attic BS. Is a big NO for me.
I have a leak sensor in the pan and hopefully I will replace it on my schedule and not at the mercy of the tank. And NO i don't like having 50 gallon of water sitting above the ceiling.
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I've had a Rinnai tankless for 15 years, so with the voice of actual ownership experience:
Maintenance: I didnt "flush" the system for the first 13 years, and I'm not convinced it is actually necessary, UNLESS you have unfiltered, hard well water.
Sum total actual maintenance i've had to do is replace the fan in the combustion intake. $150' and 1 youtube video, plus 30 mins of my time.
System Cost: $2500 installed, 15 years ago.
Reliability: Perfect for 14 years, then I had to replace the fan.
Mine is Propane, but if you have NG, then it should be an easy thing. I can't speak to pipe sizing for NG though.
Vent gas exhausting: mine uses a concentric intake/exhaust pipe and is mounted on an outside wall in my basement. The exhaust comes out at about 18" above ground at the rear of the house. Newer models seem to work differently though.
Capacity. GPM varies with the incoming water temp, and the BTU capacity/efficiency of the WH. Mine will do 6.5 GPM on my well water, which i want to say comes into the house around 52F, and I have the output temp set to 130F. The heater can sustain this for as long as it has propane and water!
Recirculation: I find this feature to be unneeded. It truly doesnt take much longer to get hot water to the most distant tap in the house on my Rinnai, then it did on the tank. The key difference, is that some water in the hot water piping is pre-warmed by a tanked water heater, whereas in a tankless there is no such bleedover of heat, so a bit more cold water has to be expelled from the pipes to get hot water to the tap.
Now, the one area, where Tankless sucks, is cost. New ones, with capacity similar to mine are Pricey, and a new install with a relocation...well, that Heat Pump WH is starting to look cheap.
S.
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In the house that I am selling in 6 days (closing next week!), I had two 50 gallon tanks in the attic. One supplied water to the upstairs, the other supplied water to the downstairs. 100 gallons in the attic regularly terrified me ... so I maintained them on a tight schedule for two things:
- I would flush both heaters every year, and check for flaky rust in the water that was flushed out.
- Every 3-5 years, I changed the anode rod out.
One of those heaters was installed in 2001. The second was installed in 2009. Both "6 year" heaters. While replacing the anode rods, I realized the 6-year heaters came with 0.75" diameter rod, while the 12-year heaters come with a 0.9" diameter rod. So I started using 0.9" rods. And placed leak sensors in the pans and had an automated main water valve.
Had to replace the elements and thermostats a couple times. But they're still going strong at 22 years and 14 years.
My new house has the heater housed next to the car port. I am so glad I don't have to worry about a 100 gallons of hot water taking out the ceiling!
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That’s why they didn’t rust out. Nobody knows to change the anode, or how simple it is to do. I just changed the one in our camper which was down to about 1/8th inch thickness from 3/4 and it cost me $8.
As far as preventative maintenance on our tankless water heater goes; I did it once on the one that was already installed when we moved into our current house and there was nothing but clear water after 10 years of neglect by the previous owners. 6 months later it died (I had already repaired it several times prior) and I installed a shiny new Rheem Platinum unit with remote temp control that I bought for $1500 at Home Depot (in stock) that can heat 9.5 gpm. I don’t plan on ever flushing it.
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It is extremely simple to do. And with an impact wrench is a one-person job. Over the years, I've changed about 8-10 anode rods - both heaters combined, and using an electric impact wrench never damaged the tanks .....
I used to purchase them from Zoro. A 0.9" diameter magnesium anode that was 44" long used to be about $12-15. They are running about $30-35 now (post-pandemic).
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So is it wrong to assume the type of failure we had (leaking tank) isn't going to be put off by any particular maintenance? Or is there anything I should start doing w/the new water heater that would help postpone issues like leaks?
- Flush it once a year.
- Replace the anode once every 4-5 years (assuming you get a 6 year tank). When you replace it, use a 0.9" anode, and it will last 5-7 years. Really isn't difficult to replace. I use a cheap electric impact wrench from Harbor Freight and this impact performance deep socket bit from Irwin.
Your tank should never leak. You may need the burner/thermostat to be adjusted or replaced, but the tank itself will stick around forever.
The link to the anodes I purchase from Zoro is a couple posts above this one.
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From google:
By replacing your anode rod every 3-5 years, your water heater can last well over 20 years without any risk of leaks and resulting water damage. Unfortunately, if you ignore this important water heater maintenance task, you may need to replace the entire water heater after only 10 years of use.
Don’t tell the wife 
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So sounds like we just had a unit on the wrong end of the normal curve.
We aren't going to take it apart to see where the leak is coming from, but is the more likely statement that water is leaking from some fitting connected to the tank inside the water heater's outer skin? I really don't know anything at all about how water heaters are put together, I should do some reading...
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Not only will I not tell her, but I will be blocking her internent access as well!
My other question is...
Why the h*ll didn't you guys tell me this five years ago?! 
We need to have a meeting where you assume I'm an idiot (easy) and tell me all the obvious stuff I should already know. 
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When the (sacrificial) anode rod has disintegrated from corrosion, the corrosive minerals start attacking the tank itself. That’s why you have to replace the anode if you want your water heater to last.
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Seriously, I don't get why I didn't know this/hadn't heard this before. I'm usually pretty good about basic maintenance around the house, but missed this boat so badly I wasn't even near when it arrived at its destination, much less seeing it leave the dock... EEK!
BTW - kind of funny, the anode (looked at @aaiyar's link) looks like a used piece of junk...this is going to send me off into some additional reading about anodes as well.
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Btw, tankless water heaters are stainless steel and don’t have an anode. Tank heaters are made from carbon steel.
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Not that it's relevant, but I have a 100 gallon tank that I use with my oil and pellet boilers. It has a 17 gallon corrugated (for Legionella) stainless steel coil inside for DHW. The tank shouldn't rust since it doesn't see oxygenated water. Of course, with that said, I just jinxed it.
I think it’s the chlorine in the water that causes most of the corrosion.
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I personally am on a well.
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Which is odd, given that every water heater has a cavity inside.
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Are you thinking of fluoride?
Which should help. My aunt had well water and when we sold her house it had a functioning gas water heater that was 25 years old.
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Oh man...just not my morning. Going to shuffle off to my hole and reflect on my failings. I will take a bag of chocolate chip cookies with me to aid in my recovery.
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