I don't know why you don't just replace the anode rod. The plumbers can also do it, but it's not something they like to do because that means they won't get a water heater job for a long while. I used to just get the 6 year ones, but now I get the 9 or 12 year ones depending on whatever they have in stock. And yes, I haven't noticed any difference in brands. I think the big box stores are made to a slightly cheaper spec in that the drain valve is typically plastic whereas the supply houses have brass but I don't think there's too much other differences, like you, I found they lasted the same amount of time as each property I bought always had a different mix of water heaters. They all normally failed in the 5-10 year range and if you were lucky, maybe 12. Since I started changing out the rods, I'm close to 20 years on one water heater, the rest are probably 10+ at this stage. As I mentioned in the other thread, get the thicker rod, I think the factory ones are thinner so only last the 5-7 year range, I changed them again after 9 years and the ones I got was the thickest they had so that's why they lasted longer.
Most of the failure I have are on 1 particular spot, closest to a dryer vent that cannot be relocated. That particular one typically last 5 year and the adjacent one (in parallel for an apartment building of 4 units) last 10 years. I suspect the reason for failure is the dryer sheet corrosion from outside, so anode won't help.
The main problem with the tankless systems is that you normally need to upsize the gas line, normally 3/4 and then you need to do the venting so it goes directly outside instead of using a chimney. And most of them just come with just a 5 year warranty on parts/labor. You can get 12 year warranty water heaters although that's only parts. And because there's so many different makes on them, if a part fails 5+ years from now, good luck trying to find a place that sells the part. Gas water heaters tanks are pretty basic and the parts are simple to get and replace. Also if you do the math, you don't really save that much money on the instant side of things and of course as a landlord, the tenants pay the gas bill so there isn't actually any reason for a landlord to switch to a tankless system. Changing the anode rod would probably get you 15-20+ on a regular gas water tank, will your tankless last that long? Anode rod is still relatively cheap, you can get one at Home Depot in a ship to store for $22.44. If you have a bunch of water heaters, there's still places that sell them for $13.99 but they charge shipping unless you buy enough to avoid shipping, that would mean buying 18 of them. Either stock up on them or split it with a landlord group.
When I had a plumber do it, it ended up costing me about $50 a water heater including the anode. He did 4 at a time and I think I paid him about $150 and it took less than 2 hours. Now maybe about the same as I just have my handyman do it and I rolled in the cost of my Milwaukee 2767 so he could do it as a one man job and it didn't take that long. Probably at least 75-90% of my failures are water leaks which is basically the tank rusting out from the anode rod being gone. Very rarely is it the thermostat, burner or dip tube or other issue.
Just to have a plumber showing up at your door here is about $300-500, mine install for about $600, all labor only cost. As I mentioned previously most of mine last quite long and they are often 15-25 years. Maybe I can get by with only 6 year warranty one, but I think for my situation replace when leaking (15-25 years) seems to make the most sense, and the one that fails early, fails early regardless of 6 or 12 years warranty model I install.
Plumbers around here are in the $115-$150 range. But you have to shop around. Found a hungry one on Craigslist a while ago, at the time I didn't realize he was a drunk but he was good enough to fix basic plumbing leaks and installs nothing super complex like putting in an entire system but he was only $50-$75/hour, he charged by the job mostly. The handyman, general contractor types are more like $30-$60 an hour here. Sometimes the plumber was cheaper than the handyman because the handyman would take several hours and the plumber could just bang out the whole job in about an hour.
