This is bull crap. Gas tankless are completely reliable, extremely efficient, and DO NOT require annual maintenance.

It is "recommended" to flush the heat exchanger yearly with vinegar to remove scale build up, but absolutely NOT required.
Warranties are still valid without doing this.
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We had our heat exchanger flushed a bit more than 1 year after purchase. There was no scale build up yet. Our water isn't terribly hard though (7 grains / gallon) and it's just the wife and I in the house.

Large families with heavy usage and hard water would be advised to have the heat exchanger flushed annually.

If you don't follow the maintenance instructions from the manufacurer, then yes, they can fail. In 22 years of plumbing, I can honestly say that I've NEVER seen an exploded water heater. I've have seen the aftermath of burst pipes too many times to count

I don't have anywhere near 22yrs plumbing but I have never seen an exploded w/h either. I have come across a dozen or so failed t&p. Tankless just use a pressure relief w/o temp valve, so I guess in theory a person could accidentally put one in a standing w/h.
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Hey Hamas did you experience any shrinkage?

Sounds like the hot & cold lines were reversed which is the only way sediment from the bottom of the tank could be pushed into the hot supply. If so, the fix is only temporary. When the sediment starts to build up again, it will start happening again

:hmmm: Never seen a pressure only relief valve either. Every tankless heater I've ever installed comes with a T&P valve

When a T&P fails, most times it starts to leak eventhough the WH is working as it should. When elements go bad they tend to super heat the water then blow out causing less or no hot water. 99% of the time the water does'nt get hot enough to set the T&P off

I'll take a closer look tomorrow, but I'm 90% sure they are only pressure, they install directly into the piping (I put isolation valves in and install the PRV into them).
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Did some research and found that there are pressure only relief valves, my bad. It's code to have T&P valves here which is why I have not seen any of them.