Need expert plumbing advice please
I have a 5 year old Richmond/Rheem natural gas 50 gallon hot water heater.
It has been performing fine till recently.
A few months ago we started running out of hot water during a 2nd person showering. So maybe after 13 - 15 minutes. Then it got worse and lasting only 8-10 minutes of hot water before going tepid to cold.
I drained off a few gallons a month ago; noticed a little blackish color in first "quart or so", but then clear. Funny thing though, it came out at a trickle, could barely get a good draining in my opinion. Flame through the window looks fine/perfect. Cranked up the temp just 1 notch, too.
Seemed to help for a while and 2 showers did fine.
Now back to tepid after only about 8 minutes of showering with no other draw. After about 5-7 we even turn cold to almost off and still it isn't burning water and I have the thing cranked up to about 135 degrees setting.
First coming out of tub faucet when starting and the water is indeed burning hot.
Thoughts? Thermocouple seems fine as flame seems fine.
What do I check?