Losing hot water in one facet only

If you have hot water the heater must be heating. Perhaps the switch is bad or the gas on/off solenoid is malfunctioning. But, how do you know that there is hot water, is the water facet on? If so try turning off the water.

If the release valve referred to is the pressure release valve, then a water leak out the attached vent pipe is an indication that the valve is failing. To change the valve, the water must be allowed to cool (turn off AC power or gas) and the water level inside the tank must be lowered (drained) to below the level of the pressure relief valve. Once this is accomplished the vent pipe is unscrewed from the relief valve and the pressure relief valve is unscrewed from the water tank. A new pressure relief valve with a sealant on the threads, is then screwed and tightened into place with the outlet port facing downward. The vent pipe is attached to the new valve and the tank refilled with water. A hot water facet must be opened to allow air to exit while filling. Upon becoming full (water will begin to flow from facet), turn off the facet and restore AC power or gas, therefore allowing the water heater to heat up. When the first hot water is utilized there may be some air expelled from the facet.

Is this gas or electric:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#new
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
http://waterheatertimer.org/How-to-install-gas-water-heater.html

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21dv50 is discontinued. This is ordinary tank-type 50 gallon gas water heater.
Amazon sells 50 gallon 22V50F1 Rheem water heater for $420 with free shipping ... but who carries the warranty?

Frankly you can get in-store returns, and buy same quality gas water heater at local box store since Rheem makes GE water heaters at home Depot.
Other brands are equally good, as AO Smith makes Kenmore water heaters at Sears and Whirlpool at Lowes.

I prefer the AO Smith because maintenance is easier.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

If incoming water is too cold, tankless might not have enough time to heat the water as it flows by insince the heat exchanger.

1) You can passively preheat cold water using staging tank, or tampering tank.
http://waterheatertimer.org/Tempering-tank.html

2) Buy old water heater from plumber that is not rusted out.
Replace anode rod to stop further rust.
Rinse tank with water and hydrogen peroxide to kill bacteria that results from anode rod.
http://waterheatertimer.org/Replace-anode-rod.html
Clean sediment out of bottom:
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

3) Don't forget, a tankless heater needs to be de-limed each year, and cannot be exposed to more than 11 grains hard water.

4) Since all costs are energy, when you get tired of messing with tankless.
http://waterheatertimer.org/How-to-install-gas-water-heater.html
http://waterheatertimer.org/images/Compare-water-heater-parts.jpg

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Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

There is a minimum water flow required to operate your Bosch tank-less water heater. It is around .75 gallons a minute. There are three things you can do to help the flow.
1.Turning the temperature down will help increase the flow.
2. Installing higher flow aerators on your faucets.
3. Clean the Bosch tank less water heater.
You are not alone in having this issue. The most common problems are low flow faucet aerators and low flow shower heads.

Replace the inlet and outlet pipe connections. One or both of them is blocked with rust and/or debris.

On the back of the gas valve (where you turn the temp up) there is a stem that actually goes into the tank that senses the temprature of the water it could be dirty or buried in sediment depending on how old the tank is, or just bad. if the tank is over 10 years old i wouldn't bother replacing the gas valve, the tank is probably not far behind. A new valve will run around $100.00 maybe a little less. You wont find it at menards or home depot in stock but they may be able to order it for you. Menards sells rheem products or try you local plumbing supply house. REMEMBER TO RELIEVE THE PRESSURE ON THE TANK BEFORE YOU WORK ON IT. IF YOU CHANGE THE VALVE DRAIN IT FIRST!