I turn the faucet and hear the water running, like maybe filling

The inlet must be turn on as well as the outlet side (a hot water fixture) so that water can flow through the system. Make sure that the plumbing connections are correct.

Open hot water handle farthest from the water heater. Turn water on that fills the water heater. It will take several minutes to fill the tank. Let it continue to run until all air is out of line. Turn all other hot faucets on to purge air from lines.

turn off gas
turn off cold water to heater
connect hose to spickot
place hose end outside
turn on spickot
open hot water faucets in house
wait.
when water stops, turn on cold water for 30-60 sec or so to try to stir up any sentiment in tank
turn off cold water and let drain.
close spickot
turn on cold water
let fill till water start coming out hot water faucets then turn off hot water faucets.
turn on gas and relight pilot.
finish bleeding air from hot water faucets.
set water heater thermostat

I am assuming that you are referring to a calcium or rust build up in the water heater.
If you are having this issue you will need to turn off the water to the heater. You will need a garden hose, access to a outside water soucre, cold water is fine.
Shut off the gas or power to the hot water heater before starting.
Now you will need to place the garden house on the drain fitting on the water heater, placing the end of the hose outside and lower than the drain valve. Open the valve and drain the heater, you will need to open a hot water faucet to help drain the heater. I don't suggest opening the TPR ( Temp Pressures relief )valve due to the fact that they sometimes leak or don't reseal after opening them.
Hopefully the drain valve is not clogged, you should hear and see water exiting the garden hose.
If you are not getting any water from the drain valve you are going to have a challenging task.
If no water is coming out, my recommendation is to get the required tools to remove the hot water line from the top or side or the heater, also a few towels to soak up water A bucket is helpfull also.
remove the hot water line, then you will have to use a small hose to siphon the water from the tank. Once the tank is empty you can then remove the drain valve and replace it.
once you have replaced the drain valve you should ensure that it's closed and use a Gallon of CLR. Poor this into the water heater through the hot water outlet where you removed the pipe from. Now make sure the faucets are closed and replace the hot water line. You can now turn the water back on to the water heater.
After a few minutes open a hot water faucet to let out any air until water flows out, than close the faucet.
Let the water heater set for a few hours, then drain the tank!
when the tank drains, close the drain valve again and refill the water heater. Once again open a a hot water faucet and let the air out till the water heater fills. Again let sit for a hour.
At this point open a hot water faucet at the farthest point from the heater and let the water run for 5 min. , than close it and move to the next closest faucet and repeat until all faucets have been opened and run for 5 minutes.
If you still have a slow flow in a faucet, you may need to repeat the CLR step.
I also suggest that you remove all faucet screens and clean them.
If you still have a clog than you are going to have to remove or replace a piece of water line in the house. The blockage may be hard to find but if you have access to most of the pipe or it's visable you may be able to hear the blockage with the water on. Otherwise this will result in using some water line taps and a pressure guage to find the blockage.

1) Copy following link:
http://waterheatertimer.org/Crossover.html

2) Add comment with clear description of water heater, fuel type, and pump?
Is this a pump problem or water heater problem?
Is this a water well pump?
Are you getting good stream of cold water at tap?
Which water heater do you have?
Is the heater new?
Or older heater that stopped working.

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

You have water leak that you can hear running water.
Could be broken pipe under house.
Could be water running out of tank at TP valve.

Turn off all water taps and put ear against each pipe at tank.
If TP valve is leaking, then replace TP valve.
If pipe is broken under house, then call contractor.
Copy following links for troubleshoot walk-thru.
http://waterheatertimer.org/images/Ear-against-pipe-200.jpg
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#listen

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1) Guesses: Sounds like a bad single-handled faucet cartridge, or low water flow at faucet, or clogged water filter.
Replace faucet cartridge, clean the aerator, and/or clean sediment out of pipes below sink, and clean water filter at tankless unit each month.

2) For exact measure of problem:
Typical faucet can have 2 gallon per minute flow rate.
2 gpm flow rate means faucet fills 1 gallon container in 30 seconds. Test faucet using 1 gallon container and a cooking timer to get flow rate.
Clean aerator to improve flow rate. Compare kitchen flow rate with other faucets in home.
http://waterheatertimer.org/Low-hot-water-pressure.html

Most homes have 1/2" diameter pipe running to kitchen faucet.
100 feet of 1/2" diameter pipe holds 1.2 gallons of water. This assumes no sediment build-up inside pipes.
http://waterheatertimer.org/Water-heater-recirculation-system.html

Assuming only hot is turned on: If faucet is 100 feet away from water heater, and pipe diameter is unobstructed 1/2" diameter, and faucet has 2 gallon per minute water flow, then hot water should arrive in 36 seconds.
50 feet away, and hot water should arrive in 18 seconds.

If kitchen has single handled faucet, and the handle is raised in the middle, then equal amounts of hot and cold water are drawn at same time. This doubles amount of time for hot water to arrive.

If single handle faucet has a bad cartridge, then faucet can have crossover.
Crossover means that cold water can enter the faucet even if faucet is turned to hot.
Bad single-handled faucet cartridge can delay or stop the arrival of hot water.
http://waterheatertimer.org/Crossover.html

Tankless require maintenance including cleaning combustion parts yearly, de-lime heat exchanger frequently, and clean water filter each month.
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Add a comment for more help.

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Turn main water off to the water heater, it may be a valve on cold side coming in to shut off. Next turn power off to the heater and drain the tank the drain will be located near the bottom of the tank.
You will need a water hose to connect to the drain to run water outside. Open a faucet hot side to allow you to drain the tank.

Upon reinstall connect the tank up and fill tank and open up hot side on one faucet at a time to purge out air from system before turning power back on. Make sure you have water flowing from each faucet's hot side before turning power back on.

If the lower heating element burns out the upper element will heat the top part of the tank then shutoff. homw depot/lowes etc should carry elements. 1 turn off breaker, remove element access cover (the 2 panels on the front of the heater) test for electricity at the reset button ( uppermost device in upper panel) to make sure power is off. 2 turn off water (I usually turn off the cold water main to stop water cross feeding into hot supply line back into heater) 3 disconnect the 2 wires on the lower element, test with continuity tester. shhould be no continuity between terminals and tank, but around 10 ohms between the 2 terminals. Check the thermostat for continuity if tank is cold thermostat should be closed ie zero ohms across the 2 terminals.
If stat is bad replace stat; disconnect wires push it up out of the spring clip insert new stat reconnect. If element is bad: drain tank. connect garden hose to tank drain valve, open tank drain valve turn water main on to clear rust sediment and get water flowing. Next remove filter screen from a hot water faucet then turn on ( a hot faucet upstairs or higher than tank drain). Now turn water main off, air will now go in through hot faucet and tank will drain, takes about an hour. Use a water heater element wrench about$10 to remove element(s) make sure no bits of rubber gasket are left in element opening- dig em out with a screwdriver if needed. Insert new element with its gasket and tighten. reconnect wires DO NOT TURN ON ELECTRICITY! BE SURE TO FILL WATER HEATER WITH WATER ! Turn water main back on -air will come out of the hot faucet for several minutes. When water runs without any air sputtering, turn off faucet and replace the faucet filter screen. check element(s) for leaks. turn breaker back on.

you will need to have a 115 curent for electric, run a line off the panel box, turn off water to the house drain it down, disconnect the lp gas line and cap it off, remove the water heater, install the new heater run the cold side water to the cold side {blue on the heater} the hot side to the red, take the cover plate off on the water heater run the wires, hook up the groung wire then the red and white wires to the wires comming off the panel box,, fill the water heater with water have a open faucet on the hot side, and bleed the air out, DO NOT TURN ON ELECTRIC TILL THE AIR IS OUT OF TANK AND WATER IS FLOWING THRU COMPLETELY, after you get water commming out of the faucet turn it off and turn the electric on to the water heater ,, wait about 30 min, you will have warm after an hour it will be hot and you are done,, good luck write me if you have questions, [email protected]