If your piping looks ok coming into the tank, check the water shut off valve.
The water heater shut-off valve should be located on the cold water line near the water heater.
Check the valve to be sure it is opened all the way.
The next step is to see if you have the proper pressure going
in and coming out.
If you're water heater has flex supply lines (copper or silver colored) with a large nut at each end this check will be easy.
If your water heater is plumbed with copper or cpvc you'll have to cut the lines and remove the fittings from the water heater. If the plumbing is not attached to a nipple at the heater you will need to get a 3/4 in. nipple to screw into the water heater so you can attach a garden hose to it.
Turn off the water supply to the water heater. Grab your wife's
mop bucket and drain about 1/2 gal. of water out of the heater.
The water heater drain valve is located at the bottom of the heater.
Remove the flex line from the hot side (where the hot water comes out of the water heater) of the water heater.
Take a flash light and look down the nipple where you took
the flex line off. If you see a lot of build-up in the nipple you've
found the problem.
Don't try to clean the nipple this will just cause more problems later.
Replace the nipple with a dielectric nipple. The inside of these nipples are coated in plastic to cut down on rust.
If you don't see a problem at the nipple attach a garden hose
to the nipple and run the hose outside. Make sure its not kinked
Turn the water on at the water heater. Go outside and see how strong it is running.
You should have about as much pressure as you would have coming out of an outside faucet if you attached a hose to it.
If the pressure is good you have eliminated the water heater
as the source of the problem.
The problem will be between the water heater and the faucet nearest the water heater(if the faucet has low hot water pressure). My guess would be a restriction in an elbow somewhere in the line, maybe near the water heater.
At this point the only thing to do is open the water line at the sink and clean the line back to the water heater with a snake. Should you try it? I don't recommend it. The snake could get hung in the line. Never try this if you have galvanized pipe.
If the pressure is not good coming out of the water heater you'll need to check the pressure coming into the water heater. This will involve checking the cut-off valve and the water heater dip tube.
The cut-off valve or gate valve that controls the water supply to your water heater can malfunction and cause low hot water pressure
There are a couple of ways to check the gate valve. Choose the one that's easiest for you.
Shut the water off to your house, remove the top of the gate valve,check that its working properly and for any obstructions.
Detach the flex line on the cold water side. Screw a 3/4 in. nipple into the end and attach a garden hose to the nipple. Run the hose outside, turn the shut-off valve on and check the water pressure.
It's possible that the water heater fill tube has clogged. To check it, remove the water supply line on the cold side of the water heater. Remove the nipple if there is one.
The fill tube is a plastic pipe with a flange at one end to hold it in place. To remove it insert your finger into the pipe. Wiggle your finger to loosen it and pull it out.
You will see a small hole in the fill tube about 6 in. from the end. It's suppose to be there.
This is a fill tube that I removed from an old water heater behind my shop.
While you have the fill tube out of the water heater check it for cracks. Replace it if you find one.
If you have copper plumbing it can be a pain to cut it loose. You
can check your fill tube another way.
Attach a garden hose to the water heater drain valve and run it outside. Turn the drain valve on and check the pressure coming out. If the pressure is equal to an outside faucet the cut-off valve and fill tube are ok. If the pressure is low you will need to drain the tank, remove the drain valve and check it to be sure its not clogged. Reinstall it, fill the tank and open the drain valve again.
If you are sure the drain valve is not clogged and the pressure coming out the hose is low then the problem will be incoming water pressure.
Other causes of low hot water pressure
If you have a water softener by pass it and check your pressure.
A water leak can cause loss of pressure. If your house is on a concrete slab walk around the house and look for a wet spot. Put your ear to the floor in the kitchen and bath. You may hear water spraying. This really works!
To eliminate future low hot water pressure problems. It's a good idea to flush your water heater every three months. flush hot water heater
Clean your water heater at least once a year. cleaning hot water heater
Water heater home
low hot water pressure
Low hot water pressure part 2