Hot water tank is leaking slowly and water accumulating in the drain pan. What to do?

Add a potable expansion tank

Copy following links:
http://waterheatertimer.org/Water-heater-will-not-drain.html
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Put next water heater in drain pan.
Gene

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

Probably bad news if steel tank is leaking.

1) Copy following link for do-it-yourself troubleshoot walk-thru:
http://waterheatertimer.org/Leaking-water-heater.html

2) Drain tank and set in a drip pan with drain line to outdoors.
http://waterheatertimer.org/images/Water-heater-drip-pan-2.jpg
http://waterheatertimer.org/Water-heater-will-not-drain.html

3) If tank ruptures, it will flood home

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

I recommend safety changes to prevent ruptured water heater and assorted leaks.
135 degree hot water.
Maybe replace TP valve if it is dripping.
Drip pan under water heater with 3/4" drain line to outside.
Expansion tank on incoming cold water line to absorb pressure build-up, and prolong life of tank and plumbing.

Here's why:

1) Yes you can have scalding hot water.

2) I recommend 135 degrees since that will burn the evil right out of ya. And generally kill clothes washing bacteria.
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html

3) 150 is dangerous and can seriously burn skin in 1.5 seconds. Not safe around children and elderly.

4) Temperature settings on electric water heater are approximate, since thermostats are calibrated approximately, and read temperature through tank wall.

5) Also if you want to increase amount of hot water for high-consumption household, then setting tank temperature upward is one solution since more cold water is mixed with hot, so amount of hot inside tank goes further.

6) High temperature can cause TP valve to release water.
TP valve is pressure-temperature safety valve. Do NOT cap off.
Rating should be 125 psi and 180 degrees ... but all things water heater are approximate, and TP valves need replacement now and then.
It is not recommended by water heater manufactures to use 150 psi TP valve when water heater came with 125 psi. Rating shows in small print on end of TP. New TP valves sold at local hardware. Let tank cool before installing new one.

7) Higher temperature and high pressure can rupture tank.
Pressure over 80 psi shortens life of water heaters and plumbing fixtures. Put BTG100 pressure gauge on water heater drain valve to check pressure.

8) You need a drip pan with 3/4" drain line to outdoors, or to drain vent line, to prevent flood caused by ruptured water heater.
Turn water heater off when going on vacation.
Inspect water heater each 6 months.
Drip pan available at local home center or hardware:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html#drip

9) Heated water expands and puts pressure on tank and pipes.
Water does not compress... so when water gets hotter it expands against tank wall and against pipes.
Especially in closed system (check valve present) when system is without expansion tank.
Expansion tank is installed on incoming cold water line.
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html#expansion

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

If you mean the drain valve, then buy garden hose cap at home center.
Cap will have rubber washer that seals tank.
If drain valve is plastic, the cap may not thread on without sanding down end of plastic drain valve.
Otherwise replace drain valve, which is simple if drain valve is brass, but can be difficult if drain valve is plastic.
Turn off power to water heater: Electric: turn off circuit breaker. Gas: turn knob to pilot.
Turn off water, open bathtub spigot on Hot side, drain water heater, remove drain valve with wrench.
If plastic drain valve breaks while being removed, then 'gently' break out broken part using hammer and big screwdriver.
Buy new drain valve at box store in water heater department.
Put teflon tape on new drain valve threads.
http://waterheatertimer.org/Water-heater-is-leaking.html
http://waterheatertimer.org/Water-heater-will-not-drain.html
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

Add a comment for more free help.
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.

Hot water pressure relief valves serve two functions. The first is to remove high-pressure water from the hot water tank. The second function is to act as a high temperature valve to remove water that exceeds a predetermined point for the valve. Both of these relief settings are preset at the valve factory. Many pressure relief valves will come with a metal tag that identifies these release points. If you're attempting to remove the hot water heater pressure relief valve, pay careful attention to ensure that you replace with the correct valve.

Your hot water heater has a pressure relief valve safety mechanism. When pressure inside the water tank exceeds certain limitations, the valve opens up to allow the pressure to be released. A faulty pressure relief valve will not release excess pressure, which could cause extensive damage to your home. You should test the hot water heater pressure relief valve once every six months to ensure proper operation. If the pressure relief valve fails the test, replace the valve with a new one immediately

Locate the pressure relief valve on your hot water heater. The pressure relief valve has a lever handle on top with a drain pipe attached to it. The pressure relief valve is most often on the side of the water tank toward the top. Newer models of hot water heaters have the pressure relief valve on top of the water tank.

Inspect the area around the pressure relief valve for signs of leaking water. If there is leaking water near the relief valve connection, replace the relief valve. If no leaking is evident, proceed with testing the pressure valve.

3

Examine the area below the pressure relief valve drain pipe. Most water heater closets either have a drain in the floor below the pipe or the pipe is inserted into another pipe to direct the water out of the home. If the end of your drain pipe is not in another pipe or there is no drain in the floor, place a bucket under the drain pipe to catch water when you test the pressure relief valve.

4

Pull the lever handle on the top of the pressure relief valve up with your fingers. A working lever will lift up without difficulty.You should begin to hear water going through the drain pipe. Slowly release the lever to shut off the pressure relief valve. The pressure relief valve is good. If you did not hear water running through the drain pipe, lower the lever on the pressure relief valve and replace the valve. If the lever is difficult to raise, do not force the lever. Replace the pressure relief valve.

Now with all that said if you have a closed loop water supply system and you have a pressure vessal/expantion tank it may have gone bad and is making the TP valve leak. you can call your water provider to see if you have closed loop system.

Water heater should be set in a drip pan.
Otherwise, put 1 gallon bucket under TP release pipe to catch water.
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html#drip-pan

When you say relief valve, I assum you are talking about brass valve located on top of tank or near top of tank. This is TP valve and a pipe connects to this valve and runs down to within 6" of bottom of tank. TP valve has a small lever on it.
If you are talking about tank drain valve located at bottom of tank, then add a comment.
Following information is for TP valve.

Dripping TP valve can be caused by 3 different thing:

1) Obstruction inside TP valve. Open TP valve for a moment to flush away particles and slowly lower lever back down. See if water stops dripping, if not, then replace TP valve

2) Defective TP valve: replace TP valve. Turn off electricity to water heater. Let heater cool for several hours so you do not get scalded. Close shut off valve located above tank on cold side. Open tub spout to relieve pressure, open tank drain or open TP valve and empty water below TP valve level. If TP is located on top of tank, not much water has to be drained. Tank is under press when hot, so be careful. Put teflon tape on new valve and install with big pipe wrench by turning clockwise until TP valve is very tight.

3) Overheating tank. TP valve is temperature-pressure valve. If tank water is over 150 degrees, the TP valve starts drizzling hot water to relieve tank pressure.
Lower tank pressure two different ways:
a) turn down thermostat
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html
b) install expansion tank on incoming cold water line
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html#expansion-tank

It sounds like two problems.
Problem 1) Clog inside faucet or clog inside pipes at faucet
Remove supply tubes leading to faucet and see if problem is inside pipes or inside faucet.
My bet is clog inside faucet.

If you have galvanized pipe that meets copper pipe, this can be trouble spot.
Article on clogged water pipes at Inspectapedia:
http://www.inspectapedia.com/water/Water_Pipe_Clog_Repair_Guide.htm

Article on clogged pipes:
http://www.waterheaterrescue.com/pages/WHRpages/English/Troubleshooting/Tanklets/backflushing.html

Problem 2) Water heater leak ... you say tank valve is leaking? Is that cold-water shut-off valve? Or tank drain valve? Or TP valve?

If cold water shut-off is leaking, then replace shut off.

If tank drain valve is leaking, then check if plastic valve is cracked. If drain valve is cracked, then replace with a brass valve from Home Depot. If plastic valve breaks off, then use hammer and screwdriver to gently chip out the broken plastic valve. New valve threads need teflon tape to seal pipe. If valve is just dripping, then put garden-hose cap over end of valve and screw down tight.

If TP valve is leaking, then replace with same temperature and pressure rating. New TP valve threads need teflon tape to seal pipe.

Problem 3) If pressure is low all over house:
Buy pressure gauge at hardware store that screws onto hose connection.

Test pressure on outdoor spigot.
Open faucet and check if pressure drops. This says if problem is inside pipes that enter house.

Test pressure on water heater drain valve.
This says if pressure problem is before -or- after water heater.

If problem is before the water heater, then shut-off valve located on cold water line is suspect.
If problem is after water heater, it could be in hot water outflow pipe on top of water heater, or a nearby elbow.
Remove hot water line leading from water heater and check pressure. This says if problem is where hot water leaves tank.
Read articles above about clogs inside water lines.

If your water tank is leaking, is it dripping from above or actually leaking thru the bottom?. Can you find the source? Feel around and try to find wet spots above and around it. If it's not leaking from the water pipes or T&P valve, it's your tank leaking. You'd be wise to have it checked out in the morning, before it gets worse. Hopefully your catch pan has a pipe attached to divert water to the outside or to a sump pump or floor drain. Water heaters do sweat, but you can tell the difference if it continues to leak for several hours.

If you're sure it's the DRAIN, just get a garden hose cap for it (your other option is to replace the valve - drain tank, replace valve, refill).
NEVER CAP THE RELIEF VALVE DRAIN!