Came down stairs water heater off and pan filled with water

then you need to call a plumber if you own the property, if you rent call the owner of said property

Tankless or hybrid water heater?

Tankless gas must have special fitting between water heater and vent that has a condensate line fitting. Hot toxic vent gasses traveling up vent pipe cause acidic water vapor to form inside vent pipe that drips back down vent and drips onto heat exchanger and other parts eventually burning hole through expensive metal alloy heat exchanger, rendering tankless computer water heater useless after electric circuitry gets wet and shorts out.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html#vent

Hybrid or heat pump type computer water heaters must have condensate drain connected to catch pan located just above storage tank and below compressor. Condensation can form on condenser coils and drip into catch pan where there is primary and secondary drain
http://waterheatertimer.org/Review-GE-Heat-Pump-water-heater.html#maintenance

Read product manual
Gene
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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

1. Are you on mains pressure2. Is the problem new. ( was it working once)If Yes could be a stuck HW washer.

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 you need to replace it, Actual cost can very widrly depending on your location but I would say $350-400 is probably close. Plus installation unless you can do it yourself. Hope this helps. Thanks

ballvalve may need new jet and washer

you have air lock in top floor rads must bleed it out start with the one closest to boiler and work your way out to what you figure is the end of line as best you can figure