The water turns cold in my tub after tub is about 4'' full.

Gas or electric?
Electric water heater ... then test for crossover, check for water leak, replace thermostats, test elements
Gas water heater ... test for crossover, check for water leak
http://waterheatertimer.org/Crossover.html

http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

http://waterheatertimer.org/How-to-test-water-heater-element.html

water leak: turn off all taps and put ear against pipe and listen for water leak... check if water meter advances when all water taps are off.

Gene

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

if you do not get a light showing on the topside panel indicating it is heating the first thing to check is the filters. Remove them even if they are new. If the problem persists then check to make sure both valves are completely open on either side of the pump so you have not restricted the waterflow.If the valves are open and the filters are removed and the heat indicator light still does NOT come on then you most likely have a bad pressure switch and should be changed.there are a couple of other things that would cause a hot tub NOT to heat but that is the starting place for you.
RodUS Parts Center888-521-6183

82V52-2 is typical 240Volt 50-gallon electric water heater.
Water heater has to be full of water before turning on power.
This is true for gas and electric water heater.

Once water pipes are connected, you are ready to fill tank.
Turn on cold water shut-off so water enters tank.
Open bathtub spigot on hot side.
When tank is full, water comes out of tub spout full stream.
Let the water run until there is no more air coming out.
Next, open TP lever on water heater for a moment, and this will bleed out last pocket of air.
TP valve is located on upper part of tank, on the side or on top. It is a brass valve with silver-colored lever. Lift lever for a moment and water will exit tank down the overflow tube.
Water heater is full.
Turn on 240Volt circuit breaker going to water heater.

Other links with electric water heater information:
http://waterheatertimer.org/How-to-replace-water-heater-drain-valve.html
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html#cleanelectric
http://waterheatertimer.org/How-it-works.html

Once the hot and cold pipes are connected to water heater, then open bathtub spout on hot side, turn water on so tank begins to fill.
After tub spout is running full stream without air, then water heater is full and it is safe to turn on the heater.

Turning on gas water heater before tank is full can damage tank.
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Turning on electric heater before tank is full will burn out elements. Replace elements.
http://waterheatertimer.org/How-it-works.html

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Fixya is always less expensive than a service call.

If I understand correctly you have residential electric water heater, and your shut off valve located above water heater on cold-water line is leaking?

1) Hot water would come out of this valve if it's leaking because water inside tank is hot and under pressure ... also remember, hot water rises so tank is probably heating the water inside the cold line during hours when no hot water is being used.

2) You probably need to replace the valve. But if water is leaking because a pipe is rusted, the valve may be ok, and just the pipe needs to be replaced.
Take a photo of your problem to hardware store or home center.
I used to work in those stores and could help folks get the right parts.

3) Here are some steps for replacing the valve.
Shut off main water source into house. This is usually located underground in same box where meter-reader looks at meter.
Your water heater shut off valve is called a gate valve, and it will be 3/4"
All water heater pipes are 3/4"
You will need two 10-12" pipe wrenches, maybe a new gate valve, maybe a new flex connector, probably short 3/4 pipe pieces, teflon tape for each threaded pipe.
Hardware store or home center can get you the right materials, just take a photo over to hardware store

4) When you get started, turn off water heater circuit breaker for a couple hours so water will cool down. You don't want to get scalded with hot water.
Do NOT drain the tank.
Simply open a bathtub faucet on hot side to release pressure.
When finished, turn on the water, and make sure water is flowing out of tub spout in full stream before turning circuit breaker ON

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I can think of a few possibilities. 1) Something is blocking the hot water in to the mixing valve. Fixing this would probably mean cutting out the valve (then redoing the water lines with a new valve (possible option)).

2) The hot water line is losing too much heat going to this second faucet. (If the two tubs are on different floors this is more common. Many places are plumbed with one line with Ts to all fixtures.) Insulating the water lines might help; alternatively an point of use water heater would help.

3) The hot water and the cold water in are reversed. (Test by leaving the water on full cold and full hot for a few minutes each.) If you get full hot when it should be cold, valves with a cartridge are easy to fix. Open the valve assembly, remove the cartridge and reinsert it after turning it around (side closest to you when you removed it goes in first). Otherwise, the incoming plumbing needs to be redone at the point where the cold and hot lines come into the shower.

I hope this helps.

Cindy Wells

First thing to do is wrap rag or towel around spraying joint to localize water.
Next, turn off power to water heater.
If gas, rotate knob to pilot so pilot light stays lit but burner does not come on
If electric, turn off circuit breaker.
Close cold-water shut off valve above tank.
Open tub valve to release pressure on water heater.

Wait for water to cool before working on water heater to avoid high pressure scalding water.
When finished, make sure tank is full of water and water comes out of tub valve before turning on power.

Water heater should not have plastic couplings.
I can't see your problem exactly, but water heaters have 3/4" nipples coming out of top.
Easiest solution is use flexible connectors from Lowes or Home depot.



Flexible connectors are 24" or 30" long and have 3/4" nut on each end, so disassemble existing pipes using pipe wrench.
Lowes and Home depot can help you select appropriate pipe >> take several phone-photos of your pipes to home center so they can see your problem precisely.
Attach 3/4" male adapter on end of pipe.
Attach flexible connectors between pipe and water heater nipple.
Cold water line has shut off ... make sure this attached to nipple marked cold.
Make sure hot pipe is attached to nipple marked hot.
Use teflon tape on threads.

Add a comment if you need more help.

How big is your bathtub? Is your water heater set on the hottest setting? What is the temperature of the hot water at the tap?

Keep in mind that hot water can cause severe burns. Children and those with diminished mental capacity can be seriously harmed by hot water.

The "40 Gallon" rating on the water heater is usually the amount of hot water the unit can produce in one hour. So the tank, and the amount of immediately available hot water, might be smaller than 40 gallons.

My bathtub is 2 feet wide by 1 foot deep and 4 feet long. This is 8 cubic feet or almost 60 gallons. So a 40 gallons of water at 120 degrees, mixed with 20 gallons of cold water (60 degrees), fills the tub with 100 degree water. My January cold water is 42 degrees, so it will take 40 gallons at 129 degrees to fill the tub. If the tub is cold to start it will take even hotter water.

So the water heater might be just fine.

Possible solutions:
1. turn the temperature up on your water heater- or the temperature control could be malfunctioning and not getting the water hot enough.
2. insulate your hot water pipes so that the water loses less heat on the way to the tub.
3. bigger water heater.
4. smaller bathtub, or less full bathtub.

It could be back flowing into the hot water heater tank which is not full because of the leak. Even though the valve is closed on the cold water entry side, there is probably no valve on the exit side. That is how it is returning to the tank.

There is something in the line or in the faucet.Are your pipes going to the faucet black iron or galvinze.Some of the pipe will rust up and stop a full flow of water.But I would check the faucet first to see if the screw is loose or somethng in it.