Hello we have 75 gallon Rheem hot water tank that was built in 2004. We have replaced most of the parts in the tank that control the fame opertation and we still have the tank when heat is called for, come on and off continously. please help

Tank type heater?
Replace gas control valve?
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Gene

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Whats it doing or not doing?

1) Copy following links:
http://waterheatertimer.org/How-to-replace-thermocouple.html
http://waterheatertimer.org/pdf/How-to-replace-water-heater-gas-valve.pdf

2) Inspect for broken TRD
http://waterheatertimer.org/How-to-repair-Rheem-TRD.html

3) Rheem air intake located below combustion chamber.

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

Hopefully just the thermocouple. The thermocouple is the small rod right next to the pilot light. The flame on the pilot heats up the thermocouple and keeps the gas valve on. You can replace it yourself. The thermocouple will be connect by a small nut on the underneath side of the valve, and you will have to pull the thermocouple out of the pilot assembly. Not an expensive part, you can get it at any hardware store. Hope this helps you .

You have 75 gallon gas water heater.
I install water heater insulation 3 ways:
1) Easiest: buy ready-made blanket from Lowes and use duct tape
2) Harder: wrap heater in ordinary fiberglass insulation and use tie wire
3) Hardest: Build a box out of blue styrafoam >> leaving room for air intake for gas water heater, and space around vent pipe

Instructions for gas water heater:
Do not cover TP valve.
No insulation is put over gas valve or air intake.
No insulation touches vent pipe.
Remember the gas water heater has to breathe air into the burner, and aftter combustion takes blace, hot gases escape so extra space is left around hot vent pipe to avoid fire.

Find age of Rheem: look at 3rd and 4th digit of serial number, for example 06 was manufactured in 2006.

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

I sounds like you have build up in the bottom of the tank. Sounds like its time to replace it.

65 gallon water heater should fill more than 1 bathtub since typical bath only uses 12-14 gallons hot water. First hour delivery on your tank should be greater than 60 gallons.
http://waterheatertimer.org/Hot-water-use-chart.html

Since you have such a small amount of hot water, it sounds like bottom element is burned out, or one of the thermostats is bad. There are two thermostats, and two 4500 Watt elements on 65 gallon tank.
If upper element was burned out, or water heater was not getting 240V electricity, then you would have NO hot water. So we know upper element is good and circuit is good.

Ordinary tank-type electric water heater is fully repairable by homeowner and parts are available at hardware or home center.
I suggest taking 30 minutes and doing troubleshoot before replacing any parts.
Buy cheap multimeter from home center.
When testing voltage, rotate multimeter dial to 240-250-277 VAC
When testing elements or continuity, rotate dial to Ohms.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Add a comment for more free help. I can answer questions as you fix the water heater.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on heater.

A leaking water heater cannot be repaired, assuming it's the tank leaking and not nearby water pipe.
http://waterheatertimer.org/Water-heater-leaking.html

First thing is to protect your property from catastrophic tank failure where tank ruptures and water runs until it is shut off:
Tanks are under pressure when they are hot.
Close cold water shut-off located above tank.
Turn control knob to 'pilot' so pilot is lit but burner will not turn on.
Open hot side on tub for few moments to relieve tank pressure.

Now your property is safe.
You need a new water heater.
In mean time, gas water heater makes 41 gallons hot water per hour.
Shower used 5-9 gallons hot water.
Bath uses 10-15 gallons hot water.
You can turn on water heater for short stretches and limp by until you get a new heater.

Steps for installing gas water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Serial number should be on label on side of tank.
Labels contain information like model number, tank capacity and BTUs
First 4 digits of serial number are date of manufacture: for example 0604 is June 2004
Take photo of label for easy reading and for your records

http://www.rheem.com/products/tank_water_heaters/gas_water_heaters/

Label for Craftmaster Gas 30 gallon:

Typically, this problem is solved by the installation of an expansion tank. You will be able to find one at your local home supply store.