Gas water heater running only cold water

certain water heaters had a tube that dissolved, you take 1/2 in. copper flare one end of it, drill a1/8 in. hole in pipe 4 in. below flare ,remove cold water nipple on tank install new copper tube, cut tube to 3 inches off bottom of tank depth , reconnect cold water supply you should be good.

It is probable the cold water comes in from the outside pipe so will be unaffected by anything that happens to your boiler/water heater. The boiler/ Water heater must have an issue, the problem is not a water cut I THINK.

As cold water enters the heater. its heated by the pilot flame, and takes a while before u have total hot water? what do u mean by cuts off? perhaps u have a pilot heating problem due to it going or coming off and on? are there any air drafts around the heater?

has this heater always acted like this? if so
you need to supply heater with the right water pressure or flow
try cutting back by turning inlet valve off to restrict flow or restrict flow on hot end. hope it works it did on mine

Manual Yes 12 volts may help. I can't see anything about your switch from this manual

If incoming water is too cold, tankless might not have enough time to heat the water as it flows by insince the heat exchanger.

1) You can passively preheat cold water using staging tank, or tampering tank.
http://waterheatertimer.org/Tempering-tank.html

2) Buy old water heater from plumber that is not rusted out.
Replace anode rod to stop further rust.
Rinse tank with water and hydrogen peroxide to kill bacteria that results from anode rod.
http://waterheatertimer.org/Replace-anode-rod.html
Clean sediment out of bottom:
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

3) Don't forget, a tankless heater needs to be de-limed each year, and cannot be exposed to more than 11 grains hard water.

4) Since all costs are energy, when you get tired of messing with tankless.
http://waterheatertimer.org/How-to-install-gas-water-heater.html
http://waterheatertimer.org/images/Compare-water-heater-parts.jpg

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.

You have 240volt electric water heater
Check both elements for leaking.
Check top of tank for leaking pipes.

If water heater is leaking from side of tank, then tank is rusted out and could rupture.
This tank must be replaced.
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html
My friend welded his tank to get more life out of tank, but he assumed the risk.

Hot water heaters are under pressure from heated water which expands.
If tank ruptures, water will flood out of tank and water will run until shut-off is turned OFF.
This causes major damage inside home.

Protect yourself first, and then let's talk more.
1) Turn off water at shut off located on cold water line above tank.
2) Turn off power to water heater:
Gas heater, rotate valve to 'pilot' so pilot is lit but burner does not turn ON.
Electric heater, turn off circuit breaker.
3) Open tub spout on hot side and let run a bit to relieve pressure.
4) Now your property is safe.

You can limp along with failed heater if leak is slow.
Turn heater ON when you need hot water.
Typical Gas heater makes 41 gallons hot water per hour.
Typical Electric heater makes 21 gallons hot water.
Shower uses 5-9 gallons hot water, bath uses 12-14 gallons hot water.
http://waterheatertimer.org/images/Water-heater-reference-guide-2.jpg

If you are going to install heater yourself:
Look at label on side of old tank to identify size and particulars.
I buy ordinary heaters from Lowes and Home Depot and always buy scratch-n-dent models when possible, because superficial damage to outer tank does not lower lifespan of heaters I have installed.
Buy same size heater if it worked fine before.
Buy new flexible water connectors.
Set water heater in drip pan.
Offer 10% less than price shown. Make them deal with you. Shop Ace hardware, Lowes, Home Depot. Kick the tires since you have a bit of time.

I know you have electric heater, but many folks read these answers, so I include info on gas too.
#1 rule) tankless heaters are a rich-man's toy, because tankless will never pay you back, and will become major money pit if you encounter 1 repair. Electric tankless heaters require larger electric service box. Gas tankless require larger gas line (larger gas line, more gas consumption, bigger bill unless you cut consumption)
2) How to install gas heater:
http://waterheatertimer.org/How-to-replace-gas-water-heater.html
3) How to install electric heater:
Read link above because many of same steps apply to electric.
The difference with electric, is turn off power, disconnect wires, and reconnect wires exactly like before. Set electric heater directly on insulated surface like dense blue styrafoam. Cut styrafoam in circle so it fits inside dip pan. Wrap entire electric heater with insulation blanket, except don't cover TP valve.
4) Connect bare copper jumper wire between hot and cold pipes to prevent stray current corrosion which will shorten life of tank. Make sure cold water line is grounded.

Also read:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

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Try turning the thermostat uo a small amount and see if this makes a difference. I suspect the control board is bad but I would try adjusting t-stat before I spent money on board or service call. Thank you

check diaphrapm valve or micro/switch best of luck

I pulled the water heater out of the rv and found the one way value broken and cloged.replaced with new one $9.12 and put back together.works great.