My bolier gas supply is going on and off while the water is on

you can go to get another relief valve because if its dripping its probably bad. if you have a clark supply center nearby, you can get one there! or another plumbing place or water heater supply. you may have to drain out some water to bring the level below the valve. Shut off the power to water heater. If electric, turn gas to pilot setting, if a gas water heater. Cut off water supply to water heater as well. It is not necessary to drain the entire tank. Open a hot water faucet, drain water from tank, drain for approximately three to five minutes. Prepare the new relief valve with Teflon tape. Unscrew the relief valve counter-clockwise with a wrench to remove.
Turn the water supply back on to water heater. Make sure water is running from the open faucet before turning the power back on. good-day!

I had the same problem. There's another clear rubber pipe underneath, that runs from the front right to a pump in the middle. I had to pull it off and dry it with a hairdryer before mine would light properly. Blast into the hole the pipe came off too to dry the sensor if there is one.

You won't see any red glow until the burner ignites and burns for a while. Its only an ignitor and flame sensor, not a glow plug style.The ignitor has to provide spark for gas to ignite, then it also senses the flame and keeps gas flow coming. What exactly is happening when you turn it to on?

Sounds like a flow sensor may be bad. It may be registering flow when there is none and it sending signal to ignitor and gas solenoid that there is.

The 250 MUST have the condensate line connected. EC comes from gas supply problems and CO2 levels not being adjusted. LP units require 3/4" smooth gas pipe or 1" corrugated pipe minimum size. The LP regulator needs to be turned up to about 13 inches of water column and with all the gas appliances on, measure no less than 11 inches of water column. The CO2 levels must be measured by a qualified gas technician with a calibrated (with in 6 months) CO2 analyzer.

If it is the pipe leading in or out, you can repair it yourself with a torch and solder, after draining the pipes and turning off the gas.
If it is the tank itself, you will need to replace it. The tanks cannot be reasonably repaired, so replacement is your only option. Legally, you must call a certified gas fitter (or plumber with a gas ticket) to handle the removal and replacement.
I suggest shopping around yourself for a heater as prices vary widely, and plumbers generally charge more than a home improvement store like Lowe's or Home Depot. Have the heater on hand when the plumber arrives and it will be a very quick job.

The pilot assembly should look similar to the pic above. Depending on where you live, you may be required to hire a certified plumber to replace the parts. If not you can do it your self. Keep in mind if it's not staying lit you also need to check the thermostat and the thermocoupler before replacing the pilot assembly. There is a good chance it is just the thermocoupler going out. It is one of the first things to check when troubleshooting a pilot light problem with a gas water heater. A thermocouple determines if the pilot light is lit. If it's not lit, the gas valve fails. If that's the problem, replace the thermocouple.

1. Shut off the gas valve on the water heater and the gas valve on the line near the heater. Remove outer and inner doors, and loosen the pilot and supply tubes and the thermocouple connections to remove the burner assembly.
2. Step 2 Loosen the screw holding the thermocouple to the bracket. Remove old thermocouple. Bring it with you to the store to ensure you are buying the proper replacement.
   
3. Step 3 Install the new thermocouple. Position the thermocouple so that the pilot flame heats 1/2 inch of the tip.
   
4. Step 4 Reinstall burner assembly in the water heater. Tighten burner supply tube, pilot tube and thermocouple connections. Tighten thermocouple no more than a quarter turn more than hand-tight.
   
5. Step 5 Turn on both gas valves. Check supply tube and pilot tube for leaks with soapy water. If soap bubbles, tighten connections to fix leak. Ignite the pilot light, and make sure the thermocouple tip is heating up. Replace inner and outer doors.
   

you need to change the cartridge, it is a temp control and is going bad so it keeps you from burning yourself it is like a mixing valve and is givng more cold water, that way you don;t get burned,,

First check your gas supply, gas in bottle or lines? I know, this is basic & I'm not having a shot at you, but you'd be surprised at how many times it gets overlooked.

If you do still have gas supply, evident by that low hiss when you hold the pilot feeder button in (you might have to lean in close to hear it), then you need to check that the piezo is working. This is the bit which produces the spark to ignite your pilot. You will be able to see it sparking at the pilot outlet when you push the button (if you're looking in the right spot).

Ok, you have gas & a working spark? Not sure which buttons to press?

You mentioned the manual no longer being in a helpful condition...do you know which of the 4 buttons, 2 taps, 1 slide control & 1 dial do what?
Well just in case, here we go in simple form... first button on the left is the pilot feeder (lets gas through the pilot), second button from the left is the 'ON' button, third from left is the 'OFF' button & the fourth from the left is the piezo ignite button (gives you that much needed spark!). The slide controls your flame (how much gas gets fed to it). The dial, below the pushbuttons & slide, controls how much water flows through the system which then helps regulate the heat. Make sure the taps on your gas & water lines to the unit are on. There you go...2 taps = gas & water supply, 4 buttons = pilot gas, on, off, piezo (spark), 1 slider = gas/flame control, 1 dial = water flow control.

Ok let's see if we can light it! Water & gas supply turned on, slider set about 25% from left, dial set roughly in the middle, push in button 1 (1st from left) & hold in for about 10 seconds, you should hear a very faint hiss from pilot area. Yes? OK, keep holding button 1 in & press & release button 4 several times until pilot lights, you may not see a difference but if you can't see it and it does light you will hear the faint hiss change to a soft fluttering sound as the flame consumes the gas. Still OK? if so then the next step is to push in button 2 quite firmly & then release button1 as well. If all works as planned you will shortly have hot water supply restored.

If the above has not worked then your hot water service may just need a service to clean everything up & maybe replace a few worn or excessively corroded parts. Otherwise ,if there are no blockages in supply lines etc, then it may be time to replace it.

You neglected to say how long you've had it.

Cheers, hope this has helped.

Dennis

The tankless water heaters require larger gas supply lines because of the on-demand consumption. While I don't know your units rating, most Propane Tankless Waterheaters require 2" supply lines. Moreover, pressure is directly related to temperature which sounds like your problem entirely; consider the run of the gas pipe from the tank and the ambient temperature. Now you see why plumbing is a hard profession to fake... =)