Tankless water heater back pressure issue

[Newbees]

Howdy ya'll! My camper had a portable tankless water heater that stopped working and installed another on the same plumbing that the last person installed. It worked fine at 1st then when shut off wouldn't turn back on. Called the customer service and they said it was back pressure. So I added a release valve. Problem is, it seems to back up pressure constantly. I thought maybe it was the way my kitchen sink was plumbed being that it's hot and cold with one knob and seems like the shower works fine as long as I only use the the hot water knob, but sometimes that's winky too. Any plumbers out there?

[chuckster57]

What is the water pressure going into the unit?

[Newbees]

Originally Posted by chuckster57

What is the water pressure going into the unit?

I'm connected to city water. I know it's enough to turn it on because as soon as I open the flush valve it starts right up.

[chuckster57]

Originally Posted by Newbees

I'm connected to city water. I know it's enough to turn it on because as soon as I open the flush valve it starts right up.

Are you using a water pressure regulator? I’m just thinking out loud, but trying to eliminate obvious things.

[jasin1]

It looks like it only is supposed to work when the faucet is on or open...all the pictures I see of it show it as a temporary hookup by connecting a garden hose for incoming water and just a handheld shower head for outlet.. if you are using it connected to all of the plumbing then when you shut off the faucet it will not come on and maybe is building up the back pressure and keeping it off. I seems like your trying to make a portable unit work like a permanent one

[lunge motorsport]

I have experience with heaters of this type. When troubleshooting tankless the first thing I look for is a “crossover”. In other words cold water is entering and pressurizing the hot water supply. In residential applications it is usually a thermostatic or pressure balancing mixing valve that has failed. As your photo doesn’t show the piping in and out of the heater I can’t tell if you would be able to shut off the cold water supply to the heater. If you can do that, you can check for pressure at the hot side. Pressure on the hot side would indicate a crossover. Look at the outdoor shower as a potential crossover point, insure water to the shower is off to at least one of the valves there. You have already proven that the heater functions when the drain valve is open so I don’t believe this is a heater issue, I believe it is a flow issue. Also check for reversed hot and cold piping to the unit.
Good luck