Exactly the problem and answer!
 

Our 3 month old new home in Newell had the problem of hot water turning into cold water during a shower. The installation plumbing company came out and fixed our problem yesterday!

The problem with the gas tankless water heater turning off during a shower was the low water flow…they require an increased flow to operate. The plumber opened up the shower head water flow restricter. It comes with restricter to reduce your water consumption!

Problem resolved!

Ohiobuckeye
 

I have a Tankless Water Heater but it’s outside, now isn’t that stupid. I babysit it all winter. OK, back to your question, I have the same issue but the only way I get hot water is to run the hot water for a couple of mins. until it start to run hot. Then you never run out of hot water, even if you run it for 2 hrs. I don’t agree with the person that said it’s in your circuit breaker box, unless it kicks out your breaker & you have to reset it. Run your hot water for a couple of mins. & see if you get hot water then otherwise whoever sold you the Tankless Water Heater call them back. It’s not the breaker box unless it’s kicking out your breaker for Water Heater!

Going cold
 

Over 4-5 years ago we installed a Rinnani Tankless Water Heater. We use natural gas. Never had an issue. Are you using electric. If you do not get satisfaction call the home office for the manufacturer.
The only complaint we have is it takes about 1 minute to get hot water as the pipes are in the ground under the slab.
We can run 7 gal a minute for as long as the faucet is turned on.
Good luck

If the water only goes cold using your showers, and not when filling a tub or using your sinks you need to remove the restrictors in the shower heads. If the water goes cold using the tub or sinks, I would check the screen on your inlet valve. If the plumber turned off your water service to do the install, some dirt may have been kicked up in the lines when the water was turned back on. There is a stainless steel screen on the feed (inlet) side of the heater.

If you have a mixing valve that was not change, it most likely is the problem. You can have it changed or removed but be warned without it, the water will get scalding hot and you need to careful mix enough cold with the hot.

This was happening to many of us new homeowners down here in DeLuna a few months ago. It ended up being an actual faulty part that needed replacing. It wasn’t due to flow restriction. Mike Scott, the company that installed the tanks here, was sent by warranty when this happened to us, and at first they wouldn’t admit what the issue was because we didn’t have an error code to show them. But when I mentioned that this had happened to many homeowners in our village, and they had all shared what the issue was on Facebook, they finally admitted it was a faulty part. They replaced it and we’ve never had the issue again after several months. We still have our flow restrictors too… for now. We will be replacing all of our shower heads soon, and will remove all of the restrictors. I’m sorry I can’t remember the name of the part, and hubs is up at Savannah center at the moment setting up for this weekend’s model train club show, but I will update here when he gets home.

OP, Just yesterday, we had a very good experience with Pike Electric.
Trustworthy. Professional. Very Knowledgeable.
I am a woman who felt very comfortable dealing with all of them, from Casandra at their office in Wildwood to the 2 electricians, who came into our home.
Maybe you should give Casandra a call, she's very proficient.

There are tankless models that are instant hot. I get very hot water immediately from every output.

If there is an error code 11, The ignitors tend to be a cause. If handy, they are easy to change and available on Amazon.

Rheem PROTECH Ignitor Coil RTG20235C https://a.co/d/hwzmBCZ

There's also been problems with the cheap Chinese gas pressure regulators used during construction. You may have instances of hot, cold, hot water before they fail completely.

Since they are attached to the gas line, best to use a plumber like Mike Scott Inc. to replaced those. The regulator they use is much higher quality, replacement cost is under $200.

Update - The original issue of hot water going cold during showers has stopped occurring for the last 5 months. I can't explain why it is now OK, so sorry for the mystery...

A bad igniter will stop the unit from firing up at all. 100% cold, 100% of the time.
But if that is the issue for someone else, with a Rheem, then it's a simple task to swap. Trivial DIY and no plumber is needed because nothing is being touched with the gas or ware lines.