I assume this is a gas unit?

Too low of a water flow-rate would cause it to cycle off, or perhaps if something is obstructing the venting causing the burner to shut off.

A reduction in flow-rate could occur if you have an anti-scald shower valve that's coming into play. Also if you have the tankless temperature set high and are manually turning the hot water down very low to get the shower temp you want. Try turning on faucet at the bathroom sink or another location while having a shower, see if the tankless stays on. If that fixes the problem, then flow-rate is your problem. You'll have to find some way to increase flow-rate while showering ... or perhaps turn DOWN the tankless temp so you can keep the hot water running faster. I set my (electric) tankless at 102°F for normal operating temp (occasionally turn it down to 95°F during the summer season) and run only the hot water for showering.

In addition, insufficient gas line size, supply or pressure will cause the unit to stop firing, reset itself and start again.

Try turning on multiple fixtures at once to see if you can cause this to happen. You see, these units will continue to supply endless water; they just control the flow rate to meet the temp outlet setting.

When the demand rate exceeds the gas input available, the unit will shutdown thinking there is a gas supply problem for the purpose of preventing complications of improper combustion, two being excessive production of CO or burner sooting.

Unfortunately, too many times a plumber will install a unit in place of a regular tank water heater and not account for the units BTU input gas requirements and not account for this with a new and bigger gas line, along with possibly the meter, regulator and gas main from street.

Thanks for the replies, I'll do as suggested and see what happens. For clarification, it is a gas unit and the plumber installed the gas line, etc. as new since I was all-electric before. The faucets do have an anti-scald device. I have tried setting the temp for the tank at each level from 102 (lowest)to 140 (highest)degrees and have the same problem at each setting. Again, thanks for the replies... I'm off to have a warm shower I hope.

Hi. I had this problem on a Noritz tankless water heater that I installed. In my case, there was an elevation issue. The only solution was to call customer service. Over the phone, they were able to do a diagnostic check on the unit via the remote controller. The controller threw out a series of numerical codes that allowed the technician to know exactly the gas pressures and flow through the unit. We were able to dial the unit in over the phone via the controller. Since then, it has worked perfectly.

Before calling customer service, I had checked the flow rate and gas pressure on the unit. I also had run a new gas line with more than sufficient BTU's (calculated on the high side of demand - 200,000 btus for this model). Everything was to spec. If the tankless water heater is dialed in correctly, it will not matter what temperature you have it set to or whether the faucets have anti-scald devices. All new shower faucets have them by law now. By the way, the specs used to dial in the water heater were not available to me as the installer. It could only be done with customer service.

With Noritz, customer service was a breeze to use. I would be surprised if Rheem/Ruud is any different. I believe their customer service number is: 1-800-432-8373.

I hope this Helps
Ben Bonnett

Is the Rheem a rebadged Bosch unit? My neighbor installed some type of Bosch tankless and never could get it to work correctly. Lowe's took it back. He was on the phone numerous times with tech support. Bottom line, if the Bosch unit can't deliver the flow of water at the temp desired it just shuts off. Haven't had that problem with my large Noritz unit.

Not sure if its a rebadged Bosch unit or not, it just shows Rheem RTG-74XN-2 I just took a shower with the hot water running in the sink (as suggested) and the water in the shower didn't get cold for the first time. So I guess it is a water flow problem, but how would you remedy this?
Thanks, and I'll call customer service too to see what they say.

Rheem is a Paloma unit.

Never buy a Bosch!

See if you have a flow restictor in your shower head that you can remove for more flow or, try a diff head altogether.

Just to reiterate, no, Rheem isn't a rebadged Bosch. It is a Paloma unit.

The first thing to do is try to stop guessing. Look for the error code on the control unit.

If no error code, run the shower and see what the unit is doing. Does the control unit show the burner is shutting off?

If no clues, I would try turning on another hot water faucet when the thing goes cold to see if the additional flow makes it work. They don't need much water (.3 gpm) to keep them fired.

In any case, I would call the installer and not let him off the hook unless he can prove the problem isn't his. If this was a problem from day 1, he should be troubleshooting the problem, not walking away. The job isn't done until it works.

Id go with seeing if there is a flow restrictor in the shower head... as said above.
I ran into this problem on a complete house remodel. Its was a multi-million dollar home. He had two Renani Tankless heaters in his attic. Huge shower with multiple jets. He had the same problem. Plumber before me couldnt figure it out, so I went in a changed the heads... worked great. I also had to install two point-of use heaters under both lavatories (Each one was on opposite walls) So they could have hot water.... Comes down to not enough flow to bring the heater up to deliver hot water.

I've had the same issues with a Bosch Aquastar tankless water heater. I've read a few articles regarding the pressure-balance shower valve, and they seem to suggest that switching this to a thermostatic valve would solve the problem; any opinions on this?

Thanks!

The Bosch Aquastar uses water flow to power a spark igniter so that the unit doesn't need electrical power. One of the many problems with this setup is that it requires significantly more water flow to keep it on-- 1 gpm I believe. Other heaters typically have a minimum flow between .3 -.5 gpm.

The rheem unit has a minimum flow of .5 to start and .3 to keep running.

For the Aquastar owners, you need to be moving at least 1 gpm of hot water through your shower. You can do this if you lower the water heater temp so you are mixing more hot water into flow.

Other water heaters that have a more standard minimum flow shouldn't have this problem.

The OP's issue, unless he is using a very low-flow shower head, seems like a malfunction of some kind rather than not meeting the minimum flow requirements to keep the heater on.

I've had the same problem of the water getting hotter (from 100F climbing to 110F) and then the unit shuts off leaving me in a cold shower. My incoming water is 88F. Here's what the Rheem Tech had to say:
It is a flowrate issue, we are only talking about a 12 Degree delta-t that is he is looking for. When the Delta-t is below 35F the temperature accuracy can fluctuate. Even at 120F on the T-stat the Delta-t is only 22F.

Base on Minimum Fire, at a 100F set point with a 90F input temperature; he needs 1.6 GPM to maintain the unit in the on position. His water temperature out of the tankless unit will be 109F in this scenario. If he is mixing down to letÂs say 100F for a "comfortable" shower, then his hot water GPM will only be 1.36 GPM. Based on a 2.5 GPM shower head, Not enough to keep the unit running.

If he turns it up to 120 and tries to mix down to 100 then his flow rate is going to be .9 GPM hot, based upon a 2.5 GPM shower head, but the heater is going to require 1.2 GPM to stay on.

His cold water is simply too warm.

Which goes back to what I've pointed out in the past ... electric tankless seems to be better-able to deal with lower flow rates and temp rises (delta-t). My incoming water (mid Texas coast) is often ~80°F+ during the summer season, and I've *never* had trouble with temp fluctuations or the unit dropping out, at a setpoint of 102°F (or lower). It activates at 0.4 GPM.

I did try running the sink faucet while taking a shower and it does seem to work. Having bought the unit to conserve energy, it doesn't sit well to use more water and gas to get the lower temp. I desire. While it was disclosed that the unit would not operate on a low flow rate, it was not disclosed that the unit would not operate on less than a 35F Delta-T which is most of the time with the Mississippi River water in New Orleans.

I am having the exact same problem. I have tried running the faucet at the same time and the water will stay warm enough to shower. I was thinking it was my single control Shower/bath faucet. How do I check for a low pressure shower head?

What is your summertime inlet water temperature?

You check for a low-flow showerhead by measuring how much water comes out in a minute (or in 30 seconds and multiply by 2) Normal flow is 2 - 2.5 gpm.

I'm not sure what the inlet temp is, but I did run the lines side by side. I probably should have insulated them and now they are closed up in the wall.

Well, all hot water lines should be insulated. That's a no-brainer.

That said, are you talking about 80 degree cold water like some of these other folks?

It's been in the 100's here in TX and the unit is in the attic. So 80 degrees might be possible. Especially once the heated line radiates it's heat onto the cold line.

At least the new wall isn't finished out yet, so I can tear it out. I will insulate the pipes there and then do the ones in the attic. It was working fine before It got so hot here.

An instance of checking recently, my incoming water temp (as reported on the tankless unit's digital display) was 89°F. I haven't noticed any temp fluctuations during a shower with the tankless output set at 102°F, and that's with the flow rate low as 0.8 GPM. The tankless output may throttle down low as 4% (1,152 watts) (or less) of full capacity.

Okay,

So to bring a conclusion to this thread:

An application issue for gas tankless heaters is if the summertime water inlet temps are likely to get into the 80's.
What can happen, particularly in situations where there is a low-flow shower, is that the minimum btu input of the heater can't be sustained and the heater shuts off. In other words, there isn't enough water flow and temperature rise to allow the burner to run at its minimum btu rating.

If you are in the southern US or otherwise have high summertime cold water input temps, a gas tankless unit may not be appropriate. Pay attention to the minimum BTU inputs.

As Dadoes has pointed out, that may not be a problem with electric tankless units. However, electric tankless whole-house units provide no real energy savings to speak of and require large electrical service. Except for point of use applications or where continuous hot water is needed, consumers wanting an electric water heater would probably be better served with an electric tank unit.

My shower head flows right at 2gpm. It does have what Pfister calls a "Pressure balanced valve" to avoid hot and cold water shocks. If I run the kitchen sink when I shower all is fine. I may have to replace the shower controls to have separate valves.

My plumber came back out and ran a separate line from the front of the house directly to the unit to insure we were getting enough pressure at that point. Same problems. He replaced my shower head with a new one out of the box. Same problems. He ripped out the water restrictor which is standard in every shower head- problem solved. I'm trading more water for less gas now but at least I can finally get a confortable shower.