Not enough flow is being maintained to activate the burners. They are probably doing what they do at home and trying to start with really hot water and add a bunch of cold to try to temper it down to be a comfortable temperature. Your best bet is to have one of those "NO" bar slash signs laminated and placed at the entrance to the shower showing starting out the shower with no cold water at all. Then they can add a little cold to reach what's comfortable to them. Then they might "get" how tankless works.

Your unit likely has a touch pad control. Depending upon brand you should be able to read to a tenth of a gallon what the flow is in the shower. Compare that to the minimum specs. Also, this could be a piping issue. If someone else in the house opens a hot faucet, the water may "short circuit" to the shortest path.

Hollysprings, Thanks for your response! I instructed all of our guests to do just what you suggested, and they all swore that they either adjusted it very slightly to add more cold, or never touched the adjustment at all, which is why this is so perplexing. I actually had this exact thing happen to me at my sister's house, where they also have a tankless. And I know how to fine tune the temp., but 10 or so minutes in I got a cold shower!
Jackfre, I'll check out the unit but now you have me wondering if it is a pipe problem - I didn't think that was supposed to happen, or that it would only be momentary.

An older shower valve may not be regulating the flow properly due to leak by. A re-build kit is in order just to remove this possibility. In my current home with the old piping I can be in the shower and if someone opens the kitchen faucet I am bingo hot water until the KF is turned off. I know this to be true as 10 minutes ago I was n the shower and my wife is prepping dinner;) This is not uncommon. I am in the middle of a kitchen and bathroom(s) re-model with 100% new piping. I have laid it out so this will not be a problem. Ah, new piping...we cannot wait.

My disclaimer here is that I used to represent the top manuf of tankless in the US. That said, I've had them in my homes for 15 yrs. I like them and they work well. What brand and model do you have?

We've had a Noritz since 2006. The tankless just serves two bathrooms. One shower has a thermostatic valve, while the other just has a pressure-balance valve (both installed around the same time).

In the summertime, our cold water temperature here in GA gets warm enough that very little hot water is required to get to a comfortable shower temperature.

Our heater is set at 105F output temperature (which still gives a plenty-warm shower). Even then, 5 to 10 minutes out, the thermostatic valve modulates flow low enough that the burner cuts off. Then you have to wait for it to come back on and for the HW to travel to the shower.

I don't know whether it's a pressure-balance issue as others have commented. If it's more like it always happens at the same shower duration, it's probably the burner shutting off due to low flow.

Two things you could do:

1) Reduce the heater's setting

2) Put an hourglass in the shower to encourage guests to finish up more quickly (10 minutes is a pretty decent length shower, IMO).

I'm hopeful that the controls on newer models have a lower cutoff flow, but I haven't looked into the details.

thull, I'd suggest that you increase the flow in the shower. If you look at the GPMX500XDelta T = BTU you can figure what is going on with your unit.

Snidely, tanks have to operate at the higher temperature because with the bacteria in the stored water and the accumulation of sludge in the tank you have to have the higher temps to kill the bacteria. As well, you have to have the higher temps to be able to mix the water down in temp to give capacity. A tankless is a direct fired appliance and the water is actually heated to a much higher temp than the touch pad control indicates. A water flow servo on the unit them meters the appropriate cold water after it leaves the burner area to arrive at the set point temp, plus or minus 2*f. Also, given that there is no stored water the bacterial issue is, in fact, a non issue.

I think you should keep your tank!

I believe the concern with Legionella is if the water is stored warm but less than 130F (or somewhere thereabouts). The temperature is warm enough for it to grow but not hot enough to kill it. And there's still chlorine in the water, so I'm not sure it's that much of an issue. The tankless doesn't have storage, and the piping cools down between uses.

And we live with the short showers in the late summer because we're both in the environmental business. Removing the flow restrictor isn't gonna happen. There's always the option to turn on the second head and both get in there. ;-)

For what it's worth, my recollection is that our heater needs maybe 0.5 gpm to turn on, but it turns off at 0.8 gpm (something like that). As I described before, when the cold water isn't that cold, the thermostatic valve modulates closed enough on the hot side after several minutes to get below the minimum.

There was a Fine Homebuilding article several years ago about adding a small electric tank (5 gal or so) downstream of a tankless to provide tempering for just this situation.

Here is a link that might be useful: Fine Homebuilding article

Thull, I wouldn't necessarily suggest that you remove the flow restrictor. Look at drilling it out...slightly. A very minor increase in diameter will increase flow, perhaps just enough to offset the high supply water temps. I am familiar with Rinnai, which makes at .4gpm and will hold fire down to .25. Some models have higher minimums.

"There was a Fine Homebuilding article several years ago about adding a small electric tank (5 gal or so) downstream of a tankless to provide tempering for just this situation."

There's nothing environmentally friendly about having to use two water heaters (including an electric one at that) to provide a low flow shower demand simply because the primary unit is unsuitable for the task.

Slight increases in diameter can make huge increases in flow. If you increase a hole from, say, 1/8th to 1/6th (an increase of about one-third), the flow nearly doubles.

Jackfre, you can be certain I will be keeping my tank! I hope y'all are as happy with what you have as I am with what I have. With all the costs and quirks of tankless units, I'll never understand why anyone would choose one. But so it is.

Snidely, I get what you are saying. My experience with tankless has been excellent. I had a unit in my home for 12 years and never touched it. In my case my gas bills were reduced far beyond the numbers you see in publications. Then when we had 14 people in the house for a weekend, both showers ran almost continuously until all 14 people had as long a shower as they wanted.

For the last 20 months, since moving west, I have had a tank water heater and hate it. We have to plan hot water use. I'm in the middle of framing the new kitchen and both bathrooms and yesterday I roughed-in my new tankless and can't wait to start using it. I also positioned it so that it will operate at it's best. Pipe size is correct and the longest run to any faucet is 16'.

In fairness I have to add as a disclaimer that I was a manufacturers representative for Rinnai in my business for over 20 years. In the 9 years I had the water heaters I was responsible for well over 100,000 units, trained thousands of contractors in installation, service and troubleshooting of our units. I am very comfortable with them and have seen just about every application catastrophe you can imagine, because I went out on them. The tankless segment will continue to grow in the US and m-a-y-b-e someday you too may find yourself with one;)

I want to tell you the coolest application I found with my tankless. The last winter we were in our MA home was the worst snow I had seen in the 25 years we lived there. Everyone was loosing roofs, walls ceilings and gutters to ice dams. We were two weeks from closing on the house and the ceiling/wall of the master bedroom came down. I had to do something to stop that and at 62 didn't think being on the roof with and ax would do it so instead I got on the roof with 100' of garden hose, connected to my tankless and with a nozzle that gave a shower head type pattern I melted the ice off the house, gutters and roof. I ran hot water for 3 1/2 hours, melted all the ice and snow off the roof, did no damage to the shingles or the gutters and I'll tell you it was slick! All the neighbors, who thought I was nuts (with reason) were asking if I could do that for them. My hose wouldn't reach that far;)

I have had the same exact problem with several tankless water systems (3!) that were installed in different apartments. This has an easy fix, and thull from GA is right on. All you need to do is set your water temperature LOWER, because these systems have a safety to prevent overheating. This will happen if you draw too little water for heating, which is what happens when you mix cold and hot water for showering. If you set the temperature at just a little bit higher than the shower temperature, then your guests will draw enough hot water to keep the safety from coming on and voila! no more cold showers! You may prefer a hotter shower than your guests do, and so it only happens to them and not to you.

This I learned from living in Taiwan and talking to the plumbers here. They told me that in this subtropical weather with still 4 clear seasons, in winter setting a temperature of 42 C is sufficient for most hot showers/baths, and a temperature of 36 C is even hot enough during the hot summer months, when people naturally prefer cooler water and less heat is lost when traveling through the pipes. To translate to Fahrenheit, that would be 107 F in winter and 97 F in summer. You can experiment with your own in your climate to see what works for you.

The best thing about these systems is that you can adjust the temperature at will when no one is using the water, so you can dial the temperature down when guests are expected and dial it up the minute they leave, and it takes only walking over and pushing a button. You can even dial it up for dishwashing and back down for other purposes, if the appliance is located conveniently. Then, instant-presto-chango! For me, it is only a step outside the kitchen, so it's super easy.

I think this kind of basic understanding should be more clearly stated in manuals and be disseminated by installers, but this is one major difference between the operation of tankless and tank water heaters that needs to be explained better to new users.

The possibility could exist, the exhaust vent has build up of insects inside. The air restriction causes overheating, and shut down. That happened to mine, during moth season.

I have the Rinnai R75LS. I have yet to get through a shower without a shock of cold water. After reading post, I think I may have correct way to control shower. I turn hot water on fully? Then add cold water to adjust to comfort? Seems like i have to always turn down hot water cause it ia too hot. Have temperature set at 140. Is that the issue? I am going to try again of not service call will have to be made.

Yes that is the problem. Turn the unit down to 120. I actually run mine in the summer at 115 and 120 in the winter months. Plumbing code requires that shower valves be set by the manuf at no more than 112*. If you sit in a tub over 105 you are no longer bathing, you are par boiling. 140 is dangerous. You use it in a tank to increase your hot water availability and to kill bacteria in the tank. At 140 you are using very little hot water to arrive at your shower temp. You are falling below the minimum of the Rinnai to keep operating. At 115-120 you should have a better balance of hot to cold and the unit should run fine.

Our Bosch was about 15 years old, it had always ran as advertised. Endless hot water. But started having to keep turning up the hot water, so I decided to get a new water heater. Went on ebay and bought a Marey. Decided to give it to my grandson. He complained that it went cold before he could get his daughter's tub filled for her bath. I thought it was just him and ordered another one with 3.5 gpm. He installed the first one in my house. It runs 5 min and goes cold. If you shut it off and wait about 2 min, it will get hot for another 5 min. I have had endless emails with Marey and they say this is normal, unless you set the temp for the hot water and NOT add cold water. I am so sick of Marey. Why would you want a hot water heater that had to be set for each person who uses it and never be able to adjust the temp in the shower? They said to just use the hot water and never add cold water. Most showers have only one control, so you can't do that.

I am experiencing the same issues with my Rinnai! It's about a year old and this is the first time I have had issues like this. I plan on calling Rinnai in the morning and am hoping it's not the norm!

Is the Rinnai throwing any error codes? Look at your manual for directions on getting them out of the unit. You can access the last 10 error codes, as well as a lot of other information, on the digital touch pad. You will need the model and serial number off the unit when you call in.

I do have the model and serial number. No error codes

I bought a Marey, it went cold after 5 min. Worst heater on the market. Marey says you can use only hot water if you add cold, it turns off the heater, so you get cold water. Thought they worked on water flow. Have added an inline 220V unit to heat the water. Just turned Marey off.

Sophie Wheeler has me confused. Should I be starting the shower on hot and then cooling it down? Or the opposite? The No sign with the bar through it sounds like a double negative. Help!

bshanley,

Yes, start the shower on hot flow and add cold as needed for the desired temp, without reducing the hot flow rate if possible. The point is to keep the hot flow rate high so the tankless unit doesn't shut off due to insufficient flow. A generally better way to do that is by turning down the setpoint of the tankless unit so the hot water flow is at or near the desired shower temperature, which allows full-flow hot water to be run ... that is, you don't need to turn down the hot flow to attain the desired shower temperature.

Thank you!

I had the same issue with the shower turning cold after about 5 minutes. Simple fix. It was the condensation outlet on the bottom of the unit. A plastic port that had a drainage tube attached to it. The port was screwed in too tight and was not allowing the condensation build up to drain from the unit, causing the unit to give an alarm and stop supplying hot water. Simply turn the screwed in plastic port a little to the left, watch the water drain out, and its fixed!

I have just had the Rinnai 98e installed two weeks ago and I have been experiencing the hot cold problem since the first night of using it. There seems to be no pattern so far as to when it happens but it is frustrating. Some times I will wait 5min for hot water to do dishes other times not long. Some showers are great others hot and cold. I had the installer come back and it worked for him. It is set at 140 should I be turning it down?

yes, set it to 120. Setting it higher doesn't get you anything and can cause probelms

Noralong,

You may want to reach out to Rinnai too. Do you have a recirculating pump on your system? Where are you located? What is your incoming water temperature and do you have other items using hot water at the same time?

I just installed a Rinnai tankless water heater today, after my tank rusted out last weekend and I've been without hot water for a week. I got it installed, and it seemed to work okay for all the sink faucets, but when I went to try to take a shower, the water got cold after just a little bit. Had to hop out and go check it, found I didn't have the hot water flow valve opened up all the way, so I did that and tried it again. It seemed to be working a lot better, so I got back in to finish my shower, but it wasn't very long, maybe 5 minutes and the water went cold again. I went down and checked and there was no error code, so I thought the temperature setting might have been too low, so I tried switching it to the 140 degree option, but now it just errors out with a vent error, which I was getting at first, but I changed one of the settings for the vent and it worked better. I'm probably going to go back down and set it back to 125 or maybe even all the way down to 120, but I'd like to figure out why it won't stay hot in the shower? I cranked it all the way on hot, then barely turned it down a bit to get a more comfortable temperature (I have a shower that doesn't have separate taps for hot and cold, you just crank it all the way over to get hot, and cold is barely over).

Anyway, any suggestions you might have would be appreciated, I'm stumped on this one.

To begin with, you need to start your own thread. This one is almost 5 years old

Try not adjusting the water and see if it stays hot. I ended up getting an electric on demand, when the propane one kept going cold, when the water was adjusted. THe company told me that the heater wasn't made to be adjusted in the shower. Stupidest thing I'd ever heard of. Everyone wants to adjust to their own liking.

Thank you Jake for answering my question

Homepro01 those are very good questions

I do run off well water but I do have a filtration system and the pump pressure is very good. I am located in Northern Ontario temperature has been dramatic -10 to -30.

I am turning it down and looking into if I have a recirculating pump. Let you know what I have learned about it. Thank You

noralong: when it's set too hot, you can sometimes run into the situation where there isn't enough hot water being used to keep it running.

Unlike a tank heater, setting it at 140 doesn't increase the amount of hot water you get.

noralong: I would encourage you to start a new thread if turning the temp down does not solve your problem - again this one is 5 years old and is already very long.

What is a new thread?

In other words stop posting to this that was started 5 years ago by someone else with a different problem and post it as a new problem.

Ok, so I've read through most of the posts here but was unable to locate my problem. Just had an ECO Smart ECO27 electric tankless installed 2 days ago in the lower level of my tri-level, which is where the previous hot water tank was located. Every faucet except the upstairs bath tub produces hot water. When the upstairs tub tap is turned on the output pipe from the tankless unit goes cold. I tried running hot water in the kitchen, which it was hot, then turned on the upstairs tub tap and the water cooled off.

Any help provided would be greatly appreciated.

Please see above

I know this is an old thread but I wanted to sign off by saying I turned down the temperature to 120 and have not had a problem in the past 4 months.

Thank you everyone.

Glad I saw all this. Tankless is obviously more hi-tech expensive junk that wastes enormous amounts of time. I will stick with a tank. Of course the company rep is going to push it.

I love my tankless. Have one in garage that will feed whole house, one under kitchen sink so always have hot water in kitchen sink, one in master bath but plumber who hooked it up, somehow screwed up the water lines and it is bypassing the heater. But it worked before he messed with it. So I highly recommend the tankless. I get mine off ebay. They are much cheaper than the box stores.

Haven't had any trouble with shower temperatures in 14+ years of tankless, set at 102°F to 105°F. Need to raise the temp for a specific task? Go into the broom closet in the laundry room, next door past the kitchen, press a button for a few seconds. It auto-resets to the lower setpoint after 30 mins.

Richard Howell - don't know how you came to that conclusion. It's not correct.