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Tankless water heater - one shower got cold

Posted by debn1951 (My Page) on
Wed, May 29, 13 at 13:57

A few years ago we installed a tankless hot water heater and we love it. However, we have had several house guests get ice cold showers, even after tutorials on how to adjust the temp. Several claim they didn't even touch the temp., and that the shower started out hot, then turned ice cold after about 10 min. and never got hot again. This happened to at least 4 different people. What could be causing this problem? It's embarrassing to have this happen to guests!


Follow-Up Postings:

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RE: Tankless water heater - one shower got cold

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.


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RE: Tankless water heater - one shower got cold

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.


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RE: Tankless water heater - one shower got cold

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.


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RE: Tankless water heater - one shower got cold

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?


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RE: Tankless water heater - one shower got cold

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.


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RE: Tankless water heater - one shower got cold

I can only scratch my head about how you guys could love a water heater that supplies cold water after 5 minutes, or gets cold if someone else turns on water, or never gets hot at all. Hot water use by guests shouldn't require a training session.

A guaranteed solution to these problems is to get a water heater with a tank.

PS - I wonder if using a 105 degree setting should have you worried about bacteria growth? Health officials seem to encourage using at least 130 degrees, and even higher temps are better and can be safe with the use of a tempering valve.


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RE: Tankless water heater - one shower got cold

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!


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RE: Tankless water heater - one shower got cold

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


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RE: Tankless water heater - one shower got cold

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.


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RE: Tankless water heater - one shower got cold

"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.


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RE: Tankless water heater - one shower got cold

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;)


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RE: Tankless water heater - one shower got cold

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.


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