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Thread: Tankless Water heater problem...advice needed

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  1. #1
    DIY Member jed1154's Avatar
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    Default Tankless Water heater problem...advice needed

    I have several water heaters in the house. One for the master which is a 240v unit and a couple of smaller units in other places in the house. The master bedroom one is on the fritz, so we have been using the shower that is on one of the smaller tankless units. The nightly routine is usually to bathe the child, then the wife and i take showers one after the other. The water is scorching hot until about 10 mins before the last person showers. It seems to get cool, then luke warm....maybe it will heat up a little, but its not near as hot as it is when the routine starts. By that time the water has been running for maybe 30 mins or so.

    Whats the deal? Why would a unit work great for so long, and then just peter out like that? Im on a well, but it doesnt have problems usually.

    Any ideas? The unit is 5 years old or so...

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Is it possible that the thing has an internal overheat sensor, and after running for an extended period, it shuts itself down? If so, it should be described in the manual.

    If the water gets extremely hot, it sounds like you do not have a tempering valve installed on the outlet, or if you do, it is malfunctioning. From what I've read, this is a requirement. You may want to check that.

    It's also possible that after you've used up all the water in the bladder tank (that's had a chance to warm up from the deep well water's temp), the now much colder water directly from the well means the tankless just can't raise the water to the same temperature. Say the water in the tank warms up to room temp, that's around 70 and the water from the well is say 50, the tankless can only raise the temp so many degrees as it passes by and now it's starting out 20-degrees colder.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member jed1154's Avatar
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    I guess thats possible.

    What is a tempering valve? I mean its scorching hot because i have the temp dial set that way.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A tempering valve goes on the output of the WH. You set the max temp, and if the incoming water is higher, it automatically tempers it by adding enough cold so it never gets above that maximum. A typical setting is 119-120 degrees, but it is adjustable. A tankless, depending on its sophistification, can vary the output if the flow changes before it can react and can produce dangerously hot water. The tempering valve solves that and protects users. It is required in many places and is probably specified by the manufacturer to be installed on your tankless system.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member jed1154's Avatar
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    Im not aware of anything like that on any of ours, unless its internal somehow. Im not really interested in it anyhow, more interested in the problem im having with the unit.

    I love my THW heaters, but the one in the master bath has been replaced once, neither unit has lasted longer than 4 years, but thats a whole separate issue.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Look at the installation instructions, I'm betting it specifies one should be installed. What do you have the output temperature set to on the tankless system and how hot does it actually get? Don't take safety issues lightly, hot water can kill older people or children as their skin is much thinner and burns can happen in moments not counting the fall that could occur if a spurt of excessively hot water came out of the shower.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Retired Defense Industry Engineer jadnashua's Avatar
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    It might be funny until you get sued...then you find out who your real friends are. Unless you are using this as a commercial item where they dictate a minimum water temp for say a dishwasher, there is no point in running it at max. That's like turning the house thermostat up to 100, and opening a window if it is too hot inside - the furnace or whatever is just working much harder than it needs to. One of the beauties of a tankless is continuous, which you're not seeing. Plus, replacing one like did in what, 4-years?, means it's not providing you with the service you expect. you're spending much more for this than needed, IMHO.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member jed1154's Avatar
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    I havent messed with the thermostat because thats just what it was set on when i moved in....Ill try turning it down. Either way, regardless of the setting, it should maintain that temp for any amount of time. It makes no sense that it works great for 40 mins....and tehn trails off. in fact, now that i think about it, it quits working at about the same time every night so far. I can't say it hasnt always done this as this unit feeds a shower, a dishwasher and the kitchen sink. the shower is hardly ever used, and the rest arent used for that long, except the dishwasher, but i have no way to tell how its performing.

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    Master Plumber Redwood's Avatar
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    I venture to say your whole problem is probably due to the high temperature setting. It might work okay in the winter with colder incoming water temperatures where you are using more hot water, But, This time of year the cold water is quite warm and you are mixing a small amount of hot water to have a hot shower. The amount you are using is so low that the flow switch is barely activating. Set the temperature to the lower temp and use more hot water and I'll bet your problem disappears.

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