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I have a tankless water heater. After I finish to use hot water and I close the faucet, the water heater starts a fan and make a typical noise, it's very typical and expected. I guess it blows off excess moisture or similar.

I noticed however, that when I flush a toilet (only the one of the three toilets I have), the moment toilet valve stops the stream of cold water, my tankless heater starts the same noise as when you shut-off hot water. This leads me to believe that some hot water is pulled in when flushing a toilet. I closed the output hot water valve on the heater and - obviously - it does not make the noise when I flush that toilet. Also, the "in use" light of the water heater does NOT turn on when the toilet is being flushed, unlike when you open hot water "explicitly".

I also made another experiment and if I open the faucet of the tubs, fully on cold, when I shut it I also get the same reaction from the water heater.

I suspect I'm literally draining energy down the toilet every time I flush, and I'd like to understand what is going on.

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Do you have a mixing valve somewhere in the supply to the toilet ?

In cold climates it is common to mix some hot water in with the cold so that the water supplied to the tank does not cause condensation on the outside of the tank do to a temperature difference between the water and the ambient room temp.

Thus if your toilet is calling for water it will use some hot water from the hot water heater.

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    Are such toilet mixing valves generally outside the wall and visible? – Jim Stewart May 26 at 20:23
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    @JimStewart you are correct in that they are usually not buried inside a wall but many times they are not apparently visible because they are in a crawlspace or some other out-of-the-way location such as near the water heater. That having been said I just demolished a wall last week that did have one inside the wall with only just a small adjustment screw port poking through the sheet rock. I had no idea what that tiny screw was poking out of the sheet rock until I open the wall. – Alaska Man May 27 at 2:59
  • This is a question I asked some time ago - could it be related? Sorry I did not bring it up earlier but your mention of mixing valves revived it in my head: diy.stackexchange.com/questions/160664/… – Alessio Sangalli May 27 at 18:49
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May need back flow valve.Yes water being pulled back.If a A back flow is installed you need to have a thermal expansion tank. According to manual layout

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    Well pulled back from where? As far as I know the water heater will start when there is water flowing inside but... – Alessio Sangalli May 25 at 3:39
  • Pulled from heater cold water feed and it looses at little presser thinks hot water was turned on .Look into that adjustment may be the answer.Not knowing all the facts just giving pointers to check. Have kids flush toilets. – user101687 May 25 at 4:04
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    The bathroom is the room adjacent to where the water heater is installed. The total pipe runs must be in the range of 10-20ft so relatively close. – Alessio Sangalli May 25 at 18:16
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    The pipes are copper, 3/4" until they stay "common" and then 1/2" when they branch out to go to the single toilet or tub. Unfortunately I have not found a setting to adjust the flow rate. The manual says however if a backflow preventer is installed, then a thermal expansion tank is necessary. That would be something I do not want to get into right now :( – Alessio Sangalli May 26 at 17:33
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    Thanks I will try that – Alessio Sangalli May 27 at 18:39
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What happens if you turn the cold water on the tub on full and then shut it gradually? Does the WH react? There may be a sensitivity setting on the water heater. The heater is not supposed to come on if a small amount of water is flowing. It may be set too sensitive.

Does the automatic valve on the cold water supply for the clothes washer also cause this? You can get supply lines for the washer (and I also think for the toilet) that restrict the flow to provide protection against a ruptured hose. One brand is FloodSafe. You could put one of these on the one toilet this is causing the WH to react. If you want to test this try turning the cut-off valve to that toilet to three quarter open or half open and see if this stops the WH reacting.

Putting a backflow valve on the cold supply to the water heater might or might not abolish the pressure pulse (aka water hammer) when you have the cold water on full somewhere and then shut it off quickly. First see if you can adjust the water heater. Does the kitchen faucet also do this? Outside faucets? Bathroom lavatories?

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    I need to perform some experiments. Now I have wife and kids at home and it's absolutely impossible. – Alessio Sangalli May 25 at 3:42
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    This is not a deficiency that requires immediate attention. Just operate the water heater and see how it performs over time. According to your original post the heater is not turning on when this pressure (or flow) pulse occurs. – Jim Stewart May 25 at 11:11
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    Well I've installed the new water heater few months ago and while I understand it's not as urgent as a water leak or - even worse - a fire, I'd still like to understand what's going on. I will probably pull the water heater cover to see EXACTLY when it is starting, because so far I've only relied on the indirect evidence noise the heater usually makes when the water stops flowing. – Alessio Sangalli May 25 at 18:19
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    So you have the hard copy of the operating instructions. Look in there to see if there is a sensitivity setting. Does this heater have remote control of temperature? What is the current temperature setting? – Jim Stewart May 25 at 19:59
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    I have a hard copy but I use the soft copy in PDF that allows for easier text search: media.rinnai.us/salsify_asset/… I have not seen a sensitivity control for the flow (even if it references the flow sensor several times in the document). The heater has a "wifi" controller for temperature and recirculaton settings but I am not sure that is what you are asking for, and I have not ins – Alessio Sangalli May 26 at 17:40

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