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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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4 Answers 4

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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? May 26, 2019 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, 2019 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/… May 27, 2019 at 18:49
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I had a tankless water heater installed in November 2023. When I flush either of my 2 toilets the WH makes a noise. I pulled the cover and discovered it's the purge fan for the combustion chamber that's kicking on at the very slight movement of water because when the toilet flushes there's a slight differential pressure so the hot side which is still at full line pressure flows ever so slightly backwards toward the cold side and the sensor switch for it senses that slight movement and starts the purge fan in preparation to ignite the burner.

I called the WH customer service and they said, "you have a plumbing problem. The unit is functioning properly". I asked if there is an adjustment for the sensitivity of the flow switch that starts the purge fan and they repeated the same thing.

I found a plumber/technician who suggested that a single handle shower/tub valve may be cross leaking a little which may let the hot flow to the cold when a toilet is flushed. So I change both my shower/tub single handled valve cores. I also have a single handle valve on my kitchen sink which I isolated with the valves under the sink. Neither action helped.

After many calls to the manufacturer and customer disservice I finally found a sales rep that made several calls and found one of their WH company experts who'd recently had the same problem and had to install a check valve & surge pot on the COLD WATER line near the WH and that solved his issue.

I'm about to contract the plumber/technician who helped me to begin with to install a check valve & surge pot.

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  • Very interesting. Yeah in the meantime I remodeled the bathrooms and kitchen and replaced pretty much all valves (and toilets) but the problem is still there. I stopped thinking about it, but now your explanation makes sense. I just have no idea what a surge pot is, I will look it up Feb 29 at 0:53
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    This condition is not using any gas since the burner is not firing up. The electricity use is negligible. The only negative is wear on the fan in the water heater.. Probably it will not affect the serivce life of the fan but it could. Feb 29 at 1:08
  • Amazing what hitting the Enter key a couple of times here and there does for readability.
    – FreeMan
    Feb 29 at 13:58
  • I was thinking the same thing about the purge fan starting,"just let it run. what's the big deal? When it wears out the manufacturer can replace it on the 25 year warranty." I said that to the WH Rep and he said,"that fan is only covered by a 5 year warranty. The 25 years is only on the fire box and EXPENSIVE components." So I'm going to think about it for a while but I'll most likely put it the check valve and surge pot.
    – Don
    Mar 4 at 16:22
  • surge pot is small vessel that looks a little like a bbq propane tank. has a rubber bladder in it to keep the water and air separated. When installing it you use an air compressor to pressure the air side to 5-10 over incoming water pressure. This is a plumbing code requirement anytime you have a check valve or back flow preventer in your cold water line which would prevent your cold water from freely back flowing into the city water system if for some reason the WH continued to fire even if no valve in your house is open.this would pressure up system if there were a check valve in the line.
    – Don
    Mar 4 at 16:25
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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... May 25, 2019 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, 2019 at 4:04
  • You seem to have pressure problem do not know how close heater is to toilets
    – user101687
    May 25, 2019 at 4:11
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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. May 25, 2019 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 :( May 26, 2019 at 17:33
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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. May 25, 2019 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. May 25, 2019 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. May 25, 2019 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? May 25, 2019 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 May 26, 2019 at 17:40

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