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Hello I recently (few months ago) installed a tankless water heater following a failure of the old 55gal gas heater.

Performance has been fantastic, it actually resolved a number of problems, but I also noticed an increase in water hammer, most likely due to the fact that there is no tank to damped the water movement. I installed a couple of anti-hammer devices in strategic points and now that problem has been resolved too.

However, sometimes I still had noises coming out of the water heater itself, especially late at night. With the recent warm weather, I started leaving the windows open at night, and the other day I realized that those noises coincided exactly with my neighbor's sprinkler system becoming active. The noise repeated again 9 minutes later when the sprinkler changed zone.

The water meter of our houses shares the same location near the road, about 100ft from my house. I measured water pressure in my house while my neighbor's sprinklers are active and when they are not and it's similar at about 75PSI.

Two questions: should I suspect maybe his sprinklers, for some odd reasons, are connected to my own water lines? Pretty unlikely also given his house is much closer to the road than mine. Also, how do I fix the noise in the water heater? Note the noise comes from within the heater, not from the pipes.

As far as I know, my house does NOT have a pressure regulator, nor an expansion tank.

Also, I have never heard any noise from the heater during the day, and as I worked from home pretty often recently, I had plenty of times to stay in a very quiet house during the day.

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Oops tried to edit and deleted. To me it sounds like you are hearing the pressure changes when his system kicks on your main pressure may drop, as you said no tank you hear more and it could even be the controls in the tankless that tanked water heaters do not have. You might want to add a regulator or pressure reducing valve on your main incoming line and reduce your pressure 5-10 psi this should be enough to prevent noise caused from main pressure fluctuations, if you think he has tapped your line when his sprinklers start go out and shut off your main to the house at the meter. If his sprinklers keep going it is just the pressure change. I doubt you need an expansion tank with a tankless as all the ones I have installed require a flow to heat, a regulator set at 70psi should eliminate the noise caused by pressure shift since the pressure is 75 when he is running the sprinklers.

Added because comment was two long , have you ever had the water temp in the shower change when someone flushed or turned on the water? A regulator on the supply lines to the shower valve(s) can eliminate this it’s the same principal the regulator set below the minimum pressure eliminates the change. I think a flow in your main supply is causing a pressure fluctuation that you can hear because of the type of water heater you now have has controls that are pressure sensitive flow or static. Eliminate the changing pressure and no more noise.

  • In a static scenario, no water flowing in my house, would a pressure regulator really make a difference? If there is no flow I'm not sure what happens, I shall try to lookup how a pressure regulator works. Thanks! – Alessio Sangalli Sep 3 at 4:07
  • With a change in your main line pressure your water heater may think there is “flow” , not going to get into all the what if’s but since this problem occurs with a high flow the main line pressure will change it’s just a reaction of the flow with a limited supply. Installing a regulator or pressure reducing valve below the pressure swing should extent the life of your valve seals and Valve packing. 60 psi is a normal pressure setting. Everything on your side of the regulator stays at the lower set pressure flow or not, while the supply side may have some wild swings that is causing your noise. – Ed Beal Sep 3 at 4:27
  • I just monitored again the sprinkler's action and, very precisely, when it changes zone I saw a spike to over 95PSI. I still have to make 100% sure the irrigation lines go under their meter. However I am not sure I really want to put in a pressure regulator. I'll definitely think about it after I understand how they work internally, but in the meantime a correctly sized water hammer arrestor on the main input line to the house could be an alternative solution? – Alessio Sangalli Sep 3 at 7:09
  • To test if they are on your line turn your main off while they are running if they keep running turn your water back on they are on your neighbors meter. If it is a pressure issue an arrest or only dampens the change , I can’t hear the noise so I can’t say if an arrestor will help but I can say a pressure regulator would help. – Ed Beal Sep 3 at 13:32

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