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I presume there must be plenty of protections for such a case, but is there any reason to be concerned if I turn off (or my fortrezz value and smartthings turns off) my main water supply and my Rinnai tankless isn't turned off? I looked at the instruction manual and it doesn't mention any thing about this situation that I saw.

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Tankless/combi just won't start if there's not enough flow (it's well written on tech label), usually minimum flow that let fire light is around 2L/min, that's intrinsic in their way of work. Most use a microswitch to sense the flow and a temp. sensor to adjust the flame, other just measure the flow and adjust the flame according to that. Some have minimum pressure sensor, other do not but on DHW it's not needed.

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There are flow sensors in tankless units so if you are simply shutting down the valve on the water supply and then reopening it I wouldn't be too concerned. That being said shutting down the water supply can change the pressure in the system as temperatures of the water change. I don't think it would cause any trouble. But, anytime you close the valve the water is in a closed system. It's a good idea to install a potable water expansion tank.

This is a potable water expansion tank enter image description here www.amazon.ca

They are almost exclusively 3/4" NPT. You would need to measure your static pressure(pressure with the taps closed and the main open) with a gauge. Then pump air into the air nipple (bottom of the tank pictured above) with a bicycle pump or preferably air compressor to match the static pressure before installing it. It's a good idea to install a service valve as well.

  • Some have flow and some have differential pressure switches. But both are doing the same thing,,, no water use no heat. – Ed Beal Mar 6 at 5:30
  • I've never run in to one on the water side. They must be fairly reliable. I use Navien exclusively. They use a flow transducer/hall effect sensor. I've changed a couple. But I've installed hundreds of units. I still have a faulty sensor in the garage. I connect to a 5vdc power supply and an LED while blowing in it and it knocks my kids socks off watching the light blink. Is it a variable pressure transducer? Or just a switch with the gas throttled via another sensor? – Joe Fala Mar 6 at 5:47
  • It can be both, on some system is used as variable transducer, in others is just on/off, usually the case is the second and the flame is adjusted by a temperature sensor: if it uses only the flow to determine the heat needed boilers won't be able to guarantee output temperature. So if acquaduct is 5° and boiler is set to 60° you may have 55°, while on 20° acquaduct you may have 75° DHW because tempes are calibrated with 10°C inlet. Usually old, mechanical only, instant-boilers have this kind of control wile newer with 'electronic control' use temperature and on/off flow-switch. – DDS Mar 6 at 8:44
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    Also in tankless system expansion tank is not needed because water is cold and heated only when there's flow, so there's no thermal expansionin 'no-flow' situation. – DDS Mar 6 at 8:47

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