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Low flow hot water

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I have a gas fired Rinnai hot water heater. I was told it requires at least 1.5 GPM in order to trigger the unit. No problems with showers/high flow but how do I maintain hot water in a low flow situation such as a bathroom sink or washing dishes?

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  • mattmia2
    mattmia2 Member Posts: 9,747
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    Looking at a few Rinnai spec sheets, it looks like it is more like .5 gpm. There are some types of flow switches that collect magnetic contaminants and fail over time until they are cleaned. Is this a new problem or has it happened ever since it was installed?

    A recirculation pump is a way to artificially maintain the minimum needed flow.
    GGross
  • ortho1121
    ortho1121 Member Posts: 3
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    It has been since day one. I first changed out the flow restrictors in the showers but otherwise seem to have this low flow problem. I do flush the unit yearly. If I install a recirculating pump will that cause the unit to continually turn on?
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,884
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    You could consider a tiny tank or two - they make them in the 2 to 8 gallon range that you can use for the lowest flow applications.
  • nekdahl
    nekdahl Member Posts: 12
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    Get a Magnetic Filter like a Boiler Mag or Caleffi and a small ECM re-circulator that is tied to a time clock and Aquastat. You will get hot water flowing through your plumbing system but only when you typically use it and only when the return water temp drops during that set time. You will have a nice efficient system.
    Nick Ekdahl, CPD, GPDDirector of Training & EducationDawson Company
  • rick in Alaska
    rick in Alaska Member Posts: 1,457
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    If you are having problems getting it to run with a low flow situation like a sink, then it is most likely the turbine inside that senses the flow might need cleaning. They can be taken out and cleaned if that is the case.
    Rick
    Larry Weingarten
  • SummitMechanic
    SummitMechanic Member Posts: 25
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    Rinnai water heaters require .4-.6 gpm to ignite initially, and .26gpm to maintain the flow of hot water. I would venture a guess and say either your overall water pressure in your home may be too low, or the flow sensor on the water heater itself is failing and ohming incorrectly. put a gauge on a hose bib or anything similar. if pressure is lower than 50lbs i would address that first and move on from there.
    Experienced Boiler Mechanic In Summit County, Colorado.
  • Jamie Hall
    Jamie Hall Member Posts: 23,438
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    Ah... if our OP is on a private well, it's likely that the well operates between 30 and 50 psi, although a few are set up for 40/60. In which case, a reading below 50 psi is entirely normal. It ain't broke. Don't fix it. Look elsewhere.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SummitMechanic
    SummitMechanic Member Posts: 25
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    Ah... if our OP is on a private well, it's likely that the well operates between 30 and 50 psi, although a few are set up for 40/60. In which case, a reading below 50 psi is entirely normal. It ain't broke. Don't fix it. Look elsewhere.

    I agree, but with modern flow rates on faucets combined with required flow rates on modern tankless heaters it can cause the flow sensor to not detect enough flow when only 1 faucet is being used at 40-50lbs. which is why they should know house pressure before they start throwing money at the problem
    Experienced Boiler Mechanic In Summit County, Colorado.
  • Jamie Hall
    Jamie Hall Member Posts: 23,438
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    Ah... if our OP is on a private well, it's likely that the well operates between 30 and 50 psi, although a few are set up for 40/60. In which case, a reading below 50 psi is entirely normal. It ain't broke. Don't fix it. Look elsewhere.

    I agree, but with modern flow rates on faucets combined with required flow rates on modern tankless heaters it can cause the flow sensor to not detect enough flow when only 1 faucet is being used at 40-50lbs. which is why they should know house pressure before they start throwing money at the problem
    Oh I quite agree. The modern low flow rate faucets and showers can be a real problem on private systems, particularly when combined with modern tankless heaters which won't work at low flow rates.

    There are several workarounds, none of them really good. The one I use is to take the flow restrictors out of anything advertised as low flow -- or avoid buying it in the first place. That is not always possible. I'd also avoid tankless heaters, but only partly for that reason (those things are power hogs). The other is to boost the pressure switch settings, but the pump may not be able to do that and even if it can, it's pretty tough on the pump, and reset the pressure tank pressures to match.

    The first option is the best.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • rick in Alaska
    rick in Alaska Member Posts: 1,457
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    Also, on the models that are high efficiency, like the RUR models, they need higher water pressure because of the internal pressure drop of the unit. If I remember right, it is at least 50 psi.
    Rick