Water heater shuts off when cold water open

Discussion in 'Tankless Water Heater Forum' started by Chris Chervenyak, Dec 18, 2019.

  1. Chris Chervenyak

    Chris Chervenyak New Member

    Dec 18, 2019
    New Jersey
    I have three bathrooms in my house, 2 full and 1 half. The hot water and cold work together in both full bathroom's sink and showers. My half bath is on the first floor, directly across from the laundry room here the water heater is. I have the control pad in the hall and can see it from the bathroom door. When I turn on the hot water, I see the flame icon appear on the control panel. Once I crack the cold water, the flame icon disappears and the water becomes cold. I have no idea why this would be. I thought maybe it was because the cold water feed was 3/4" or maybe because it was from being so close to the feed to the water heater. I rerouted the cold water to be further away and being fed by a 1/2" line. This didn't work. I have a four year old Rinnai RUR 98i.
    Any suggestions would greatly be appreciated.
  2. Bob Y.

    Bob Y. New Member

    Mar 25, 2019
    Hi Chris. On your system there is a temperature set point adjustment to set the temperature of the hot water. It seems that the set point is set too close to the cold water input temperature. Check the manual for set point adjustment and readjust it to a higher temperature set point. If that does not work, check the thermal couple. It may be open. Most good plumbing supply houses would carry this item.

    Chris, please let me know how it works out for you.

    Bob :)
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Most vanity faucets, at least those produced in the last 20+ years, are flow restricted. It could be that the mix of temperature you're trying to produce at the sink is not flowing enough volume to keep the tankless on. If you run full hot, does the unit turn on and stay on? If so, as you change the outlet to cooler, at some point, the tankless will turn off because there's not enough flow. One of the reasons I'm not enthused with today's tankless systems. They can work, but not for everybody and every situation.

    Running a shower will usually draw enough volume to turn the unit on, and keep it on, unless you like cold showers purposely.
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