Buffer tank to eliminate cold water sandwich

Discussion in 'Tankless Water Heater Forum' started by Willy101, Jul 23, 2021.

  1. Willy101

    Willy101 New Member

    Jul 10, 2021
    Does anyone make a small buffer tank that I can install on the hot water outlet of a Rinnai RL75I?
    I have a recirculation line with a grundfos pump. I use a repeat cycle timer to run the pump on a schedule similar to the heater program. The system is working great except for the cold water sandwich if you happen to be taking a shower at the end of the cycle. I want to install a buffer tank to mitigate the temperature drop. Haven't been able to find one except for a small electric water heater. Anyone know of a small buffer tank without electric?
  2. fitter30

    fitter30 Well-Known Member

    Feb 2, 2020
    Retired service tech
    Peace valley missouri
    Water heater would be the best bet. Its insulated, has a diptube for mixing so the water doesn't get stratified.
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  4. Willy101

    Willy101 New Member

    Jul 10, 2021
    Thought of that but haven't been able to find a small one with 3/4 fittings. Everything has the sink fittings. Don't want to reduce the flow. A small buffer tank would be a easy way to reduce the cold water sandwich effect. Thought someone would be making one.
  5. wwhitney

    wwhitney Well-Known Member

    Mar 17, 2019
    Berkeley, CA
    You can get a 2-1/2" gallon hot water heater with 1/2" IPS fittings and then adapt to 3/4". The short section of 1/2" IPS won't have a significant additional pressure drop.

    But you should be aware that the way it will reduce the cold water sandwich effect is by spreading it out over time. E.g. if you currently get 6 seconds of water 30F colder than target, you might spread that out to 60 seconds of water 3F colder than target (made up numbers).

    Cheers, Wayne
  6. Bannerman

    Bannerman Well-Known Member

    Mar 19, 2014
    Ontario, Canada
    Not sure what this means as cold water sandwich should not be occuring while hot water is flowing continuously unless the flow rate is reduced lower than the rate needed to keep the gas burner operational.

    Because a tankless heater has no hot water preheated in storage, when hot water is first drawn, some cold water entering the WH will pass through the heat exchanger before the gas burner becomes activated and heating water to capacity.

    Although the cooler water exiting the WH will be not much of an issue when no hot water has been used for some time and so the water within the hot water plumbing through the house is also cold, it will be a problem when hot water has been used, then shut off and then ran again while the distribution plumbing remains hot.

    Since some cold water will again pass through the heat exchanger during startup, hot water within the distribution lines will be hot initially, which will be then followed by the cold slug that exitied the WH during burner startup which will be then followed by the hot water produced after the burner has refired and is up to capacity.
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