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Cold sandwich

Can anybody explain cold sandwich, I have a customer interested in tankless and I want to make sure we don,t run into any proublems.


  • icesailor
    icesailor Member Posts: 7,265
    Cold Sandwich?

    Yeah, explain that to me too.
  • bob eck
    bob eck Member Posts: 930
    cold water sandwich

    16. What is a cold water sandwich effect?

    The term “cold water sandwich effect” is a term that is used to describe the introduction of cold water into the hot water supply line during frequent on/off operation of an instantaneous water heater. The cold water sandwich effect, when present, appears as a momentary drop in hot water temperature as it is discharged from a hot water supply outlet (i.e. shower, tub, or faucet). This phenomenon is present in the operation of all instantaneous, tankless style, water heaters, but is minimized with the high tech design of Rinnai water heaters.

    The technology built into the Rinnai water heaters is designed to minimize the cold water sandwich effect. Rinnai water heaters are microprocessor controlled and when water flow through them ceases, they remain in a “ready to fire state” for approximately 1 minute. If water flow through a Rinnai water heater begins within the first minute following water flow stoppage, the water heater will fire back up within 1 to 2 seconds. This minimizes the cold water sandwich effect that would otherwise be experienced with a low tech tankless water heater. It should be noted that the cold water sandwich effect cannot be removed completely from tankless style water heaters. The safety standards developed to insure the safe operation of water heaters require a delay in the ignition sequence of all gas water heaters.

    While the cold water sandwich effect cannot be completely eliminated from standard plumbing systems, it can be eliminated from plumbing systems that have a supply and return hot water circulating system. Rinnai has developed 2 methods to eliminate the cold water sandwich effect in residential hot water circulating systems.

    The first (and preferred) method to remove the cold water sandwich from circulating systems utilizes a small electric tank water heater (powered up) that is used with a dual purpose. The small water heater acts as a mixing tank to eliminate the cold water sandwich effect from the Rinnai water heater and it uses its electric heating element to offset the heat losses from the hot water circulating system.

    The second method to remove the cold water sandwich from circulating systems utilizes a small 2 to 6 gallon storage tank installed on the hot water outlet of the Rinnai water heater. This tank acts as a mixing tank to blend the cold water sandwich with hot water and eliminating its effect at fixtures.

    This is what Rinnai has out there on info of the cold water sandwich Noritz technology also keeps the cold water sandwich to a minimum.
  • Jack
    Jack Member Posts: 1,047
    In my house

    The other day I took the second shower of the morning, I had the existing hot water in the piping. Given that the water flow had stopped, the burner must shut off. When I started the water flowing for my shower it was still a hot shower so I entered and began, well, showering. A few moments later I felt the water begin to cool as that slug of "cold water sandwich effect" start-up water get to my shower. Recognizing what was happening, I stepped aside for about 5-7 seconds and the cooler water passed and I had a nice hot shower. That is it!
  • EricAune
    EricAune Member Posts: 432
    Great example Jack

    That's it in a nutshell.  Explaining that the tankless will allow a small amount of cold water through while in sequence of lighting the burner is the key.  It's not a malfunction of the heater, the newest high quality models have dramatically reduced this scenario over earlier models. 

    I am partial to Rinnai for many reasons, Navien has a model with a small holding tank that is designed to eliminate the sandwich.  Rinnai, as far as I know, does not have this feature available.  One thing to consider is local parts (as always, but) these things have a lot of them!  Make sure you are buying from a supplier that will be available with a good stock of parts if there were a breakdown.  Most of the major manufacturers have certification classes (usually half day events), definitely take advantage of this.

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