Tankless,Cold Water Sandwich

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MichaelBukay

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any ideas on a device or piping option to eliminate the cold water sandwich effect on a tankless system? I'm in a dispute with my employer over the issue. Need backup!
 

Terry

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Navien makes a unit with a recirc that takes care of that.
Or you can add a small water heater and recirc with that. Otherwise, yes, there is a cold water sandwhich. Wait for Dana to check in though. He knows a lot more than me on this.
 

MichaelBukay

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thanks Terry. That much I know. I'm installing a Rinnai. I heard something about some expansion tank device that solves the problem. But when I called my supplier, the device was discontinued. not much in the net about all this. Thanks for the reply.
 

Dana

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What is the nature of the dispute with your employer regarding the cold water sandwich issue?

The only way to truly eliminate the cold water sandwich is a storage tank for the hot water. There is always a flue purge delay prior to ignition once the flow has been detected, which both cools off any residual heat in the heat exchanger, and allows some water to flow through unheated before the burner fires. If that unheated water can be mixed with locally stored pre-heated water, the sandwich effect is pretty small. But there is no way to do it with an expansion tank, or exotic plumbing exterior to the unit that does not include some quantity of already-hot water. The amount of hot water storage needed is small, on the order of a quart or so, but it needs some thermal mass of pre-heated water. The larger the volume of stored water, the longer the ignition delay can be without experiencing a cold water sandwich. You can't cheat the basic physics.

In Navien's case it's a tiny storage tank and recirculation pump inside the cabinet. I wasn't aware that Rinnai had ever offered a solution. Due to increased standby losses, any storage tank solution cuts into the as-used EF efficiency, but it does fix the problem. To improve efficiency Navien made heating the internal storage tank programmable to time-of-day, on earlier models and it could be disabled completely if desired. But from a user point of view having to program yet another feature is a PITA. I haven't really looked at how the newest versions can be tweaked (if at all.)
 

Leon82

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I use an ariston 4 gallon mini tank with a stainless pump in a loop .there are check valves so water only flows into the loop. the tankless heater does not run while it circulates. only whe the fixture opens
 
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