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Old 10-23-2015, 10:36 AM   #11
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Thanks for the responses, guys.

This builder is one of those Mr. Wonderful types that is often clueless but believes that technology has an answer for everything.

I was pretty certain that there was no free lunch in this situation and your responses have confirmed that he's asking the impossible.
Unless this builder is trying to be super GReen, the only other explanation is cheap.

The smart arse response running through my mind is, don't worry about under sizing the tankless unit. Global warming will catch up eventually and make up the delta in water temp rise. Lol

Kudos to the OP for at least considering the possibility, that's what drives innovation.
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Old 10-23-2015, 11:14 AM   #12
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Put a cheap electric tank downstream of the tankless with a mixing valve and recirc it back to the tankless. It won't lower the delta t it will lower the gph which will have a similar effect but let the tankless run more efficiently. It will also act as a buffer tank to prevent "cold water sandwich".
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Old 10-23-2015, 12:39 PM   #13
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you can recover some warmth from channel water, on the other hand I would suggest simply knocking up a unit.
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Old 10-24-2015, 06:56 AM   #14
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So you're worried about bacteria growth in a tank running at lower temps but not in a tank that would sit and allowed to be brought up to ambient temperature? Sounds like basically the same problem to me.
Tell the builder to stop being cheap and pay for a properly sized unit.
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Old 10-26-2015, 11:43 AM   #15
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So you're worried about bacteria growth in a tank running at lower temps but not in a tank that would sit and allowed to be brought up to ambient temperature? Sounds like basically the same problem to me.
Tell the builder to stop being cheap and pay for a properly sized unit.
I'm worried about both. I know that legionella can be an issue in warm water systems that aren't hot enough to kill the bacteria, but part of my question was if it was a concern in a ambient temp tank as well. It would seem so to me.
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Old 10-26-2015, 06:31 PM   #16
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I'm worried about both. I know that legionella can be an issue in warm water systems that aren't hot enough to kill the bacteria, but part of my question was if it was a concern in a ambient temp tank as well. It would seem so to me.
This is from OSHA

Q. What water conditions are best for growth of the organism?

A. Warm, stagnant water provides ideal conditions for growth. At temperatures between 20C-50C (68-122F) the organism can multiply. Temperatures of 32C-40C (90-105F) are ideal for growth. Rust (iron), scale, and the presence of other microorganisms can also promote the growth of LDB.

That is just legionella. I'm sure there are a lot more bacteria that would love to live and breed in a nice room temperature storage tank.
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Old 10-26-2015, 07:06 PM   #17
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Am I not understanding something? Why not put in a Grundfos comfort system circ pump w/ bypass and set the timer to the most used times? Naviens have a dip switch for when a pump is in use. Or use the internal one that comes on the 240a. Won't that help by pre-heating the water before using it?
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Old 10-27-2015, 08:36 AM   #18
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Am I not understanding something? Why not put in a Grundfos comfort system circ pump w/ bypass and set the timer to the most used times? Naviens have a dip switch for when a pump is in use. Or use the internal one that comes on the 240a. Won't that help by pre-heating the water before using it?
I don't think that will work. Are you talking about recirculating hot water back to the tankless while you're using hot water?
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Old 10-27-2015, 10:15 AM   #19
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I don't think that will work. Are you talking about recirculating hot water back to the tankless while you're using hot water?
Sure. Why not? I'm asking seriously. Would the pump not help? I guess it wouldn't work well after a minute or two of use. It would seem like the tankless wouldn't have to work as hard at first but once the GPMs increased on the outlet side it would exceed the limits the pump.

Hell, now that I think about it more I guess it's not such a great idea. Never mind.
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Old 10-27-2015, 01:25 PM   #20
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Sure. Why not? I'm asking seriously. Would the pump not help? I guess it wouldn't work well after a minute or two of use. It would seem like the tankless wouldn't have to work as hard at first but once the GPMs increased on the outlet side it would exceed the limits the pump.

Hell, now that I think about it more I guess it's not such a great idea. Never mind.
That works to avoid a "cold sandwich", so your hot water is actually on demand. The pump has nothing to do with the demand the unit can handle. Each Unit is rated for a certain GPM that can be produced, in this plumbers case, his incoming temp is 50 degrees, so you actually have to adjust your GPM down, as most of those ratings are based on 70 degree water. if not sized properly the unit will still produce your 120 degree water, however flow rates will drop dramatically once you exceed the designed GPM on the unit, same thing happens if you crank it up to 140, great, but you just lost GPM as it is now working 20 degrees hotter. And this is why the customer who bought their tankless water heater at Menards doesn't like it, the 18 year old know it all has no idea what a BTU is
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