Tankless with small tank: how to

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hondochica

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I have a Tagaki tankless wh. I don't know the model # off hand - I am looking for a solution to two problems: 1) I 'waste' about 2 gallons of water waiting for the hot to hit the faucet (only about 20 ft away!) . . . and 2) I would like to have hot water w/o turning on the faucet high enough to kick on the tankless - like, a little warm water to wash my hands for 10-20 seconds. My Tagaki needs a flow rate of 3/4 g/m to kick on. I am thinking a small - 10g tank would help at least #2 and maybe reduce the amt of water I lose in #1.

I found a nice diagram here: (from fine homebuilding - you've probably seen it): http://www.chandlerdesignbuild.com/files/fhbDecJan08.pdf

My concern here is that if I don't use enough flow to kick on the tankless, I'm not doing anything but installing a small tank water heater, cuz the tankless won't kick on with a low flow.

Is there a way to install a tank - after the tankless - with a float; such that the tankless will only feed the tank when it's about 1/2 empty - thereby increasing flow to the tank so the tankless actually kicks on! - Yes, I'm hiring a plumber to do all this.

Thanks for your help
I tried a 'search' for help with a tank/tankless combo - didn't find anything easily!

Kelly
 

Leon82

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I dont know about a float switch but I put a 4 gallon on mine with a recirculating loop acording to my bosch diagram. I can be done without the loop also.. I get instant hot water and no cold water sandwich.
 

Dana

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Most Takagi units are rated to turn on at about 1gpm- none were ever as high as 3-4gpm.

A small electric tank near the sink, in series with the tankless is probably the "right" solution, or a small tank and a recirculation loop with the pump controlled by an aquastat on/in the tank. (The contacts of the controls of standard electric tank can handle a pump load, eliminating the need for an aquastat, but hacking it in would violate all sorts of codes.) Both the supply and return plumbing should be insulated to R4 (5/8" wall closed cell pipe insulation, not the crappy 3/8" wall stuff found in most box stores.)
 

hondochica

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thanks for the help! My house is pretty small so I think I'm just going to have a small tank installed.

fyi - Dana: that was 3/4 or .75 gpm - not 3 or 4 . . but certainly easy to mis-read - thanks for the help

Kelly
 

MrStop

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In adding a small tank, is there risk of legionella bacteria forming at lower temperatures? It would require the tankless unit to be set at 140 degrees, but that would decrease the unit's efficiency correct?

In the first solution, with just the buffering tank, shouldn't a tempering valve be added also to drop water supply temp down to 125 degrees?

In the second solution, with a tempering valve and recirculating pump, it states that the tank temperature will fluctuate from 125°F to 140°F. Are we introducing risk of legionella forming with this design?
 
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