# Tankless inline with tank for high output / demand



## ferncreekjeep (Apr 3, 2012)

I cant find much info on this. Has anyone installed a tankless that fed a 50 gal tank? I want to run a high output shower valve (moen io digital) where the capacity can reach 18 gpm. It will most likely not be run all the time at that capacity, but would like to have the ability to. I am also interested in only running the tankless when the demand is above 2 gpm. That way I limit how often the tankless kicks on, saving gas. I haven't found a tankless with that capability. Should I be looking for a volume specific diverter valve or maybe a control valve to switch the tankless on? Can the tankless lower limit be modified? If this is going to be too complicated, then what are my other options? I live in KY where the incoming water temp is an average 55 degrees. I also have a 1" gas feed, will I encounter any problems there?


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## KillerToiletSpider (May 27, 2007)

Hot water is over rated.


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## ferncreekjeep (Apr 3, 2012)

Well thanks for that wonderful wisdom. Does any one have any real ideas? 

I have the drain squared away. I am installing a 5' linear drain with two 2" outputs that feed two 2" traps and then into a 3" main line. I also have a 1.5" vent within 2.5' of the drains. 
I will most likely only be using 6gpm of hot water. I just wanted some kind of a reserve capacity (hence 50 gal tank) if i were to turn everything on and probably only for just a few minutes. 
The idea of bypassing the tankless was to keep it from coming on during low volume draws (ie. shaving, washing dishes, washing hands and etc.). 
I have spoke to four plumbers and no one seems to be familiar with this moen valve. That is why I am asking here, kind of a last resort. 
Any other ideas? 
Thanks!


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## Jackfre (Mar 1, 2012)

*Won't work as you describe it!*

A tankless thinks it is the only thing in the system. As such it has the technology to regulate flow in order to maintain temp. If you put more water thru the tankless than its maximum input can handle the flow control valve on the cold water inlet will close enough to maintain the proper hot water output temperature. You will depressurize the hot water system as you try to pull more water out of the tank that is in series with the tankless. 

For your type application, any tank set-up is not going to operate as well as tankless only, properly designed and installed.

In order to get 18 gpm you will want to put, depending upon model, 3-4 tankless units together. That way you can enjoy the "flow" as long as you choose. No tank set-up will do this and it makes no sense to maintain storage to "fulfill your desire" for a long high flow shower. Fact is, at some point, I don't care how much you like your shower or how much money you have, you have to get out of the shower and go pay for the system. The real advantage of the multiple tankless in this situation is that if you had 3 units that could produce 5.4gpm each you are approaching your 18 rpm max. By the time you mix it down to your 110-112F temp for the shower it will give the 18gpm. The big advantage is that while you are in the shower you get the satisfaction of the desired flow. Once you a get out of the shower, if you want to just wash your hand the tankless system will fire only the amount of gas to satisfy that flow rate. Very cool system...if you can get the gas supply to feed it.

I've done showers up to 25 gpm. 5 tankless units for that one. the guy was going to put in a 200kbtu boiler and 5 120 gallon tanks. Nuts! Remember as you flip out at the cost of supplying this amount of hot water that you built it. Now you have to feed it 

I cannot tell you how many people have asked me, "why didn't they tell me how much it would cost to heat my hot water?" My reply is, "because you would not have bought the fixture". Let us know which way you go!


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## Seven-Delta-FortyOne (Mar 5, 2011)

ferncreekjeep said:


> Well thanks for that wonderful wisdom. Does any one have any real ideas?


:laughing: That's just the way he is. He is (was) actually a very knowledgable plumber, he's just a PITA sometimes. :bangin:


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## KillerToiletSpider (May 27, 2007)

Tankless water heaters don't think, they are machines, and no one heats hot water, it's already hot.


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## wyoming 1 (May 7, 2008)

Jack fire just currious when you put 5 together do you split the cold to feed all five or cold from one to the hot on the other?

An ATC what do you mean sometimes? lol


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## Tinstaafl (Jan 6, 2008)

KillerToiletSpider said:


> Tankless water heaters don't think, they are machines, and no one heats hot water, it's already hot.


They do if they need it even hotter. :whistling


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## KillerToiletSpider (May 27, 2007)

wyoming 1 said:


> Jack fire just currious when you put 5 together do you split the cold to feed all five or cold from one to the hot on the other?
> 
> An ATC what do you mean sometimes? lol


Separate cold feeds, a tankless has an incoming temp sensor, and won't kick on if it is close to it's programmed output temp. Takagi makes a setup that manifolds the group so that they match the demand, if demand only requires the output of three then only three will operate. Piping water heaters in series is for tank type heaters, and is a sure fire way to burn up the first one in the series.


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## wyoming 1 (May 7, 2008)

Thanks I have seen them in series in regular tank type and I always thought that it would over work the first and basicly use the second as a storagetank/ maintainer


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## matttheplumber1 (Apr 26, 2012)

what about pipe size??? that fixture needs 18 gpm... hope u increase pipe size to meet the demand... if needed that is. good luck tho, let us know how it turns out. i can test it for ya:thumbup:


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