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Thread: tankless recirculation

  1. #1

    Default tankless recirculation

    I was hoping to get a few expert opinions on how to provide a hot water recirculation system to my new tankless installation. There seems to be few different ways to provide instant hot at all fixtures. I have considered them all but since I have an existing 1/2" recirculation line I think it might be my best option. The attached schematic shows what I'm working with.
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  2. #2

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    Why did you decide to go takless then add a recirc pump?

  3. #3
    Master Plumber master plumber mark's Avatar
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    Talking that looks like great fun

    I dont know how a tankless would work with a recircualtion
    line.....

    woudent the flow of water through that system automatically get that burner going ?????

    in theory wouldent it be going all the time as long as their was some flow inthe pipes.....

    I would also consider the extra wear and tear on
    the tankless unit

    i know that the normal tank type gets over worked in a
    recirualtion system and burn out early...

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Most tankless systems I looked at (that was only a few) strictly prohibit use with a recirculation system. Carefully read the specs on the one you have.

    All of the savings you thought you might have by putting one in will go up the flue if it has to run continuously. If you really want instant hot water, you probably want to do something else...
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    Master Plumber master plumber mark's Avatar
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    Talking another tankless dream crashes and burns

    I hope that their is a way to do this but
    I think its just another reason not to buy one

  6. #6
    Master Plumber Dunbar Plumbing's Avatar
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    Default Hey don't say that!!

    I might of landed a job putting in a HUGE Rinnai tankless.

    Guy just bought the house, has 8 people living in the house. 5 kids and 3 adults.

    No chance for any water heater supplying that demand.

    Told him to go with an oversized tankless for his needs and only because he wants tankless.


    But when I told him that it'll cost as much as the heater to install it, he pulled back.

    And when I told him that the sections of stainless steel piping near $100 a 4 foot section, he sat down.

    Then I told him that he needs to clean the compartment regularly for hard water, he took medicine.

    When I finally told him that no one in the area works on them and that hot water would take days, not hours to accomplish, he passed out and fell out of the chair.





    In all seriousness though I told him that he would be one of the better candidates for the application figuring the requirements of so many back to back showers. Two water heaters I don't believe would work given the confined space the area allows.

    I'd kill to have his tax status though; government probably pays him to have created all those social security numbers!!!
    Last edited by Dunbar Plumbing; 12-11-2007 at 08:51 PM.
    Read what the end of this sentence means.

  7. #7
    In the Trades Bob NH's Avatar
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    Default

    Quote Originally Posted by jjfta View Post
    I was hoping to get a few expert opinions on how to provide a hot water recirculation system to my new tankless installation. There seems to be few different ways to provide instant hot at all fixtures. I have considered them all but since I have an existing 1/2" recirculation line I think it might be my best option. The attached schematic shows what I'm working with.
    If you put a 2.5 or 4 gallon point-of-use heater at the end of the line where you were planning to put the recirculation point then the system will provide the hot water you need and will work with your tankless heater. They run on a 120 Volt circuit and don't waste as much energy as a recirculation system. Check out Ariston.

    The only problem with them is that they are so simple, reliable, and inexpensive that nobody promotes them very much. Not enough whizz-bang or high tech.

  8. #8
    Plumbing Designer FloridaOrange's Avatar
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    Default

    Quote Originally Posted by Bob NH View Post
    If you put a 2.5 or 4 gallon point-of-use heater at the end of the line where you were planning to put the recirculation point then the system will provide the hot water you need and will work with your tankless heater. They run on a 120 Volt circuit and don't waste as much energy as a recirculation system. Check out Ariston.

    The only problem with them is that they are so simple, reliable, and inexpensive that nobody promotes them very much. Not enough whizz-bang or high tech.

    I've desigend a system with a tankless and a 6 gal for recirc purposes. AFAIK it worked rather well.
    Matt
    Semi-professional plumbing designer
    Enjoying life in SW Florida

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    Which brand and model allows recirculation? Those I looked at specifically prohibited it. It's been awhile, I haven't looked recently.

    One of the big advantages of a tankless is you can put it closer to the point of use since they are all pretty much closed systems and need minimal clearance. Then, you won't have huge lengths of lines to heat. If things aren't centralized, you may be best served with several, though or you'll run into the same problem.

    WHen I experienced them in Germany, it was located in a closet right next to the bath. It took a little longer for heat to get to the kitchen, but that wasn't as big a deal.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #10
    In the Trades AZ Contractor's Avatar
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    Default

    The Rinnai's allow circulating pumps.

  11. #11

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    Rinnai does indeed allow for a recirculation line but only if you do it the way they show you in the installation manual,which is with the small point of use tank.Any other method reduces the warranty on the Tankless by one half.They teach this in their installer certification class.Ive heard it said lately the certification is no longer needed to purchase their units,dont know if its true.

  12. #12
    DIY Senior Member Marlin336's Avatar
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    Default

    Quote Originally Posted by FloridaOrange View Post
    I've desigend a system with a tankless and a 6 gal for recirc purposes. AFAIK it worked rather well.
    You're probably basically using a tank less to fill a tank, correct? I don't see why that wouldn't work without putting undue stress on the tank less.

  13. #13
    Plumbing Designer FloridaOrange's Avatar
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    Default

    Basically yes. I'd have to dig through some archives to find the exact routing/valving that we used but yes, the tankless filled the tank. The recirc was back to the tank, not the tankless.
    Matt
    Semi-professional plumbing designer
    Enjoying life in SW Florida

  14. #14
    In the Trades Bob NH's Avatar
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    Default

    Quote Originally Posted by FloridaOrange View Post
    I've desigend a system with a tankless and a 6 gal for recirc purposes. AFAIK it worked rather well.
    . . . . .

    Basically yes. I'd have to dig through some archives to find the exact routing/valving that we used but yes, the tankless filled the tank. The recirc was back to the tank, not the tankless.
    Unless the "tank" is a water heater then the recirculation will eventually cool off the tank and the result will be only to delay the cooling. Furthermore, that 6-gallon tank will become a "tepid tank" after a while and that 6 gallons will have to go down the drain before really hot water arrives at the faucet.

    A heat and mass calculation would show that addition of a recirculation system AND a 6-gallon tank that will eventually be filled with tepid water will not be an improvement over putting a smaller point-of-use heater at the point where the hot water would be recirculated. It could me made to work in that it would deliver hotter water sooner for cases where there is limited time between water usages, but even in that case it will waste more energy.

    Simpler is almost always a better design, and one small point-of-use heater is simpler than a tank + recirculation system. The only justification for the recirculation system would be if analysis showed that it actually improved on the simpler system, and that can't be shown by thermal loss or any other analysis.

  15. #15
    DIY Junior Member gmt's Avatar
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    Default

    I've installed recirc systems with tankless. You definately do not want to utilize the tankless as part of the recirc system or, you cut the 10 year warranty in half. But if you use a little Ariston, (say eight gallon) with a pump and expansion tank you will have little problem. The small electric w/h supplies the recirc, then once the call for hot water is triggered the Rinnai flows through the Ariston, ensuring it is kept hot.
    I have one where the Rinnai is approx. 100' or so from the master bath. The cust has never complained about waiting for hot water. This was installed about 2 years ago

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