tankless recirculation

Discussion in 'Tankless Water Heater Forum' started by jjfta, Dec 11, 2007.

  1. jjfta

    jjfta New Member

    Messages:
    6
    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.

    Attached Files:

  2. elvisclock

    elvisclock New Member

    Messages:
    35
    Why did you decide to go takless then add a recirc pump?
  3. 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. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    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...
  5. 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. 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. :eek:





    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: Dec 11, 2007
  7. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    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. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida

    I've desigend a system with a tankless and a 6 gal for recirc purposes. AFAIK it worked rather well.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    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.
  10. AZ Contractor

    AZ Contractor In the Trades

    Messages:
    90
    Location:
    Phoenix, AZ
    The Rinnai's allow circulating pumps.
  11. Lancaster

    Lancaster New Member

    Messages:
    164
    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. TMB9862

    TMB9862 New Member

    Messages:
    206
    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. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida
    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.
  14. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    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. gmt

    gmt New Member

    Messages:
    26
    Location:
    Pennsylvania
    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
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    It's not as simple as just adding a recirculation system!

    So, basically, with a tankless, you need another WH, albeit small, to supply the recirculation loop. that makes sense, as keeping the tankless on the whole time the recirculator is running would add signficantly to the typical duty cycle. So, you can't or at least shouldn't JUST add a recirculator - there's more to the story.

    Thanks, you've confirmed my thoughts.
  17. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    If you put the 8 gallon Ariston AT THE MASTER BATH then you don't need the recirculation system. The small amount of water in the supply pipe mixes with the 8 gallons in the Point-of-Use heater to keep the temperature up where it is delivered and the tankless heater will supply the makeup.

    Avoiding the recirculation will avoid the constant loss of heat due to recirculation of hot water from the heater. Because the objective is to keep the water in the supply pipe hot there will be heat loss from two pipes that must be made up by electric heating.

    The recirculation pump and pipe are unnecessary.

    Keep it simple. Skip the recirculation!
  18. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    If there were multiple bathrooms you wanted to keep near instant hot water to, you'd still want the recirc, but I think the complexity and energy costs may make it kind of expensive. Then, I suppose if you've got a house big enough to need it for multiple bathrooms you may be able to afford it, too (at least for now!).
  19. jjfta

    jjfta New Member

    Messages:
    6
    Tons of good discussion here.

    So the option of an on demand type recirc system (Taco or Ready Temp) that push the hot back up the cold line is not a good option for the Rinnai system. I did not like the idea of having luke warm water in the cold line anyway. Certainly do not want to lesson my warrantee.

    Using a point of use tank with out a recirc, I do not think will work for me as the tankless is located 35 feet from the master bath, and 60 feet from the Kitchen. I would most likely need two instant hot tanks (still may be more economical though).

    It sounds like my best option would be to install a small electric tank in accordance with the Rinnai schematic shown in their installation manual. In my case I all ready have the old existing recirculation line. I will just need to extend it 30 feet.

    If I go that route, what size electric tank do I install (Rinnai recommends 2 -5 gallons)?
    What size recirculation pump? (I plan on using the UP10 series from Grundfos). This is a small pump with temp sensing controls; I hope it is enough hp.
    In addition, what size expansion tank should I use? I have know idea why an expansion tank is recommended from Rinnai

    Thanks again to everyone for all of the good discussion.
  20. gmt

    gmt New Member

    Messages:
    26
    Location:
    Pennsylvania
    On the size of the water heater you need to make sure that it has 3/4 inch inlet AND outlet, because the main hot line sytem also runs through this. I have found the Ariston 8 gallon model to fit the bill.

    A normal Grundfos recirc pump to be sufficient. The timer model is nice in that it doesn't have to run at times when there is no need.

    The expansion tank is around 2 gallons and just a normal water heater safety tank. It's main purpose is for the "closed" system of the recirc line.
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