Any way to have a gravity recirc loop with a tankless water heater?

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by benze, Jul 4, 2012.

  1. benze

    benze New Member

    Messages:
    63
    Location:
    Montreal, Canada
    Hi,

    I've got a 60gal hot water tank with a gravity recirc loop that is working well. My home currently has 3 full baths and 1 half-bath. I am now looking to add an extension to my home that will include a new master bath and an additional full bath. With 3 girls living in the house, I don't expect that a 60G tank will be enough to handle the load/supply.

    I am thinking that perhaps the best solution will be to drop the temp of my hot water tank to 80 or 100F and add a tankless water heater on the output of my hot water tank. Given that the tankless will not need to heat as much, I am hoping that I will be able to supply enough hot water for the home.

    So a couple of questions arise;
    1) Is this a good concept?
    2) Is there anyway to maintain a gravity loop in this type of scenario?

    Thanks,

    Eric
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,806
    Location:
    New England
    Don't think it will work as you hope for two reasons: the tankless needs a minimum volume to turn on and I don't think gravity would do it, plus, for a gravity loop to work, there has to be a difference in density of the water. It gets that difference by being hotter than you plan on the storage tank. How is the tank heated? To increase the 'effective' size of the tank, you can run it at higher temperatures. This would necessitate adding a tempering valve, if you don't already have one. If that tank was an indirect and you ran it higher, it may be large enough.
  3. mikeplummer

    mikeplummer Plumber

    Messages:
    190
    Location:
    Ontario, Canada
    you should be thinking of using a tankless as your master water heater using the 60 gallon as a buffer/storage tank. this may be beyond the scope of DIY however. you would need a "pumped" heating loop, pump operation would be activated by t-stat on tank, but again, not a "simple" piping system for the average guy...
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,256
    Location:
    Cave Creek, Arizona
    No for two reasons. First the tankless needs a positive flow of water and a gravity loop does not provide any where near enough velocity. Second, the gravity loop needs a constant supply of "hot" water to maintain its flow. There is NO hot water in a tankless heater to maintain the circulation. 80 to 100 degrees is "tepid" water, and for all practical purposes not much more than the ambient temperature in some cases.
  5. benze

    benze New Member

    Messages:
    63
    Location:
    Montreal, Canada
    Interesting concept, although I don't really see how this "buffer" would work too well. In a standard setup (ie: no tankless involved), when I turn on a hot water faucet, the pressure from the incoming cold water into the tank pushes the hot water out. So my output pressure is directly related to the input pressure.

    With a tankless in front, sourcing the tank, the hot water output from the tank would be controlled by the pressure from the tankless. If the tankless has to heat from 10C->50C, then I am limited by the flow/speed of the tankless. Unless I lower the output heat from the tankless and only heat to 30C, and let the HW tank heat from 30C ->50C.

    I'm not sure I follow your idea on a pumped heating loop though; what would the loop be used for?

    I guess my goal when updating the system is 2 fold:
    1) to keep "instant" warm water at the faucet (via a recirc loop)
    2) to ensure that I have a constant/steady stream of hot water even with multiple showers running and 3 or 4 girls in the house using them (ie: long hot showers).

    Thanks!

    Eric
  6. mikeplummer

    mikeplummer Plumber

    Messages:
    190
    Location:
    Ontario, Canada
    the loop would be used for heating the tank in the hot water tank..your existing piping would be the same except your tank would now be fed from the hot outlet of tankless. the water would return to the tankless, usually adding a tee at drain port of tank. without a circulating loop between the tank and tankless, the water in the tank wouldn't get hot as it would only be heated as you use it. so the pump is operated by the thermostat on the tank to turn the pump on when the tank drops to set temp.
  7. benze

    benze New Member

    Messages:
    63
    Location:
    Montreal, Canada
    Why not use the heating elements in the tank to keep the heat in the tank constant, instead of refeeding it back to the tankless? Additionally, if refeeding it back to the tankless, it would likely have to tee into the cold water supply line (I assume with a check valve on the cold), hence instantly losing some temp as they mix, thereby making it more inefficient. Unless there is some way to have a Y valve that flows only water from the HW tank when the recirc pump runs vs pulling water from the supply line when there is a demand for HW.

    But again, I wonder what the advantage of this setup is vs just using the HW tank to keep the HW heated as it currently does?

    Thanks,

    Eric
  8. Winslow

    Winslow Plumber

    Messages:
    450
    Location:
    Hawaii
    I have piped systems similar to this and it works great but only done it with gas systems.
  9. lifespeed

    lifespeed Member

    Messages:
    320
    Location:
    California
    Gravity loops are for tank water heaters. I can't think of a simple way to control a tankless to heat the slow trickle that results from heat siphoning. You could do something Rube Goldberg in complexity, like heating a small tank with the tankless to feed the gravity loop. But recirculators that are not timed or on-demand waste energy anyway, which may be undesirable.

    What I did is install a redy-temp recirculator and put low-voltage momentary switches in the kitchen and bathrooms. The switches supplement the programmable timer in the redy-temp, which has hot water recirculation scheduled for morning showers and dinnertime. This retains the efficiency of the tankless by limiting recirculation to short times, and allowing for on-demand use with the switches.
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