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Want a storage tank to make tankless water heater more efficient.

My tanked water heater just went out and i have a tankless to replace it.
My problem is, is that in winter the ground water here gets down to just above freezing and the tankless has a hard time heating it. Works fine in the rest of the year. I was hoping i could just put some kind of tank before the tankless heater so the water would be allowed to warm naturally to room temperature, but i can't find anything. Everything is either pressure tanks for wells, or tanked water heaters. Do they make storage tanks that can be plumbed into system? Too much pressure?


  • kcopp
    kcopp Member Posts: 4,298
    What you are referring to is a " daylight " tank.
    You could use an electric water heater. Prob the most cost effective way. You could get a straight storage tank but it will be more money..
    To allow the water to "heat " up you prob need to strip the jacket off of it. Be aware that you have the potential to make a bit of a mess as the outside of the tank will most likely condense.
  • hot_rod
    hot_rod Member Posts: 20,720
    If you are looking for an uninsulated pre-warm tank, these galvanized well tanks are affordable.

    Depending on where you locate it, it may sweat.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • njtommy
    njtommy Member Posts: 1,105
    Would it also make sense to do it the other way? Put the tankless before the storage tank/ electric water heater? Use the water heater has storage. Idk if it's better or worse.
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,135
    Hello, Some of the old books call this a tempering tank. If you have a warm place to put it, then putting it upstream of the heater is good. Another possibility depending on your water pressure and space is to put a coil of large diameter poly pipe or something along those lines ahead of the heater. You'll get much more surface area to heat from this approach and condensation won't damage it. Two inch pipe holds about 17 gallons per 100 feet.

    Yours, Larry
  • gschallert
    gschallert Member Posts: 170
    I'd second njtommy's suggestion to place the buffer tank after the tankless in series. In addition to minimizing the lower gpm flow to taps with the higher temp rise required in winter you'd eliminate the short/low volume draw wait & waste for hand washing etc.
    EBEBRATT-Ed Member Posts: 14,455
    Putting the tank after the tankless won't work. Water is going to cool off with no way to reheat the tank it if it's in series with the tankless

    Is it possible to pipe the tankless into a tank?? Like a primary secondary buffer tank??
  • Leon82
    Leon82 Member Posts: 684
    You would have to recirculate a somehow. And there's a chance that can void the manufacturer's warranty.

    Sounds like his goal is to supply the tankless with warm enough water on the coldest nights so it'll have the maximum flow rate. But I think that might make it a breeding ground for bacteria.
  • Leon82
    Leon82 Member Posts: 684
    They do have heat exchangers for shower drain piping that would increase the incoming water for a heater
    bob eck
  • gschallert
    gschallert Member Posts: 170

    Putting the tank after the tankless won't work. Water is going to cool off with no way to reheat the tank it if it's in series with the tankless

    It can be done in-line either of two ways, the cheapest route would probably be a small one element 120V electric tank, 10 g or so depending on how much of a buffer you need before the tankless can replenish it with the lower gpm flow during the coldest inlet temps. The more expensive route would require a circ, wiring and & more plumbing. If the OP's volume (gpm) needs that the tankless can't currently meet is modest then a small electric easily and cheaply solves the problem. Wasn't it hot rod that had a similar DHW small tank setup inline for his combi? I can't remember who for sure but someone had done it. The benefit of storing heated water after the tankless mitigates the gpm loss when inlet temps drop plus has the added benefit of almost eliminating the low volume draw issues tankless without recirc's have.
  • bob eck
    bob eck Member Posts: 930
    30, 40, 80 and 120 gallon fiberglass storage tank.
    Cold water in the bottom then let the water warm up to room temp comes out the top of the tank.
    I would look at 80 gallon
    If incoming water temp is 40 degrees can you get the temp up to 55 or 60 degrees. This would help a tankless water heater out in the winter.
    How warm is the room the tank will be in?
  • bob eck
    bob eck Member Posts: 930
    > @Leon82 said:
    > They do have heat exchangers for shower drain piping that would increase the incoming water for a heater

    There is a place in Colorado that sells 3” copper with 3/4” copper coil around the 3” copper. Must be mounted vertical in the main drain line and would work best if the main shower runs into that drain line. If the house has pvc drain lines should be able to use fernco coupling to connect to the copper.

  • njtommy
    njtommy Member Posts: 1,105
    I would just install an electric water heater after the tankless. The electric wouldn't cost all that much to run do to only handling the load of stand by loss and a bit of temp rise.
  • Jack
    Jack Member Posts: 1,046
    What is the firing rate of your tankless? Has the system been flushed and maintained? How many and type of fixtures are you trying to run. What supply temp are you trying to maintain? What is the actual output in gpm on your unit? You should be able to read that on the display. Prior to suggesting mods, let's make sure it is running correctly first.