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Tankless or boiler w/ indirect for DHW, radiant loop run off coil??

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  • rbphhc
    rbphhc Member Posts: 126
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    Also, spoke to Takagi yesterday, and they specifically endorse the simultaneous use of their HWHs for both heat and DHW. It degrades the warranty somewhat from 15 to 10 years on heat exchangers, etc. Here is one schematic they suggest in their own manual:
  • WMno57
    WMno57 Member Posts: 1,268
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    Rinnai tankless install from Journal of Light Construction article. What is the downside to this?


    I DIY.
  • GGross
    GGross Member Posts: 1,049
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    Yes most tankless manufacturers approve their units for simultaneous space heating and DHW usage, they do this to allow their product to be sold for more applications, the same reason you will see flow dependent boilers approved to be direct piped even though 90% of the time they cannot be piped that way. The issue usually arises because corners are cut, pumps are not sized correctly, or the overall application is not a great fit and may cause early failures. Usually tankless jobs are either for a small amount of space heating, or the ones I see which are whole house installs that are piped wrong, sized wrong, pumped wrong etc. As with all things, if you do this correctly you can have a great install, in my opinion I would go with different equipment but only you know your circumstances best, and if your installer has faith in the system and everyone will support the sale then I say go for it.

    In my experience the cost difference to install a "real boiler" vs using a tankless and actually installing it correctly is negligible. The reason most people (not all) do it is because they plan to cut those corners. For instance, note that you must maintain between 2-4 GPM in the unit, should your flow rate fall out of this spec and the heat exchanger fails, it is completely reasonable for a manufacturer to deny that warranty if they can determine the failure was due to improper flow rates, which they can .
    rbphhcWMno57
  • rbphhc
    rbphhc Member Posts: 126
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    GGross said:

    You must maintain between 2-4 GPM in the unit, should your flow rate fall out of this spec and the heat exchanger fails, it is completely reasonable for a manufacturer to deny that warranty if they can determine the failure was due to improper flow rates.

    Understood. But *if* I have the circ pump for the indirect set to flow at 3gpm, say, to 'recharge' the tank for the radiant coil to draw from, wouldn't that keep it within spec?

    Also, what does any tankless company do about 'trickle' DHW usage - doing the dishes the way our 16yo does, for example, despite our entreaties (letting the HW run slowly while s/he nonchalantly wipes at individual spots on a glass or bowl with a slightly damp sponge)?
  • rbphhc
    rbphhc Member Posts: 126
    edited February 2023
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    Current thinking, as of 2/15. I have a good local source for Takagi HWHs and Stiebel-Eltron indirects, so this is getting realistic.

    Basically, this would be a completely conventional tankless HWH install *except* we'd be inserting a single loop to maintain the indirect somewhere between 90f and 130f. Radiant hydronic would run off the coil in the indirect.

    Obviously, DHW would function as intended, large Delta T, on demand, hydronic usually offline.

    When indirect tank falls to 90*f and calls for heat, initially there is a fairly large DT (90* to 130*f), but it diminishes as the tank heats.

    Questions:

    - Does the tankless automatically modulate its flame as incoming water temps rise?

    - Is there a minimum Delta T?

    - Assuming the circ pump can be set to maintain min 2GPM, and then shuts down when 130 is reached, we shouldn't have 'trickle flow' problems, correct?

    - If radiant happens to call for heat WHILE someone is using DHW, that would push an additional (preheated) 2GPM through the tankless, correct? Seems like that shouldn't hurt too much, esp if we are using a tankless rated for 8 to 10 GPM flow?

    - Any other issues I am failing to consider?

    Seems like we'd be asking the tankless to perform one additional burn every so often to raise 40gal from 90 to 130. That doesn't seem outlandish, does it?

    Thanks, all!



  • rbphhc
    rbphhc Member Posts: 126
    edited February 2023
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    BTW, anyone have experience w/ Stiebel-Eltron "hydro-shark" pre-fab panels? They have options for radiant in-floor w/ integrated DHW, using gas boiler/tankless HWH. https://www.stiebel-eltron-usa.com/products/heating/hydroshark-modular-panel-system-radiant-floor-heating
    Kokocolumbo
  • yellowdog
    yellowdog Member Posts: 157
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    i would be curious on what rinnai thought about that piping configuration.
    rbphhc
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,852
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    Rinnai tankless install from Journal of Light Construction article. What is the downside to this?

    Nothing, you’re just building a combi from hand. Why not skip the indirect? A radiant floor is a good bit of buffer. 
    GGrossrbphhc
  • rbphhc
    rbphhc Member Posts: 126
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    yellowdog said:

    i would be curious on what rinnai thought about that piping configuration.

    Me too. Seems like a recipe for short-cycling. Am I wrong about that?
  • rbphhc
    rbphhc Member Posts: 126
    edited February 2023
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    NTI makes a small wall-hung indirect, about the size of a combi, that we can 'stack' below the tankless (the S20W) to keep floor space clear. Just spoke to them today, and they actually were pretty excited about the application, and said go for it.

    Also spoke to Rinnai about using their RU180iP to run a recirculating loop off the DHW lines to maintain an indirect tank between 90 and 130 for radiant in-floor hydronic, and they were also gung ho. They said it would reduce the warranty from 15 to 10 yrs, but no reason not to.

    So we are going to go ahead with it. Old tanks removed, and space ready for new gear. Order goes in tomorrow.

    Question about propane line sizing I will post in a separate thread. Thanks for the feedback, everybody!