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want feedback piping WOOD boiler gas mod/con backup via reverse indirect BUFFER tank

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by rainmaker, Dec 2, 2013.

  1. ewdudley

    ewdudley Minister of Fire

    Joined:
    Nov 17, 2009
    Messages:
    1,627
    Loc:
    Cayuga County NY
    With a reverse indirect, reserve DHW capacity is in the buffer tank, not the coil. With an Ergomax E23/E24 you have up to a 26 gallon head start, with the E44/E45 it's 48 gallons.

    If the Ergomax size is insufficient you can plumb the backup boiler to heat some portion of your main storage instead of going directly to the reverse indirect, and let the backup boiler get ahead of potential DHW shortages that way.

    Also if the Ergomax seems costly, you can look at installing a 'tankless coil' in the top of main storage and doing away with a separate water heater.

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  2. rainmaker

    rainmaker New Member

    Joined:
    Nov 4, 2013
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    Loc:
    between Hudson Valley and Catskills NY
    Ergomax smaller tank is limited to about 3 or 3.5 GPM max flow. That's enough for maybe a shower head and a faucet. But our bath tub spout which is nothing extravagant flows 5 GPM. Even with nothing else running hot water, I just don't see that working very well. Even with the next size up boiler at 100KBtu and the next size up Ergo... still not quite there. After reading dozens of forum threads on the subject, I see about half saying it works for them (without specific use details) but also half warning specifically that it won't be enough. We have the BTU but the bottle neck seems the size of the HX coils... (edit) ... combination of small boiler with small HX coil... according to their chart if we had 150kBtu it might flow enough but probably at 180*F ... but the whole reason for buffering in the first place was the boiler already being too big!

    ewdudley: I would be curious to know how your situation works and compares to the Ergo in spec?
    I want to make this work, i do...

    Now if the Thermo2000 people would actually call back, I would consider a Turbomax with extra ports. The product rep emailed and says they do but then won't reply back or return phone calls... (maybe by luck it is vacation week?) and the other sales rep Rathe way down in NY did not know and deferred to another rep who has not called back... maybe i just don't know the "secret knock "...

    No worries. I can fab my own buffer or get a BB-30 boiler buddy and just pipe the regular indirect DHW tank, or P/S and buffer the micro zone off the indirect like the book shows... just messy and more parts and labor.

    Last edited: Dec 12, 2013
  3. rainmaker

    rainmaker New Member

    Joined:
    Nov 4, 2013
    Messages:
    17
    Loc:
    between Hudson Valley and Catskills NY
    Here is my latest version with a lot of credit to Caleffi idronics_10.
    I have not given up completely on the buffer tank/ hydro-separator/ low-loss header, but decided to sketch an alternate layout that is more "conventional" using primary/ secondary closely spaced Tees for comparison. Link to piping diagram:[​IMG]

    My previous posts in this thread with some good feedback about wood boiler storage helps me feel confident that bypass might actually be redundant, so I'll plan on pulling hot water right off the top of the storage as soon as it is available... hoping it won't have to heat up the whole thing before it is useful.

    I hope this piping allows either boiler to run independently, and each zone run independently also.
    DHW indirect domestic hot water is pumped as a zone also (after the Tees) so it can pull from either boiler source without interrupting that primary flow...
    The micro zone is pulled off the indirect tank on another loop to help buffer. Thanks to Triangle Tube piping examples in "Prestige Application Booklet".

    Anybody have any ideas to improve upon this layout?How would I control this thing? (I can post the idronics wiring schematic next...)
    3 thermostats total (2 zone valves shown and 1 micro zone)

    thanks again.
  4. ewdudley

    ewdudley Minister of Fire

    Joined:
    Nov 17, 2009
    Messages:
    1,627
    Loc:
    Cayuga County NY
    The 'tankless coil' DHW HX in the reverse indirect I built (photo above) is rated nominally at 5 gpm, whatever that means. I set it up with stainless steel circulator to keep a conventional 50 gallon electric DHW charged. It turned out that with 140 degF in the indirect tank the unit can supply hot water for two low-flow showers and the clothes washer, not much demand really, but it's all we need. So the electric water heater is now valved-off, I run the pump from time to time and heat the tank once a year or so just because. The performance specs on this stuff very hard to come by, so all I can offer is a hunch that a 5 gpm finned tankless coil probably has more capacity than the coil in the Ergomax E23, their smallest unit.

    It looks like your storage will be fairly close to to your DHW loads. You may want to look at putting a 'tankless coil' in the top of one of your storage tanks. The fittings are reasonably priced and the welding shouldn't cost a lot either. The coil itself is a couple hundred dollars.

    Threaded style like so:

    ahona_tanks.jpg

    Or neck and flange type like I used could be installed in the top of a storage tank:

    reverse_indirect.jpg

    I thought you had some high temperature loads, but it looks like you're feeding all lower temperature radiant loads. As I see it the big advantage of a buffer/hydraulic separator tank is that it makes it easy to do a good job of minimizing return temperature to storage. With low temperature loads fed by mixing valves the problem of minimizing return temperature to storage pretty much goes away. Therefore I believe your heating loads could all pull directly from storage, and then you don't need the buffer/hydraulic separator at all. Storage becomes your reverse indirect and your hydraulic separator.

    And your backup boiler can also connect to storage directly, same as the wood boiler. You would only heat up enough of storage to supply heat to the DHW coil. I can testify to the fact that the hot water stays a the top of the tank quite well.
    Last edited: Dec 12, 2013

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