How many gpm for DHW from your wood boiler/storage tank

Beno Posted By Beno, Dec 16, 2007 at 5:52 AM

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  1. Beno

    Beno
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    Feb 26, 2007
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    At peak time I'll need to have a flow of 6.5 gpm of DHW, mainly for showers and sinks.
    From this site: http://www.stsscoinc.com/Products_HeatExchangerCoil.aspx I get the following:
    1. STSS copper heat exchangers are designed and engineered to transfer heat efficiently at low temperatures.
    2. Domestic Hot Water Output Ratings:
    Part No. 425 Temperature rise from 47 Deg. F. to 105 Deg F. @ 2 GPM.

    Will I be able to get 6.5 gpm from a wood boiler/storage tank ?

    Thanks,
    Beno
     
  2. TCaldwell

    TCaldwell
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    beno, it looks like they reduced the flow to 2 gpm to raise the 47 deg water to 105deg running through the coil in storage of 120deg. if you increase the flow through the coil you will extract less than the 58deg temp rise. they reccomend a longer coil in order to increase the flow rate to the 5/6gpm range for your desired dwh temp. you could with a shorter coil if stored water was higher than 120deg, bet if you called them with your perameters they would wind a coil for you.
     
  3. Beno

    Beno
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    Is there a formula to get the length of the copper coil I need for DHW for this flow ? One 180 ft hx is about $1000 ? What is your personal experience with the gpm of DHW from your tank?
     
  4. TCaldwell

    TCaldwell
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    beno, look on line there are many many companys that wind coils for aftermarket applications with technical support that will advise . I found a coil for a different application through this method, there is a formula that most have software and plug in your parimeters $
     
  5. Beno

    Beno
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    I checked again the STSS web site and found this:

    "part# 427: Temperature rise from 47 Deg. F. to 105 Deg. F. @ 3 GPM. One Coil. 180 Feet.
    Average tank temperature 120 Deg. F. Transfer rate 86,965 BTU’s per hour. "

    So, I guess that if I put in serial 2 coils of 180 ft each I'll get the 6 gpm for DHW.
    Also, regarding the tank shape. Can I choose an STSS 806 gal tank with diameter of 5.3 ft and height of 6 ft ? It fits better my utility room.
    Thanks,
    Beno
     
  6. Nofossil

    Nofossil
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    For the money that you're looking at, I'd consider mounting a cheap DHW tank (electric, maybe) on a stand so that it's above or at least mostly above the storage tank. I'd plumb it to thermosiphon with a very small/cheap HX. It might only extract a few thousand BTU/hr, but the DHW tank would cover your surge demand. I might even connect the electric element as a backup. It wouldn't come on in normal operation, but it would be nice to have hot water whether or not you've built a fire recently.
     
  7. Beno

    Beno
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    Thanks, nofossil, I've looked on your web site and noticed that your house size is similar to our next house. You heat with baseboard at 160F. Is the EKO giving enough hot water? Or you preheat the water with solar in order to achieve this temperature? Also, an unrelated question. In order to minimize the costs and simplify the hydronics I'd like to use also hot water baseboards, and have also a VC cat stove. Is there a limit to the size of a zone/circuit? Can I have one zone per each 1800 sq.ft. floor? If you have one zone, can you turn off individual baseboards w/o affecting the others?
    If I'd go with radiant floor heating I'll need (probably) 9 zones per floor (also I might've made a mistake with the calculation). Thanks again!
     
  8. Nofossil

    Nofossil
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    As I mentioned on the site, I made the mistake of calculating exactly how much baseboard I'd need to make up heat loss at -30. Those calculations were based on 170 degree water, and worked out to 52' of baseboard. It's exactly the right amount to maintain temperature, but way too little to change the temperature, and not enough to work well at anything less than 170. The EKO provides more than enough hot water, but the baseboards cann accept only 30,0000 BTU/hr.

    Solar is used only during the summer, and as such, only to heat DHW. The sun doesn't shine in the winter, and my panels are under 20" of snow at the moment.

    We have one zone per floor. I don't see why you couldn't do the same with radiant. I think a radiant zone usually has a manifold with several loops plumbed in parallel. If a baseboard loop is too long, the baseboards on the return end will see colder water. You could oversize them a bit to compensate, or run a couple of parallel paths in the same zone. Joe might chime in on the details - I haven't done it that way. In my setup (serial baseboards) you can close shutters that reduce the airflow through the baseboards, but you can't shut them off.
     
  9. BrownianHeatingTech

    BrownianHeatingTech
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    Individual radiant loops should be kept under 300 feet, for hydraulic reasons and to avoid cold spots.

    So yeah, most systems will have a number of loops, in parallel. Most decent manifolds have actuators available - zone valves for each loop. You can put actuators on all the loops, then wire loops 1, 2, and 3 together, to act as one zone. Loop 4 is going to be its own zone. Loops 5 and 6 will be tied together. Or whatever you want. You can even re-wire later to change which loops are connected together, and to which thermostat.

    Joe
     
  10. Beno

    Beno
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    I estimated a 25 BTU/hr heat loss for a very well insulated house, the total heat loss will be 3600 x 25 = 90000BTU/hr.
    Considering also the DHW I consider Tarm Solo 40 or EKO 40.
    For 160F hot water, with 450 BTU/hr/lin.ft of baseboard, I'll need a total of 200 ft of baseboard or 100 ft of baseboard per floor.
    Is the limitation "Individual radiant loops should be kept under 300 feet" valid also for hot water baseboards? If this is correct for baseboards, I might need 2 zones per floor for baseboards.
     
  11. BrownianHeatingTech

    BrownianHeatingTech
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    No, that's due to the flow and temperature drop characteristics of radiant.

    Baseboard will depend upon the amount of piping, fittings, and such, as well as the size pump that is being used. Slower water will make less noise, so it's better to split it up into more zones that can deliver heat without upping the water speed. More zones also results in better comfort.

    Joe
     
  12. atlarge54

    atlarge54
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  13. atlarge54

    atlarge54
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    I seem to hav made a mistake using the quote feature. The last five sentences in previous post aren't Joes. Sorry
     
  14. BrownianHeatingTech

    BrownianHeatingTech
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    3/4" is sometimes used in things like greenhouses, where they can get away with wider tube spacing use less footage of tube (and yes, use longer loops). The wider spacing reduces comfort, as there will be cold spots in places, and it isn't quite as efficient. But it's cheaper to install, and the plants don't tend to care.

    3/4" gives you up to 500 feet per loop.

    Joe
     
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