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Overpumping through boiler

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by BoilerMan, Jan 3, 2013.

  1. BoilerMan

    BoilerMan Minister of Fire

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    In the thread about variable speed and fixed speed circulators overpumping through the boiler was mentioned. I understand that it will run a burner type boiler (gas, oil) more efficiently by having longer burner on time. The question is is it harmful or make any difference in a solid fuel boiler? I overpumped on purpose, thinking inherrant return protection. The boiler loop is the primary loop with 1-1/4" copper about 30' total and a 0010. The secondary loops dump low temp water into the fast-flowing high them for return protection. I monitor the return temp and use it as an imput to my injection mixing controller for the radiant slab. Most of the time I have a 10-12 degree delta due to this large flow of water through the primary loop and smaller flow from the slab which is usually in the range of 120F.

    It has been working well and return temps are always high, but would slowing the flow through the boiler help anything?
    Thoughts?

    TS

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  2. JP11

    JP11 Minister of Fire

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    My setup is very similar. 30 foot run of 1.5 inch copper. 0011 pump.

    Rated boiler output is about a 15 to 17 degree delta t.

    many times i'm around 10 or 11.

    especially now I'm burning softwood.

    seems to run fine.

    JP
  3. TheMightyMoe

    TheMightyMoe Minister of Fire

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    The colder the primary return the better because heat will transfer more efficiently.
  4. BoilerMan

    BoilerMan Minister of Fire

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    I understand how this would lead to higher heat transfer, but is there anything else that it would effect? My stack temps have never been over 300F and that seems to indicate good heat extraction from the flue gasses.

    TS
  5. TheMightyMoe

    TheMightyMoe Minister of Fire

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  6. ewdudley

    ewdudley Minister of Fire

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    The boiler is a heat exchanger. Its heat transfer rate increases as cold stream inlet temperature decreases. If the rate of hot gas flow is constant and the inlet temperature is raised, then the rate of heat transfer decreases and more heat goes out the flue.

    You can compensate by reducing the firing rate to bring stack temperature down to some optimum stack temperature, which is what I do since my system design requires fairly high return temperature. The end result is that my nominal 150000 btu per hour boiler has a net output of about 70000 btu per hour when flue temperature is maintained at 320 degF.

    This restores the efficiency of the system by sacrificing capacity.

    TS: Since your stack temperatures are low and hopefully you're not supplying too much excess combustion air, then you should in great shape.
  7. BoilerMan

    BoilerMan Minister of Fire

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    How can I tell if I have too much excess air? I've dialed the fan back to 70% with about a 10F drop in stack temp, less than 70% and things drop fast. Generally the high 280s seem to be the sweet spot for my wood (16% average MC Beech).

    TS
  8. ewdudley

    ewdudley Minister of Fire

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    For me it's more like 320 degF, if I go too low then like you say it gets feeble and unstable depending on the boiler.

    To do it right need an exhaust gas analyzer. Or just eyeball it. I set primary and secondary to factory and waited until I had a good steady fire, then decreased secondary air very slightly, waited a couple minutes, studied the flame, and repeatedly decreased a little more until I had a reducing flame. Then leaned it out a little and called it good. Had to make a temporary lower door out of foil faced mineral wool with a little window in it in order to see the darn flame.
  9. Bob Rohr

    Bob Rohr Minister of Fire

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    Consider all the requirements. Low flow, high delta t are a good way to operate the boiler. Most manufacturers will give you a flow rate or acceptable delta T across the HX in their installation manuals, 30- 40° is a common range for non condensing boilers, cast, steel, copper tube.

    But you need to consider the acceptable low return temperature to the boiler, with wood, maybe as low as 125° with super dry wood.

    Then you need a distribution system that can return those low temperatures and still meet the load.

    The Germans, Viessmann and Buderus have built some fossil fueled cast iron boilers that they claim can handle direct 120, even 90° return temperatures on a continuous basis. Run that boiler to a high limit of 180 and now you leverage a 60°- 90° delta T. Although the Euro boilers generally limit at 75°C 167F, they see no reason to run boilers above that as all their heat emitters run at low temperatures, radiant and panel rads for the most part.

    If efficiency is the goals you are chasing, always start with the structure, lower the load! next, in my book, would be low temperature distribution, large radiant surfaces on floors, ceilings, and walls, or panel rads. I like skin temperature, noise free heat distribution.

    Don't forget thermal solar. My 5 collector array kicked on just before 9:00 AM today, 20° outside, it will supply 100- 115° all day long today. I'll let the sun handle the combustion today.
  10. PassionForFire&Water

    PassionForFire&Water Minister of Fire

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    Good advise, ... as always.
  11. BoilerMan

    BoilerMan Minister of Fire

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    Attack's manual says 149F for min return water. Thats pretty high, but considering the low flue temps I get with 165+ return temps I'm not concerned. I know the condensation temp in the HX are much lower then that especially as the burn progresses. I do have the ability to return 90 degree water to the boiler as I'm all radiant slab and CI rads for the bedrooms on the second floor.
    When I design and built the system, I set it up to have some type of fossil fueled backup boiler, oil was the cheapest then so I bought the Toyotomi in sig, but never piped it into the heating system and use it solely for hot water in the summer, burn around 50 gal a summer with that. Now LP has come way down in price due to competition in the suppliers (there was only one back then) I'd go with a modcon wall hung and use my indirect in the summer.
    As it is I only have wood as the heat source, and enjoy the work involved and the freedom that storage and super-insulated houses have. Thanks for the input guys.

    BTW Bob, I've toyed with the idea of solar as another heat source as well, coupled with the slab and connected to the second HX coil in my large indirect. I have been leaning toward flat plates though as I've read they have a long proven track record and are less prone to glycol boil in the summer. I did run two 3/4" insulated copper pipes fom the boiler room into the attic when I built the place, with the idea of solar someday. Direct water heating with drainback, or glycol with the other HX in indirect and flat plate HX for radiant. Any thoughts?

    TS
  12. Bob Rohr

    Bob Rohr Minister of Fire

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    I like the drainback systems, especially if there are low load conditions possible. With DB basic water is all you need, just slope all the piping tht is in areas prone to freezing. Cove the DHW load first, usually 50- 65 gallons is plenty for most homes, then add to the heating load.

    If you like wood heat, as a RE, you will really like solar. Now wood to cut, split, and haul. No ash to clean up, etc. Sun shines, energy flows into the system.

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