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Need input for a gravity feed hot water heat system

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Danny D, Nov 24, 2007.

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  1. Danny D

    Danny D New Member

    Joined:
    Nov 24, 2007
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    Loc:
    Louisville Ky
    Hello all, This is my first post here. I am planning on building an OWB and want to use a gravity feed to circulate the water. My OWB design will be based on a return tube boiler and opperate at atmosphere pressure. The shell is a stainless tank 24" X 72" laying horizonal with several flues running between the heads. The firebox is under one end of the tank( the front). The gasses will travel back under the tank up the rear head, thru the flues, out the front head and up a stack. This will give me a hot fire on the outside of the water tank with a lot of heating surface. I can locate the OWB about 25' from the house and about 15' feet below the main floor. I have plenty of 1 1/4" copper pipe to run for lines.
    Can I expect to circulate the water with no pump using radiators for heat exchange? The hope is to have heat with no power.

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  2. Eric Johnson

    Eric Johnson Mod Emeritus

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    Central NYS
    Welcome to the Boiler Room.

    Sounds to me like it would work, at least to some extent. The big trick, it seems to me, will be making sure that the water circulates fast enough to dissipate enough heat from the boiler to keep it from overheating. You might also have problems controlling the amount of heat going into your zones, depending on how that's set up.

    My first wood-fired boiler was a Marathon Logwood 24 that I used to replace an old coal/oil conversion dating from around 1910. The house had lots of cast iron radiators and large diameter iron pipe for headers. I believe the rads were piped with 1" or 1.25-inch iron. Anyway, it worked like a dream. All we had to do was build a fire (no blower or pumps of any kind on the boiler at all) and let convection take care of the rest. I controlled the heat going into the system with a couple of big ball valves on the (two) main supply lines feeding the house. Cold days, you open them up pretty far. When it's warmer, you throttle them back.

    I sold the house to my boss. He replaced the boiler (sprung a leak at 12 years) with an identical pressure vessel, and continues to heat the house the same way.

    It's nice if you can manage it. If you get enough dry wood, you'll get through the winter no matter what.
  3. Danny D

    Danny D New Member

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    Nov 24, 2007
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    Loc:
    Louisville Ky
    Some questions I have are; how much rise should I maintain to get good flow? I know the steeper the better, but what would a minimum pich be? Will cast iron raidators work with this type of system. Will I have any electrolisis problems using stainless, brass, copper, steel and cast iron in the plumbing system? How can you figure the BTU output of a cast iron raidator? How are the BTU requirements figured for rooms with 8' celings, good insulation and so-so windows? Is anyone using a gravity feed system of there own? Thanks for any help, Danny
  4. Nofossil

    Nofossil Moderator Emeritus

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    That's a bunch of questions. I con't help much, but here goes:

    The greater the height difference between the heat source and the cold sink, the better it will work. I don't think that slope matters much, except that you have to think like a bubble and make sure that there's an uphill path to a vent. There's no induced flow to flush bubbles out, and they can cause big problems.

    Cast Iron radiators should work, but you might have to work out how to get flow to start. If there's no temperature difference between the top and bottom of your heat source and none between the top and bottom of your heat sink, then there's no reason for flow to start in one direction vs. the other. Most cast iron radiators are like baseboards - the inlet and outlet are at the same height. Maybe someone with actual experience could chime in on this one ;-)

    I've gotten by with brass as an intermediary between iron and copper, but they do make dielectric unions that probably would be a better idea.

    BTU calculations are complex, in part because you want enough capacity to change the temperature in the room as well as making up for the steady-state heat loss. Do you have any history on oil or gas usage?
  5. Eric Johnson

    Eric Johnson Mod Emeritus

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    You can download a heat loss calculator from http://www.heatinghelp.com that should help you determine your house's needs. I've never done it, but it's supposed to be the best way to size a boiler system.

    My understanding of cast iron rads vs. baseboard radiant is limited to "the more water in the system and the bigger the pipe, the better it will convect." Put another way, I don't think you'll have luck getting anything other than cast iron radiators to work in a gravity feed system. And direction of flow shouldn't matter with a gravity system, although the water is going to squirt out the supply pipe on the boiler, so it will tend to flow the normal way, i.e. out the supply and back into the return. If there's no pump, I wouldn't put a flow control on a gravity feed line. As to heat differential, once you get the boiler cranked up, hot water will flow out and cooler water will flow back in, assuming it has the ability to do so.

    On venting, my Rule of Thumb is to put a vent any place where the flow of water heads down. If you want to play it safe, vent all lines that head down, regardless of the direction of flow.
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