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Using anti-freeze for the opposite reason

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by slowzuki, Dec 17, 2007.

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  1. slowzuki

    slowzuki Feeling the Heat

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    I'm going to put anti-freeze in my system to try to avoid freezeup but the elevated boiling point is the other big benefit it looks like I'd get 50-60 F additional degrees to charge my water tank with. That means I can use a smaller heat exchanger and run my boiler loop hotter.

    Anyone else design their storage or HX around this idea?

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  2. Nofossil

    Nofossil Moderator Emeritus

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    Interesting idea. I've actually been involved in another discussion about lowering the water temp to reduce heat that's lost up the chimney. Since my boiler at least is not a counterflow design, the flue gas will be hotter if the boiler water jacket outlet is hotter. I'm running about 250 stack temp and 185 at the top of the water jacket. I think if I increased the water jacket temp, the flue would increase by about the same amount.

    Make any sense?

    If I could run the water backwards (top to bottom) in the boiler, then the flue temp could be lower than the water outlet temp. Maybe more experiments to be done. Any thoughts?
  3. Gooserider

    Gooserider Mod Emeritus

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    You have more expertise on it than I do, but I would think the flue temp would only go up if you weren't extracting as much heat out of the flue gasses as possible. You are probably doing pretty well, so yes the flue temp might go up, but would that loss of heat transfer be OK if getting the water jacket hotter allowed you to charge the storage tank to a higher temperature?

    As to the water flow direction question, the standard way to maximize transfer is to use counterflow plumbing where the coldest water enters at the point of least heat in the firebox, and exits at the point of maximum heat. This should maximize the average temperature differential along the entire water flow path. Would reversing the flow direction through the boiler do this? You know it's anatomy far better than I do...

    Gooserider
  4. Nofossil

    Nofossil Moderator Emeritus

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    In the EKO, hot gas enters the HX tubes at the bottom, and exits at the top. The water flow on the other side of the tubes is also bottom-to-top, so the coldest water is in contact with the hottest gas and vice versa.

    If you reversed the flow of water through the boiler, it would be just as you say - the coldest water coming in at the top of the HX tubes, and the hot water exiting at the bottom.

    This might cause all sorts of other problems. Does anyone know of reasons why this would be a bad idea? One that I can think of is that natural convection won't work in the water jacket, so you might get isolated zones or loops of convection that never mix with the reverse flow, causing hot spots.
  5. slowzuki

    slowzuki Feeling the Heat

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    Raising the jacket temp will loose some efficiency unless I pop in a few feet of heat exchanger at the outlet to compensate. I could probably get enough to heat a slab directly, or preheat return water from the slab.
  6. Eric Johnson

    Eric Johnson Mod Emeritus

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    If you've got pex somewhere on your supply and return lines, reversing them to see what happens would be pretty easy, if you opened and/or removed any flow controls.

    I'm still pondering the implications. Remember that the temp probe is on the top of the pressure vessel. What would result from the greater temp swings it's likely to see if you reversed the flow? Would the blower kick on and off a lot more frequently, as cooler water was returned to the top of the boiler instead of the bottom? Would the boiler be able to store more energy, since the water would all presumably be about the same temp? What would be the practical consequences of that?

    My guess is that the boiler would get up to 80 throughout, resulting in no drop in the exhaust gas temp. So you wouldn't gain anything, other than being able to stuff a few more btus into the pressure vessel.

    As I think you alluded to, nofossil, the boiler ain't designed to be run that way.

    You could always plumb a Heat Saver into the chimney.
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