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Overheating control on a National 300 Boiler, solid fuel

I installed a National 300 boiler to supply my house (all radiant floor + heat exchanger in oil furnace plenum) and 2 garages both with water to air exchangers (modine). The boiler works great provides lots of hot water, some times too much to handle, (especially when the wife has to fix the fire). The closed system boiler is in the garage and is separated from the house system with a shell and tube heat exchanger.

I have Aquastats set to 180 on the blowers, a few times this still wasn't enough to get the water calmed down. What is the best way to get this under control?

I am considering some type of storage tank which could also serve as a fail safe during a power outage by transferring the heat through thermo-siphoning. Or possibly just plumbing a pump to discharge the overheated water outside, allowing cooler water (which I would want tempered with some of the discharging hot water).

I already burst the underground 1 1/4" pex lines once and since, switched it over to copper tubing.

What do you guys think is the best solution, and the best way to plumb? Thanks.


  • Jamie Hall
    Jamie Hall Member Posts: 20,416
    Apologies for sounding stupid, but how is this puppy normally controlled? Does it have fans for draught, or is it controlled by dampers, or... both? And are they powered or manual?

    Back in the bad old days of solid fuel boilers -- which were mostly for steam -- there was a variety of interesting mechanical contrivances which controlled the dampers in relationship to pressure. Nowadays, there are a number of wood stoves which control the dampers in relation to temperature, using rather beefy bi-metal controls.

    But... how is yours controlled? In principle, you can control the energy output of the fire by closing the input dampers (do not close an output damper -- you may get smoke where you don't want it). If you have forced draught fans, you might be able to come up with a way to slow them down -- or if you have more than one, turn one off completely -- on rising temperature. I can't recommend turning down or off an induced draught fan, however.

    You mention an aquastat on the blowers. Is this then controlling the forced draught fans?

    Whatever you do along those lines, though, be aware that your efficiency will drop and your emissions may rise.

    You should also be aware that any change in draught will take some time to affect the heat output of the fire (either way -- up or down).

    It seems rather a shame to actually dump heat, so in addition to the draught controls your concept of a really big storage tank, heavily insulated, has merit. Thinking somewhat off the top of my head, I think I might pipe that as part of a primary loop off the National, and draw water from it (and return it to it) with a secondary loop feeding your heat exchangers.

    Unless that tank is arranged for gravity flow to the National -- that is, set well above it -- I doubt that you would get enough thermosiphoning if you had no power for the pump.

    Hopefully there is someone on here with some real hands-on experience, though!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • icesailor
    icesailor Member Posts: 7,265
    Do I understand you correctly that your aqustats on the blowers are set to 180 degrees? Why? That means that until the water temperature gets to 180 degrees +/-, the fans won't run. If you are using the fan units as a heat dump, you aren't. You should have the aquastats set at a much lower setting. If you blew a piece of PEX in the ground, it was probably because the water temperature was too hot.

    Heat emitters are like the radiator in your car. And without airflow from the fan when you are sitting in traffic, the engine/cooling system will overheat. A lot of Aquastats are set at 100 degrees. Its just so they don't blow cold air.
  • Zman
    Zman Member Posts: 7,433
    Low mass systems and wood boilers are a bad combo. I think you are heading in the right direction. A storage tank and some restraint on the stoking (good luck with that, she's cold) would go a long way.
    Have you read this? http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_10.pdf

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein