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Oversized Wood Boiler Solutions

mikeg2015 Member Posts: 1,187
I have a customer that installed a Central Boiler Classic Edge 750. In reality he only has connected load for a small model like the 550, and the heat loss in design condition I estimate is still less than that.

Any other solutions? Guessing you can’t downfire these boilers at all. WHat is the storage strategy for these? Right now it’s off for so long that the fire get cold and has to be manually lit again.

Not sure what dealer sold him this boiler and why someone thought that bigger was better. Total peak connected load will be (house expansion still under construction) around 120k I estimate in design conditions. The boiler output is 240k. I could add a sidearm to heat a radiant floor system (connected to a propane fired combi boiler right now) and add about 25k BTU of load.


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,966
    You're going to need lots of storage.
    Probably all the info you need is here:
  • hot_rod
    hot_rod Member Posts: 18,190
    Not uncommon, the sales pitch is the larger size has more water content/ storage.

    I suspect that 120K is a design day, so 80% of the years it will be lower, sometimes much lower.

    It may take 1000 gallon of storage to make that behave the way you expect. Math is in the journal @STEVEusaPA linked.

    Wood burning is a pretty interactive hobby, you cannot just load and walk away. I try to burn mine to the load of the day, small fires on low load days.

    Mine is an indoor type, small firebox compared to those smoke dragons :) 120K size, my load is around 40K on design day. I have 500 gallon storage. Not quite enough to get thru the evening, but is is enough to help lessen cycling.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • GroundUp
    GroundUp Member Posts: 1,495
    Keep in mind the 240k rating of the 750 Edge is max output, not continuous output. If the design load happens to be 120k, he's only got enough firebox for about 17 hours with perfect, dry hardwood. That is the correct boiler in the long run, and additional storage atop the existing 330 gallons would be no more than an extremely expensive band-aid that still requires lighting a new fire every day to batch burn. Why is he burning gas to heat the radiant while fighting the OWB simultaneously? That should have been connected from the start. Have him tighten the differential on his FireStar to 5 degrees until the expansion is complete and let it ride- then tie the rest in at that point. Odds are his loading technique needs some help as well, and he is bridging the nozzle which is never allowing the necessary coal bed to build. What species and moisture content wood is being burned? Splits or rounds? These things are picky
  • mikeg2015
    mikeg2015 Member Posts: 1,187
    They are avoiding adding a heat exchanger For the radiant. They don’t want to buy 200 gallons of glycol to mix 50/50 so boiler is just water.

    I agree they should put have put in one propane backup boiler and installed a indirect and hydronic air handlers instead of furnaces.

    So instead it’s 2 combis, 2 furnaces, 1 coil and a lot of excess wood boiler capacity.

    I think his issue is setup of the air adjustments so it keeps the coal bed hot between cycles. He shouldn’t be having to relight it manually between loads. I agree he’s probably loading it wrong and using too much green or otherwise wet wood.

    I spent almost an hour reading through the manual and it talks about all of this. Message boards clearly
    Show it’s a learning curve, but once you have it dialed in it’s a great boiler.

    Thanks for the info!

    Good news, my boss at least agreed to allowing me to use a aquastat to automatically select the heat source so there only 1 thermostat on to furnace. Since it has a hydraulic separator it will also bypass the coil automatically if the temp drops.
  • GroundUp
    GroundUp Member Posts: 1,495
    Why would they need to buy 200 gallons of antifreeze? That's the purpose of the heat exchanger
  • EYoder
    EYoder Member Posts: 60
    I rarely use antifreeze in the Heatmaster units I install. Water treatment, but not antifreeze.
    As was mentioned BTU rating on an outdoor boiler is typically a max btu rate, not an 8 hour burn rate. They are designed to cycle on and off, with an occasional spike in output if needed. We don't use storage beyond the 1-200 gallons in the boiler.
    If the heating load of the building is 120,000 BTUs it's not oversized. He just needs to know how to load it in milder weather. If settings aren't correct he'll have trouble. The dealer should be helping with this.
  • leonz
    leonz Member Posts: 682
    Fill it half full of full dimension fire brick and you will
    have the benefit of a huge heat sink that will save you
    firewood and labor and increase the burn temperature
    to burn cleaner as well. The other benefit is that it will
    release heat back into the boiler slowly as well.
    Larry WeingartenGroundUp
  • GroundUp
    GroundUp Member Posts: 1,495
    leonz said:

    Fill it half full of full dimension fire brick and you will
    have the benefit of a huge heat sink that will save you
    firewood and labor and increase the burn temperature
    to burn cleaner as well. The other benefit is that it will
    release heat back into the boiler slowly as well.

    So by that logic, why wouldn't he fill it 90% full of firebrick and really save wood/labor and really increase the burn temp? Sorry, but it doesn't work that way. Adding firebrick is like adding more water- it does nothing for fuel consumption.

    Brent H.
  • leonz
    leonz Member Posts: 682
    edited February 2021
    Well I can and will tell you I cut my wood and coal use by half after installing the full firebrick in the firebox.

    I purchased a scrap 2 inch by 12 inch piece of channel iron and laid it down on the shaker grate mounting frame and then stacked the fire brick on top of it and continued to stack it right up to the flue breech filling it with 3 cubic feet of firebrick.

    I was still able to shake the coal grates and my draft improved quite a bit and the boiler was always hot enough that there was little wood smoke due to the increased firebox temperature from the firebrick.

    I cut my wood to 12 inch lengths one inch thick and the firewood dried quickly and burned with little smoke.

    You could fill it up with more firebrick but the wood would have to be cut even shorter and you would have to load the boiler or furnace more.

    The volume left in front of the hand fed boiler allowed me to fill it with more than a cubic foot of anthracite nut coal when I need to switch to coal for heating fuel and my wood and coal fires lasted all night after I banked the fires with wood or coal and yes it does work as I did it for 5 years before I switched to using a coal stoker boiler.

    Many or most solar heated homes have large windows facing the west or near west with clear sightlines and they use huge amounts of stone floor tiles laid on thick beds of concrete to soak up the heat from the sun and after the sun passes over the home to the east it radiates the stored heat in the stone tile and concrete back into the home.

    Thermal mass is thermal mass it does not matter if its water, stone, sand or concrete its in how it is used to the most benefit so it does work that way.
  • Jamie Hall
    Jamie Hall Member Posts: 20,416
    Just remember, folks, that a BTU is a BTU. If you have a smaller firebox, two things happen: first, the maximum BTU output will be less. Which, if your boiler is oversized to begin with, is just fine. Second, to maintain a given fire for a given time (burning y pounds of coal per hour), if you can load less, you will need to load more often.

    To store heat -- to even out the output, perhaps -- is a very good thing. But... you have to get the heat input in the first place. Physics is nasty that way.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • psb75
    psb75 Member Posts: 634
    Firebrick lining of the firebox holds the heat longer and would keep more live coals for quicker start-ups after reloading.
  • leonz
    leonz Member Posts: 682
    All true Jamie, this is one more reason for lining the floor of my coal stokers firebox with full firebrick this coming summer to soak up more heat too as the base of the coal stoker is not insulated.
  • GroundUp
    GroundUp Member Posts: 1,495
    We aren't talking about a coal stoker or shaker boiler, this is a 750 Edge gasification wood boiler. Adding firebrick is as useless as adding water. Neither will have any positive effect.