Harman sf-260 wood/coal boiler

First time coal burner and have some questions
How do you keep a coal fire burning without over heating the water. My boiler reaches it's set point of 180 deg. the automatic draft closes and the temperature continues to rise to around 195 to 200 deg. When my house t-stat calls for heat the temp in the boiler starts to drop but only 4 to 6 deg. Then the process starts all over, the water temp. starts to rise again. The automatic draft does not get a chance to do it's job robbing the fire of o2.
Please help I have been at this for two weeks and am not having any luck

I'd drop the target water temp down to 160-170*. See if this works. It is hard to slow down a coal fire once it is going strong, just cutting back on the air takes at least 30-60 minutes to slow the fire down significantly. This is why stoker boilers work better, they control the amount of coal burning as well as the air to the coal.

Usually droping the target water temp will help with the overshoot and still provide hot enough water to keep your house warm in average temperatures. If it gets below zero, you probably will have to adjust the water temp back up to it's current setting.

Let us know how it works.

greg L

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Thank you greg, I will give a try first dropping the set point to 170 deg. I should have also mentioned that I am heating a domestic coil. I assume I have to be careful here not to lose to much heat. Another thing I have tried is, setting the manual draft controls to almost nothing. (two manual drafts on the loading door 1/2 turn open, on the automatic draft control there is a manual draft with 2 holes about 3/4 in dia. these are almost closed.) My goal is to cause the fire to die enough to drop the water temp so the auto draft can open and allow the fire to get hot again. This seems to be working or is this the wrong way of going about it Thanks again

Usually any air entering above the fire through loading door vents is wasted on a coal fire.. The coal burns with the air that enters from below the fire, through ashpan door vents or your automatic vents. Personally I'd close the vents in the loading door so they are providing very little if any air... They are really there for burning wood or Bituminous coal. You must have a good chimney to still have temp overshoots and a good fire with the loading door vents open any at all.

One thing I did not mention, do you have a barometic damper in your chimney flue pipe?? If not I would install one. You want to have the baro so that if it gets really windy or cold or both that the increased draft in the chimney won't pull extra air through the coal and cause temp overshoots. Set the baro at about .05 WC or whatever the owners manual says.

You should have plenty of domestic hot water with 160-170* water. Setting back the temp is just an attempt to increase margin between target and boiling.

Hope this helps, Greg L

I do have a baro damper but not a draft guage so any settings is just a guess. I tried to find a draft guage but my dealer does not have them. (His answer was, that is a tool installers use and I would not need one.) Any suggestions on where I could buy one. Thanks again.

Grainger sells them, but with your install my personal opinion is that a rough setting is close enough. What you want to avoid is a scenario where you have the air set 'just right' at say 9pm, the wind is calm, temp say 25*, but at around 2-3AM a cold front come through, the wind picks up and the temps drop, the draft in the chimney can double from say .05-.06 to .10-.12, This will cause a significant increase in air through the stove and probably cause an overshoot.

With a Baro installed, even if it is way off, say at .08 then in the above scenario it still will 'save the day', and break the high draft and help keep the fire even.

The settings on the Baro sliding weight are usually fairly close, +/- .01-.02 wc. So as long as you can see the flapper moving when a strong gust of wind pulls on the chimney, then I think you will be alright.. The settings are more critical with some stoker stoves.

Greg L

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The settings on the Baro sliding weight are usually fairly close, +/- .01-.02 wc. So as long as you can see the flapper moving when a strong gust of wind pulls on the chimney, then I think you will be alright.. The settings are more critical with some stoker stoves.

Greg is right on- this is how I guesstimated my stove until Santa ordered me a gauge from Grainger's this fall, and I wasn't too terrible far off, though I'm more confident of my setup now that I have a reference to check.

The idea I gather is to break an over-powerful draft and minimize the forge effect of unwanted air being inhaled uncontrolled through the fire- if you are ever able to see a fire when it does that, it's something

My baro damper does not have a sliding adjustment, just a screw with a weight on the other end, there for just guessing at the settings . I now have the setpoint set at 165* and the baro damper opening about 3/4 inch at a stack temp of around 250*. This seems to be working, but to soon to tell. I will keep you posted.

Other than reducing size, what is the purpose of a firebox reducer. With little heat demand (2 yr old house, outside temps in the mid 30"s) do I really need such a large bed of coal 16"x24"x6"