Hello - "New" Owner of Tarm 502

Welcome to the Forum, someone should be along soon with some knowledge about your system, have you tried finding, locating the previous owner?

I would contact Tarm directly

Maybe I can help. I have, since 1996, had a Tarm 502 heating our house. I also have coal baffles for my unit and have burned coal.

I think the instructions for using the coal baffles call for removing just the last (rear most) sectiion of the grate. I did that for a number of years and can tell you its a PIA. I found that if I just removed the shoulder screw that connects the back grate to the shaker mechanism I could use the coal baffles without removing that section of grate. The 502 is by design a cross draft burner, but coal requires a strong updraft through the mass of burning coal. The coal baffles sorta convert the 502 to an updraft burner. The coal baffles are comprised of three pieces: a small steel weldment which is about the width of the grate and eight (or so) inches long, and 2 cast iron pieces that are ~16 inches long and 10 inches wide. The steel weldment is placed over the rear most section of grate and extends back to the sloping steel piece that directs hot combustion gases up into the fire tubes, blocking the flow of combustion gases into the fire tubes. The two cast iron pieces sit on top of the steel weldment. and about 2 inches forward of the back wall of the combustion chamber. With the coal baffles in place combustion air and gases are forced to travel up through the grates and up through the fuel, over top of the cast iron pieces and down through the space behind the cast iron baffles then into the fire tubes. You can also burn wood successfully with the coal baffles in place, but not as efficiently in cross draft mode.

I switch to coal on the really cold winter days because a load of good anthracite would last 12 hours whereas I could only get 4 - 6 hours out of a full charge of wood. To get a coal fire going I first build a wood fire, then slowly add small amounts of coal. Once a coal fire was going I could burn coal continuously for two or three days. The 502's grates are not optimized for handling coal and the larger chunks of rock and clinkers would eventually fill up the space above the grates to the point where I had to let the fire die then shovel them out by hand. I quit burning coal a couple of years ago because I was no longer able to get decent anthracite in my area. I also added heat storage, so now I can go much longer without the oil burner coming on. I have the manufacturer's instructions for using the coal baffles that I could email to you if that would be helpful. Let me know.

Somewhere in your system you should have an expansion tank that functions to keep the boiler pressure relatively constant as the water in the system expands with temperature. In my system the boiler pressure is about 15 psi at 140 degrees and rises to 20 psi at 200 degrees. My expansion tank has the general shape similar to a propane tank that you use with a barbique grille. You might need to check that your expansion tank is functioning properly.

A functional Sampson draft regulator is essential. I have mine set such that the draft door closes at about 200 degrees.

Regarding the Honeywell triple acquastat: Low Limit sets the temperature at which the oil burner kicks in when there is no heat demand (i.e. no thermostat is demand heat and no zone valve is allowing flow). Tarm recommends setting the low limit to 140 degrees during heating season and 100 - 120 during the summer. High Limit is the temperature at which the oil burner shuts off when there is heat demand. [High Limit - Diff] is the temperature at which the oil burner comes on when there is heat demand. I keep my High Limit around 150 degrees because I only want the oil burner to kick in when the fire goes out.

Do you have the Tarm 502 Owners Manual? If no, I believe you can find it as a PDF online.

You should also get a flue temperature thermometer. By monitoring the temperature of the flue gasses you get valuable feedback about how efficiently the boiler is operating. Westwood makes a nice one for about $40

SRT4nier said:

TimFromPitt - Thanks for your reply and information. Sorry for my delayed response (holidays etc..). I cleaned out the fire box which allowed the coal baffles to sit in the weldment better. I've had some pretty good success with the coal so far, building a large wood fire first and trusting the system to not over heat. I've been able to maintain the coal fire for about 2 days without it dying out, but we just had a stretch of warm weather which led to 77 F temps in the house!

I lowered the low limit to 140 and high limit to 160, kept the over heat at 215. This set up has been great! Oil burner only turns on when the fire goes out.

I do have an expansion tank, barometric damper and flue thermostat installed already and all seem to be operating fine. I have not seen flue temps above 300-350, not sure what that means. Also not sure if coal produces cresote or just wood. I'll look more into that. I adjusted the damper to draw more air from the boiler because the coal fires were not burning very hot, barely 180 F.

I find that if I shake the grates three times a day and shovel out the ash pan, I can load the box pretty full and continuously burn.

Click to expand...

I saw flue temperatures in the 300 degree range also when burning coal, but the boiler could easily keep up with heavy demand even at those low flue temperatures. I think when burning coal more heat is transferred from the firebox into the surrounding water jacket than with wood. I could not burn coal on warm days as soon as the primary air was cut back the fire would die.

Coal does not produce creosote but does produce a lot of fine ash that accumulates in the flue pipe. I noticed that after several days of burning coal the inside of the firebox would be nearly completely free of creosote.

My only complaint with my Tarm 502 is not with woodburning but that when the oil burner comes on it is annoyingtly noisy.