Trying to keep it simple

Can you make a simple hand drawn diagram of your existing system and scan/post the diagram?

coal boiler addon would be ideal, maybe a catalytic wood boiler, if it exists

In your diagram, try to show how the heat emitters are piped to your system, and each zone scheme (both radiant and baseboard). Not sure how you are teeing into only one part of the main, or if the pump location described is best for steady system pressure and air elimination. Heat load calculation and radiant piping considerations like tube spacing and allowable floor temperature are critical for a project like this.

Assuming you have a domestic coil for your hot water. The absolute simplest would be to tee the supply from you wood boiler in just after your air scoop. Return from the wood just before the boiler on the return side. Put an aquastat on the wood boiler to a 2 pole no/nc relay, when the wood reaches 145-150 the relay energizes, one side opens tt to the oil burner, the other side closes and starts the circ from the wood boiler to the supply side of the heating system. When the wood cools down the relay de energizes shuts off the circ and allows the oil burner to operate. All the time this keeps your boiler hot so your domestic coil will still supply hot water. Hope this helps

You want to shut down the oil burner, because when any zone calls for heat it will try to make 180 degree water. If your wood boiler is only supplying 160 or 170 ( which is plenty to provide heat especially to your radient loops). So you will burn oil unneccessarily.

I assume you have a coil in your boiler that provides your hot water, so you want the wood boiler water to circulate through your boiler too to keep it hot enough to make hot water.

If you search parallel and series hook up you may find some info on this topic. Wood still has a value to it. If you plumb in parallel it is more efficient system. There is no reason to keep the oil boiler hot all winter. The plumbing is not that much different. All you need to add is an aqua-stat placed on the wood boiler or on the supply piping after the wood boiler. The aqua-stat will turn on the oil boiler when the water temp drops below a set temp like 130F for example. This way the oil is completely off and is not full of hot water and loosing heat.
Rob

My tarm is connected to my gas boiler with the same type no/ nc relay. It is VERY simple and very effective. At about 4 am on a cold nite the gas boiler kicks in after the wood boiler gets below 140. It is also very important to implement some type of overheat protection system. I use an aquastat on the wood boiler. It simply opens all of the house zones when the boiler gets above 200. This only happens on warmer days if I over fire the boiler. After a few yrs of experience loading it rarely happens. If my boiler reaches 200 with a full load of wood, and the zones are all satisfied it will reach the boiling point in less than 10 minuets, making lost of noise and spewing steam through all of the relief valves. I have a total of 4 pressure relief valves on my system just in case one fails. Over heating is less of a problem if you use storage. I also have one separate zone that runs around the basement ceiling connected to a normally closed zone valve that will open if the power is out and release any heat through gravity flow. that zone should be at least 15 percent of your boiler max output. (I think). The parts do start to add up, but it is better to be safe than sorry. Forgot to mention my boiler is in the basement and is a closed system.
Mike

Is it common for gassifier temps to run higher than they should? The electronic controls aren’t designed to keep temps in line? I understand if you lose power the fire can runaway but under normal running conditions the boilers are prone to “Run Away”?

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I've never had the experience described by the prior poster on boiling water, pressure relief valves blowing, etc. On the Tarm the firebox draft fan shuts down between about 180-190F water temperature (adjustable) and then turns the draft fan back on when temperature falls. I have never had a "run away" fire. I have had a few occasions when the boiler was still heavily fueled and I had little or no load. On those occasions the draft fan shut down at 190F (that's where I have mine set), the fire dies back through lack of air, but the fire die down time still left enough fire to cause water temperature to continue to rise above 190F, the overheat valve opened to add a load to the boiler, fire died back, water temperature fell, the draft fan came back on, and normal operation continued. The highest water temperature I have seen on my Tarm is 205F.

At 30 psi, the pressure relief valve on my Tarm, water won't boil until it reaches 250F, and it is 240F boiling point at 25 psi. As mentioned, the highest I have seen is 205F: no boiling, no pressure relief valve venting, no problem.

Having an overheat zone is important though, because I could imagine overheating to the point of the pressure relief valve releasing if a person had a full fire going at the time the draft fan shut down and there was no heating load on the boiler. The overheat zone simply adds the necessary load to prevent any overheating of the boiler until the fire dies down and boiler water temperature falls.

I forgot to mention my tarm is not a gasser, it is an older unit, natural draft. The gassers do have much more control over the fire, but they can still overheat. As jebatty said, his boiler did get hot enough to cause water temperature to continue to rise above 190F, the overheat valve opened to add a load to the boiler, fire died back, water temperature fell, the draft fan came back on, and normal operation continued. Had he not had the overheat valve set up the way he did he may have overheated. Overheat protection is very important for any wood or coal fired boiler.

the learning curve can be pretty steep when it comes to making your own heat and energy. I have been heating with wood for over 20 yrs. the past 8 or so I have used wood heat almost 24/7 in the winter. You can ask almost any question here and get a pretty educated answer. There are many educated heating engineers associated with this site. You can also search the board to get answers to the many questions already asked. Occasionally there are disagreements and heated debates on the methods and such. ( thats the good part, the drama, this forum should have it's own reality show.)! One other thing I would say about burning wood is that seasoned wood is really the key to getting the most out of your wood boiler. No matter what any manufacturer says about how their boiler can burn green wood, it is a deception. You will use up most of the btu's in the wood driving out he moisture and burn more wood, and money, (water don't burn). Unfortunately, you can rarely buy seasoned wood. You need to buy it or collect it one yr in advance to dry it properly. I am still learning and can never seem to get enough wood together dry enough for the next season. I burn about 10 full cord of wood a yr. Mostly scavenged from side of the road or gotten from a tree guy or 2.
Mike

You may want to consider some sort of control that monitors the water temperature from the wood boiler. If you run out of wood and have the pump running 24/7, your oil boiler will be heating the water in the wood boiler. I have a pellet boiler connected to my LP boiler. I have the temperature set lower on the LP boiler and if the water temperature drops down to that temperature in the pellet boiler, I shut off the pump until the pellet boiler raises it's temperature back above that temperature.