Outdoor wood boiler wood requirements

[rt014]

I would guess a minimum of 10 full cords/year. I've been averaging 8-10, but I'm not always there and thus keep the thermostat low most of the time.

 

[Larry Caldwell]

yes, but efficency is measured under ideal conditions. How close do the 2 types of heat match ideal conditions. a wood boiler can be designed to match it fairly well if it has a thermal resivior, but most wood boilers have very inefficient burn chambers. The other inherent innefficency with a wood boiler is that they are usually outdoor boilers, so there is heat lost to the environment before it even gets to the house. In general wood boilers are sold on their capacity, not efficency.
A problem with putting the heat into pex coils in the floor instead of a baseboard or forced air heater is that the heat exchanger is is surounded by concrete in the floor. heat radiates around. there is more insulation going down, but there is also the concrete insulating the coils from allowing the heat to go up.

If you have an efficient indoor boiler that stores the heat (allows the boiler to blaze at peak efficiency then slowly dispense the heat) inside, then use baseboards or forced air, you would have a much more efficient system.

I looked at the specs. The boiler he chose is a flame tube boiler with recirc and reburn. It should be very efficient. Yes, el cheapo water jacket boilers never let the fire get hot enough for complete combustion, but a flame tube boiler is as good as it gets.

The concrete is not insulation, it is thermal mass. Once you get the concrete hot, it takes a long time to cool off, and concrete is massive, thermally conductive, and cheap. My friend who built a solar hydronic system in Northern Idaho used 240,000 lbs of concrete (73 cubic yards) for thermal mass. The internal temperature of the house did not vary a degree under any circumstance. He used a big diesel truck radiator in the garage wall to dispose of excess heat.