Wood Boiler Radiant Floor Heating

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floorheatingSome outdoor wood boilers are either multi-fuel systems (i.e.    they can burn wood and gas or oil) or they have a built-in heat exchanger     coil to supply domestic hot water. With this style of boiler the separate storage/back-up     tank is not needed and the radiant floor can be run directly from the boiler.

The following schematic applies to the above mentioned types of     outdoor wood boilers. Just remember to bury the supply and return lines from your boiler below the frost line. Here's why...

Normally, the wood boiler is plumbed to a heat exchanger (see     drawing below). As you can see, this allows the boiler to heat a tank of potable     water, which in turn can provide domestic hot water AND floor heating (in an     "open" or "closed" configuration).

The water from the boiler to this heat exchanger flows 24 hours     a day in a closed loop, making the heat exchanger "continuously active"     (i.e. always hot). Whenever needed, the storage tank draws heat from the heat     exchanger and maintains a constant tank temperature. The advantages of a continuously     active heat exchanger loop are twofold:

1) the pipe from the wood boiler to the house can be buried in     a shallow trench (normally about 1 ft.), saving a lot of labor and/or expensive     excavation costs (obviously, with constantly circulating hot water in the supply     and return lines, freezing is impossible, even in a trench well above the frost     line), and

2) by keeping the water in the boiler constantly circulating,  stratification is eliminated. In other words, without constant flow through the boiler, the water at the top of the water jacket gets VERY hot, and the water at the bottom stays much cooler. And since most boilers have water jackets containing several hundred gallons of water, 50% of the water in the boiler  could be 185 degrees (the temperature at which the boiler damper shuts off the  air supply and sends the boiler into resting mode) and the other 50% could be  considerably cooler.

This means, in essence, that a boiler sized to provide X number of BTU''s of heating capacity is now providing considerably less than it''s design rating. Because when one of the heating zones calls for heat, a circulator pump comes on, water again flows through the boiler stirring the hotter and cooler water together, and suddenly 185 degree water becomes 145 degree water. This can really matter in a marginally sized system.

So, the point is, if you want to run a radiant system directly  off your wood boiler, always bury your supply and return pipes below the frost     line. As explained above, the water to and from your house will only be flowing  when a radiant zone calls for heat. And because many outdoor wood boilers are 30 to 100 feet away from the home, a lot of water can be sitting in a cold (though admittedly insulated) trench for a long time. If that trench is above the frost line, you could have serious trouble.

boilermulti-1Multi-fuel wood boiler, or boiler with built-in  heat exchanger for domestic hot water. The lines to and from the boiler must  be buried below the frost line.

Learn More at Radiant Company

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