In last month's article, I promised that we would look into the intricacies of the safety controls that are common to solid fuel-fired appliances. As with any appliance, the manufacturer's installation manuals must be followed to insure that the installation is in complete compliance with the manufacturer's requirements. This is not an area that should be a shortcut due to the potential loss of life and limb.
Probably the most obvious dangerous situation is the flue breaching. Remember, you are dealing with a solid fuel appliance that has a completely different set of standards as it pertains to the flue gas breaching system. In some cases, if you have a good, solid masonry chimney, all that may be required is a good initial inspection to make certain that none of the chimney has been compromised or clogged due to collapse or animals' nests.
Again, I would recommend that you defer to the specific appliance manufacturer's installation literature as it pertains to applying the product to a solid masonry chimney. Just because it is made of brick does not necessarily mean that it is compatible with the product application. If you are installing a used appliance, and the installation manuals are not readily available, it would be my recommendation that you defer to the National Fire Protection Association manual NFPA 211. This is the gold standard for constructing flue gas elimination systems for solid fuel appliances.
If you are using a manufactured chimney assembly, you must follow the manufacturer's instructions as it pertains to clearance to combustibles and vent termination clearances on or near the roof.
It is also recommended that the breaching be inspected every three months, and that it be thoroughly cleaned at least annually. With the advent of the gasifying wood boiler, the possibility of having a flue breaching fire is significantly lessened, but it remains a potential nonetheless. If you have never been exposed to a chimney fire, consider yourself lucky.
I had a fire once in a solid brick chimney, and you are lucky if the building survives. The local volunteer fire department basically stood by and kept the roof area surrounding the chimney cool in an effort to keep from burning down my house. The incoming combustion air to the appliance was completely shut down, but the fire seemed to be drawing air down the chimney, and it sounded like a freight train going through my house. It is not an experience anyone should have to endure, and with proper care and maintenance, it can and should be avoided.
Another major consideration is what happens to the appliance if you are in the process of a full burn and your dwelling is hit by a sudden loss of electrical power? With a conventionally fueled appliance, the loss of power immediately snuffs the flame, avoiding any over run or run away fire considerations. Such is not true with solid fuel fired appliances. Although with the gasifying boilers, a loss of power stops the induction blower from operating, there is still going to be a considerable amount of energy in the burning fuel to be dissipated.
Some appliance manufacturers have covered this potential in the inherent design, by surrounding the combustion chamber with a substantial amount of water to dissipate the heat and avoid a catastrophic steam event, but many of the new class of boilers do not incorporate a significant amount of water in their design. It is important to have a plan to dissipate this heat in order to keep things from going haywire.
Many manufacturers have an emergency heat dump method for a system hit with a power failure during a major hot burn event. The air supply is shut down and normally open DC-powered valves are opened up to a create a circuit where the residual combustion chamber heat can be dissipated through an emergency heat dump.
The installation that we performed depended upon the use of copper fin-tube convector to dissipate the residual energy in the combustion chamber of the appliance we installed. The manufacturer requires the use of a normally opened valve to insure proper and safe operation during emergency power loss conditions. It is also important to follow the manufacturer's instructions on the piping of the emergency dump loop, paying particular attention to upward pitch to insure gravity movement of the water. It is also equally important that the piping details be followed to insure a safe path for water to circulate by gravity between the heat source and the heat dissipation loop.
One area that should be obvious, but is still worth mentioning nonetheless, is pressurized versus non-pressurized vessel considerations. Most wood boiler manufacturers require that their systems be unpressurized to avoid the possibility of creating a high temperature (water temperatures above 212°F) condition that could lead to an explosive situation. If the appliance is rated to be pressurized, follow the manufacturer's instructions for pressure relief and expansion compensation.
So, there you have it in a nutshell. Wood biomass is a viable and green alternative to fossil fuel-fired systems. Obviously, fuel availability is a major consideration, and it is not for everyone. For the people who are willing to expend the labor necessary for gathering, harvesting and maintenance, this new class of appliances holds a significant potential as it pertains to offsetting carbon footprints from fossil fuels and also has the ability to significantly reduce cash outlay to keep people and places warm.
Tune in next month as we revisit the intricacies of interfacing alternative energy (solar, ground-source heat pump systems) into conventional hydronic heating systems. Until then, Happy Extreme Green Hydronicing!
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.