Forty years ago the phrase “the solution to pollution is dilution” was seen as a reliable method (since proven wrong) of dealing with local air pollution issues. This phrase has been reborn as a number of states, local municipalities and rural communities are in the process of considering minimum stack height requirements for outdoor wood burners (OWB’s). All parties involved should become very aware of the liabilities and consequences of such regulation.

Safety Issues:

1. Requiring a stack height of X feet above any structure within a 50’ to 200’ radius creates a very real liability issue. OWB’s should be utilized only in rural settings; thus a number of OWB’s will be located on farms where silos, barns, or other tall structures are present. Thus a vertical stack could exceed 60’ in height. At such a height, condensation within the stack will occur in the upper portions of the stack. Over time creosote will collect and the potential for a chimney fire will become significant. A chimney fire would be extremely hazardous and could lead to a stack collapse, thus triggering additional ground or building fires. Exceptions or adjustments for those situations should be addressed in any proposed regulations.


2. A single wall vertical stack serving a wood burning appliance will condense significant quantities of acidic water and creosote, throughout the stack height. The acid will corrode the stack (especially if the stack is not fabricated with stainless steel) and condensed creosote will create perfect conditions for a chimney fire. Any stack serving a wood fired appliance must be fabricated from stainless steel and be insulated to class A standards, and any proposed regulations should include this requirement.


3. Any stack serving a wood fired appliance (OWB, stove, furnace, fireplace, etc) will require periodic cleaning. Cleaning a free standing 40’ (or higher) vertical stack will be costly and/or difficult; thus cleaning will either not be done or done infrequently, leading to a critical condition with creosote loading (and chimney fire potential). Any proposed regulation should address periodic cleaning, maintenance, and inspection.

Structural Issues:

1. A tall stack may require a foundation to frost depth and at least one (maybe even several) set of guy wires (each set composed of 3 wires at 120 degree spacing) with appropriate anchors, to maintain stack stability and resist building code defined wind loads (normally 100 mph in open rural areas). Class A flue manufacturers specify a maximum vertical height which cannot be exceeded. Any proposed regulation would need to provide specifications for guy wires and anchors, as well as inspection procedures for compliance with local and manufacturer requirements. This is NOT a “structure” that should be left to the “home handyman.”


2. During storms a tall stack will act as a lightning rod. Unless properly grounded, any lightning hit could produce dangerous side flashes (injuring or killing people) and could ignite creosote, again creating a chimney fire. Proper grounding generally requires: copper air terminal; 24 to 36 strand copper or aluminum down wire; correctly spaced stand offs and copper clad driven ground rods. Again, any proposed regulation would need to provide specifications and inspection procedures.

Performance Issues:

1. A wood unit that operates well with a 15’ stack may become a bad performer with a 40’ stack. Secondly, the “draw created by the stack” will increase considerably with stack height creating a dangerous overfire condition. Indeed, an excessively tall stack may violate the manufacturers listing (as would an excessively long horizontal flue for a condensing natural gas furnace).


2. Draft inducers can increase the flue gas exit velocity of a stack (common in commercial heating equipment). Higher exit velocities increase ambient air entrainment and dispersion of fine particulates. For instance, a 30’ stack with a draft inducer may yield better dispersion characteristics of fine particulates than a 40’ stack without an inducer.

Several states allow a reasonable stack height to be 15’ to 20’ for OWBs. Existing “on the books building codes” require a stack height of 2’ above anything within a 10’ radius of the stack when a wood burning appliance is located within the building.

Given the issues that tall stacks create in safety, structure, and performance, stack height requirements beyond existing building codes should not be the first solution to address the problem of wood smoke pollution. The only real solution to wood pollution is high efficiency, clean burning equipment such as a GARN® WHS unit.

An Example of a Tall Stack

Pictured below is a tall stack (about 32’) serving a typical OWB. It consists of a class A stainless steel flue slid within a spiral pipe to provide structural support. Note the 2 sets of guy wires and 1 set of rigid supports. Note that the wood smoke is high above the ground, but it continues to travel in a rather tight formation downwind. Furthermore, the plume dropped to the ground within a hundred yards of the stack. Thus the tall stack had no effect on real emission dispersion.

A picture of the original class A stainless steel flue that was inserted within the spiral pipe is shown below.

The picture below is of the inside of the class A flue. Note the condition and build up of deposits; primarily creosote and ash. Even though the class A flue was insulated condensation occurred due to the 32’ stack height. This chimney was 11 months old when replaced.

The picture below is an overview of the entire set up. This stack was significantly taller than anything in the area and still did not function to disperse the smoke as the smoke dropped back down to the ground within a hundred yards.

The real and only solution is clean burning wood equipment.

Compare the above tall stack with a GARN® WHS horizontal flue unit as shown below. The GARN® WHS exhaust stream is white water vapor, not the brown toxic plume shown above.

To download a PDF version of this paper, please click here.