The Hearth in the House as a System

Some final thoughts

Why do chimney vented stoves and fireplaces spill and backdraft and what is needed to make them function successfully? That seems like such a simple question, but it is not. If you find this subject area complicated, difficult and sometimes confusing, you are not alone. After all, if the question were simple and the answer obvious, solutions would have been found long ago. Although venting failure has been a problem for many years, much of the formal research and analysis needed to reveal the dynamics of chimney venting in houses has been done only in the past three decades. We have just recently begun to understand all the contributing factors and their relative importance.

Incremental changes and the interplay between influences

Chimney venting design and diagnosis is challenging because it is all about increments. That is, when a venting system is in operation or even at standby, an array of influences are simultaneously at work: chimney height, stack temperature, appliance design, house tightness, wind effects, outdoor temperature and many other factors contribute to the net effect. The interplay between these influences — some driving and some adverse to successful venting — determines the result. An incremental change to one of the influencing factors can change the result. The dilemma that we often face is this: Which factor should be changed and by how much to achieve the desired result? Obviously, there is no simple solution that would apply in every case. It is clear, however, that stove/fireplace and chimney systems of good design provide a larger margin of resistance to venting failure and are less affected by external factors such as wind or room depressurization than are systems with design flaws.

What about the exceptions?

This book offers a number of design guidelines that can help to ensure successful venting. In most cases, they are just that, guidelines, not rigid rules, and there will be cases that appear to contradict them. For example, a key suggestion in this manual is that chimneys should be installed within the building envelope, yet there may be systems that seem to function fine when vented through an outside chimney. Such apparent contradictions do not disprove the principle of inside chimneys any more than the old-timer's stove pipe clearance of 6 inches that has been there for 30 years invalidates the 18 inch chimney connector clearance rule in safety codes. We know that at some point the old-timer's flue pipe is likely to ignite the wall, just as we now know that at some point the outside chimney, because it weakens the system's resistance to spillage, is likely to be a factor in venting failure. Professionals and regulatory authorities decided a long time ago not to take chances with safety. Now that we have reliable guidelines for chimney venting of fireplaces, we must decide how rigorously to apply them. The prevention of venting failure requires a new approach, one in which the lessons learned about the causes of venting failure and how to avoid it are applied with more clarity and determination than in the past.

Generalizing about modern housing

The trend towards more tightly sealed housing might lead one to believe that houses will become ever more airtight until there are no uncontrolled leaks. However, as more is learned about the cause and effect relationships involved, it becomes apparent that there are diminishing returns as one approaches total airtightness. Depending on the construction methods and materials used, a point is reached at which the additional costs to achieve tightness are not justified by any significant reduction in energy costs or increases in comfort. For those of us interested in wood heating, the issue revolves around the point at which a house becomes too tight for a hearth appliance to draw its combustion air from inside the envelope.

Now, with 30 years of experience with tightly sealed houses, thousands of which have been airtightness tested with accurate devices like blower doors, some trends have emerged and some general statements can be made, as follows:

  • Except in rare cases, even tightly sealed houses have sufficient natural leakage to supply combustion air to controlled combustion woodburning appliances.
  • When a builder uses materials and techniques designed to reduce air leakage, the resulting house can approach the degree of tightness at which the pressure inside is significantly affected by air flows produced by large exhaust devices.
  • Because of its greater surface area and greater potential for leaks, a large house of complex design normally has a greater equivalent leakage area than a small house of simple design when similar materials and construction techniques are used. Therefore, a large exhaust device will cause a greater level of depressurization in a small house than in a large house.
  • In general, energy efficient houses can accommodate wood burning appliances with modest combustion air requirements (supplied from inside the envelope) without requiring a make-up air system, provided large exhaust ventilators are not installed.
  • Because energy efficient houses can be so tightly sealed that the pressure inside can be affected by exhaust systems, the only way to be sure how depressurized they can get is to test them using the simplified house pressure test.


Summary Tools

By applying the lessons learned in this book, you will have more success in specifying or installing a system that does not fail. And, if necessary, you will be able to diagnose problems in existing systems more quickly and accurately. Use the material on the following pages as reminders of the system design strategies presented throughout the book.

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