The Hearth

  in the house as a system

© 1997 - 2013 John Gulland

This document was first published in 1997 as The Fireplace in the House as a System. In preparing this online version the title and content were updated to use the more general term hearth, which would also include wood stoves.

About the title:   The hearth, or fireplace, is literally the place within a house where a fire can be burned for heat and enjoyment. The enjoyment part mostly comes from viewing the fire as it burns. This simple definition, then, would include fireplaces and modern wood stoves, almost all of which now include glass doors. No functional distinction will be made here between fireplaces and space heating stoves, so the system design and performance issues covered in the following pages apply equally to both. This book deals mainly with wood burning hearths, although many of the concepts apply equally to any vertically vented, natural draft combustion appliance.


Preface, Acknowledgements and Introduction

The building envelope

Temperature difference

The cold hearth syndrome

The effects of wind

The effects of powered exhausts

The combustion air supply

Venting system design influences

Appliance design influences

Energy momentum

The human factor in wood burning

Spillage from open fireplaces

The spillage test

Effective make-up air systems

Some final thoughts

Defining perfection

System design characteristics

Summary of lessons learned

Summary of definitions

Appendix A: Measuring duct flows

Appendix B: Sources and resources


Snow hangs in the Spruce trees outside my third floor office window. Down on the main floor is the wood heater responsible for keeping this eight year old country home and office warm and comfortable, even when the wind howls and the temperature falls to minus thirty. The smell of wood smoke is not permitted in this house and the stove complies. It also delivers about seventy percent efficiency, keeping our wood consumption down to a reasonable three full cords each year. And it is beautiful in the bargain. I can't complain.

I don't mention this just to brag about the hearth, but to point out that perfection is possible. And even better, it is predictable — I knew before the house was built that the fireplace would work perfectly. Twenty years in the hearth industry and exposure to the best housing researchers and their findings has taught me that when the design is right, the wood heater will work. At the time this house was built, some of the research was still going on; new ideas were just emerging and barely digested. Since then the theories have been confirmed through field testing and experience.

Starting fifty years ago or more, North American housing evolved to move the hearth from the central area — the heart of the home — to an outside wall and even put the back of the fireplace outside the wall in a chase. So began an unhappy chapter in the life story of fireplaces and stoves. To provide a straightforward explanation of the pervasive problems caused by outside chimneys (and fireplaces in outside chases) is one good reason why this book was needed.

Technological change created the need for this book. Over the past twenty years house construction has changed in response to buyer demands for greater comfort and lower energy costs. Sealed doors and windows, more insulation and near-continuous air barriers have made houses far more air tight and easier to heat than in the past. During the same period a remarkable transformation of woodburning technology took place. The amount of smoke emitted by a wood heater has been cut by up to ninety per cent while efficiencies have almost doubled. Meanwhile, all the old woodburning fireplace and technologies remain, and that, as you will find in this book, is one of the problems.

This book is not about fire safety or the aesthetics of fireplaces. It deals with a neglected subject: how to make sure your hearth gives comfort and satisfaction by working properly, not smoking and not annoying. A fireplace or stove that misbehaves can be an endless source of frustration and embarrassment, but a great one gives continuous pleasure. Read on and you will find out how to achieve perfection in matters of the hearth.

Whether your reason for picking up this book is personal or professional, I hope you find what you seek. And I wish you the warmth of a natural hearth.



The author wishes to acknowledge the contribution of dozens of people in the hearth industry who reviewed and commented on various drafts that led to this book and who spent many hours discussing and debating the technical issues involved. A note of appreciation is due to the staff of the research division of Canada Mortgage and Housing Corporation who contributed generously during the preparation of an early version and whose work helped to reveal how chimneys function in the house as a system. Some of the illustrations that appear here were adapted from the Wood Energy Technical Training reference manual published by Wood Energy Technology Transfer Inc., and from A Guide to Residential Wood Heating published by Natural Resources Canada and Canada Mortgage and Housing Corporation. Permission to use these illustrations is gratefully acknowledged.

January 1997


People love fireplaces and stoves. Whenever potential home buyers are asked what features they would most like to have in their new home, a fireplace always figures prominently on their wish list. This is surprising, really, since fireplaces are so often a source of disappointment and annoyance to householders. The fireplaces in new homes disappoint by putting out less heat than expected; they annoy by spilling exhaust gases into the room when operating and spilling cold air and foul odors when they're not. Frustration with badly designed woodburning fireplaces could be one reason for the growing dominance of gas fireplaces in the hearth products market. Wood stoves installed using the same old and flawed ideas are just as frustrating.

The other thing about fireplace and stove failure that breeds frustration is that everybody is an expert, but nobody seems to have the answers. Every bricklayer who has built a fireplace and every chimney sweep who has cleaned a hearth system have strong opinions as to what makes one successful. And strong views are not limited to the professionals; just about anyone who has ever built a fire is happy to share their pet theory of fireplace function. If opinions counted for anything, all natural hearths would work perfectly.

The housing research of the 1970s and 1980s yielded the principle of the house as a system which suggests that the house functions as a system rather than as a number of unrelated parts and that its various subsystems, particularly those that move or contain air, behave in an interactive way. What a concept! It has become a corner stone of residential building science and it provided the enthusiasm to support further research. The house as a system principle forces us to look a the consequences of the equipment, material and installation decisions made in the process of house design and construction. It means, for example, that we must acknowledge when the specifications for a new house call for a woodburning hearth and a downdraft kitchen range exhaust, that steps must be taken to ensure that these two devices will function in harmony. It also recognizes that wind, temperature and other environmental conditions influence the performance of the house and its components and so should be considered.

Laboratory and field research conducted in the 1980s on combustion venting, most notably by and for Canada Mortgage and Housing Corporation, began to shed light on how fireplaces and wood stoves interact with the house at a systems level. This research, combined with the insights of North America's most experienced and thoughtful hearth and housing specialists, produced a rich mixture of ideas that eventually gelled into the contents of this book. This is what the hearth business has lacked all this time: an integrated theory of appliance and chimney function supported by scientific fact.

Hearth products, the broad term that includes the appliances, venting systems and accessories that are combined to provide the visible fire in the home, are now available in bewildering variety. Aside from the fuel they burn, hearth appliances also vary in the way exhaust gases are vented to outdoors and these differences are the most significant for the behavior of the hearth in the house environment. To help in the analysis and discussion of the myriad types of hearth systems, here are five classifications based on venting strategy.

  • Chimney Vented. This class includes woodburning fireplaces, wood stoves and furnaces, and is the primary emphasis in this book.
  • Vertically Vented Gas. Included here are atmospheric gas and propane fireplaces, inserts and stoves with dilution device, usually a draft hood. Several of the principles covered in this book apply to this class.
  • Forced Draft, Low Temperature Vent. Pellet stoves and fireplace inserts that may be vented vertically or horizontally (under favorable conditions) through pellet vent are in this class. A separate section of the book is devoted to pellet venting.
  • Sealed Combustion. This includes direct vent gas and propane fireplaces and stoves with concentric exhaust and combustion air supply. It also includes fireplace inserts with non-concentric exhaust and air supply, both routed vertically through the original fireplace chimney. Direct vent appliances are, at least in theory, immune to many of the influences that cause chimney venting to fail. They are the right alternative for people who do not highly value the natural wood fire, but wish to incorporate a hearth in the home. Because the air supply and exhaust of these fireplaces flow independently of the house in which they are installed, this class of appliance is not covered in this book.
  • Unvented. Unvented, or so-called vent-free, fireplaces and heaters are not discussed at all here. The reason is simple: their operational characteristics are incompatible with the house as a system principle. The idea of releasing the exhaust products from combustion equipment into the living space is a bad one. Unvented fireplaces pollute the indoor air; on that point there can be no argument, although their makers say it is not enough to be concerned about. Nevertheless, anyone even slightly concerned about the quality of their indoor air should avoid unvented gas appliances.

If it is to be a valuable addition to a home, a fireplace or stove should operate easily, make heat and never smoke or smell. The householder should never have to worry that it might act up. Fireplace perfection is possible and you will find the formula as you read on.

The purpose of this book is to support homeowners, builders, architects and hearth specialists who want to make sure that their fireplaces and stoves will function perfectly in the homes being built today.