3.  Advanced Combustion Wood Fireplaces

Advanced combustion technology was developed as part of a major effort to reduce emissions from wood stoves in order to meet environmental regulations in both Canada and the United States. The principal design elements of the wood stove technology are now being incorporated into new fireplaces, producing an attractive, safe, efficient and cost-effective alternative to conventional fireplaces. This new technology is also available in a form which can be installed into an existing fireplace, converting it into an excellent performer.

Design features

In these new designs, the secret of success is the use of a sophisticated secondary combustion process that ensures more complete combustion of the wood, thereby increasing the fireplace's operating efficiency and reducing the amount of incomplete combustion products that escape the firebox.

Advanced combustion fireplaces use two separate paths of combustion air, both of which are preheated. One source of air is fed directly to the burning wood, while the second is aimed immediately above the main fire to capture and ignite the incomplete combustion products that would otherwise be released into the chimney. The result is two simultaneous combustion zones and flame patterns – a visual effect that is entrancing.

Figure 2: Advanced combustion fireplace

Advanced combustion fireplaces have truly airtight, gasketed doors, a special ceramic (pyro- or neo-) glass window that allows most of the infrared heat from the flame to be transmitted to the room, and a hot air "sweeping of the window to allow clear viewing. Because these fireplaces are often installed with their back against an exterior wall, they should also have some type of insulated outer casing that prevents unwanted heat loss from escaping.

What makes them better performers?

Aside from their superior performance, these advanced combustion units differ from conventional fireplaces in that they have better heat exchange properties. Room air is drawn in through a grille under the firebox, from where as quirrel cage fan sends it through a heat exchanger and back into the room through grilles at the top of the fireplace. With some units, the heat can also be ducted to adjacent rooms or into the basement, where an auxiliary fan can send it through ducts to other parts of the house.

As a result of their enhanced combustion features, the air requirements for these new fireplaces are low. Advanced combustion fireplaces require as little as 7 L/s (14 cfm) of air from the house in order to operate properly. This small amount of air is about one tenth of the ventilation required for a typical house. Such an air requirement for advanced combustion fireplaces is about the same as that required by high-efficiency gas or oil furnaces.

However, even with this low air-exchange rate, it is a good idea to supply the fireplaces with direct outside air for combustion (this is mandatory in some provinces).

Advanced combustion fireplaces have little or no interaction with the house air, so the chances of combustion products spilling into the home are minimal. In addition, they greatly enhance the safety and effectiveness of supplying outdoor air directly to the fireplace. The maze that is used to preheat this air before it is released to the firebox is so complex that it is difficult for the combustion gases to use this route as an exhaust.

Energy efficiency

Advanced combustion fireplaces can offer high levels of energy efficiency – approximately 50 to 70%, and even higher when they are installed in a major living area with an open view to other parts of the house (this allows the homeowner to lower the average temperature in the house). In such an installation, these fireplaces can reduce overall energy demand and heating bills in a home. In particular, dramatic cost savings and enhanced comfort are possible for many homes currently heated by electric baseboards.

Table 1
Typical Heating System Efficiencies and Energy Savings

Energy Source	Technology	Seasonal Efficiency (AFUE) %	Energy Savings % of Base**
Wood	Central furnace	45-55
	Conventional stove (properly located)	55-70
	"High-tech" stove (properly located)	70-80
	Advanced combustion fireplace	50-70
Oil	Cast-iron head burner (old furnace)	60	Base
	Flame-retention head replacement burner	70-78	14-23
	High-static replacement burner	74-82	19-27
	New standard furnace	78-86	23-30
	Mid-efficiency furnace	83-89	28-33
	Condensing furnace	85-95	29-37
	Integrated space/tap water mid-efficiency	83-89	28-33 space 40-44 water
Natural Gas	Conventional	60	Base
	Vent damper with non-continuous pilot light	62-67	3-10
	Mid-efficiency	78-84	23-28
	High-efficiency condensing furnace	89-96	33-38
	Integrated space/tap water condensing	89-96	33-38 space 44-48 water
Electricity	Electric baseboards	100
	Electric furnace or boiler	100
	Air-source heat pump	1.7 COP*
	Earth-energy system (ground-source heat pump)	2.6 COP*
Propane	Conventional	62	Base
	Vent damper with non-continuous pilot light	64-69	3-10
	Mid-efficiency	79-85	21-27
	Condensing	87-94	29-34

* COP = Coefficient of Performance, a measure of the heat delivered by a heat pump over the heating season per unit of electricity consumed.

** Base represents the energy consumed by a standard furnace.

Environmental benefits

The environmental benefits of the new technology are also impressive. The use of two combustion zones reduces emissions of incomplete combustion products by ten fold compared to a conventional fireplace. The new technology thus enables the environmentally friendly use of a renewable energy source. The low levels of incomplete combustion products and creosote also virtually eliminate the potential for chimney fires.

An added benefit of these units is that in case of an electrical power failure, they can still operate as emergency heat sources for the house. Even though the electrical circulating fan on the fireplace will not operate, the heat from the fire will radiate into the house along with natural convection from the fireplace, supplying a significant amount of heat to the house.

What to look for?

Advanced combustion fireplaces will usually cost between $1500 and $2500, with installation.

If you are purchasing a new home and want a fireplace added, take the opportunity to choose an advanced combustion unit. If you already own a home with a conventional fireplace, consider a retrofit with the new technology to ensure comfort, energy efficiency and safety.

If retrofitting, look for an advanced combustion unit that is especially designed to be inserted into an existing fireplace. You will likely have to put a liner in your existing chimney to make it compatible with the new technology. Use an approved stainless steel chimney liner to ensure a good draft and prevent condensation of combustion products.

A new installation should use one of the high-temperature chimneys designed specifically for wood-burning appliances. These metal chimneys are designed to withstand continuous gas temperatures of 650^o C, which is higher compared to chimneys intended for other fuels. This chimney type has a thicker wall, more insulation and a higher grade inner liner than other chimneys.

It is worth noting that only these types of advanced combustion fireplaces may be installed in Canada's super energy-efficient R-2000 Homes, since the low emissions level is generally a sign of high operating efficiency and low air requirements suitable for operation in airtight houses.

What does the future hold?

The design and manufacture of energy-efficient wood-burning appliances, including fireplaces, is an ongoing process. Work is continuing to create integrated wood heating systems that are energy-efficient, environmentally friendly and safe to operate in today's homes. As well, the federal government plans to develop seasonal efficiency standards for wood fireplaces. Over time, the introduction of regulations and standards for fireplaces could make the use of advanced combustion fireplaces mandatory.

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