Heating with Wood: Vermont s Renewable Resource Skip Hayden Integrated Energy Systems Better Buildings by Design 2009 Burlington, VT, February 2009
Objectives Understand performance advantages with advanced woodstoves and pellet stoves Appreciate the technology limitations and installation/operating requirements of central wood-fired boilers and furnaces Understand why you should be wary of many outdoor boilers Appreciate air supply, venting, pollutant emissions, IAQ problems, other drawbacks with conventional fireplaces Recognize why advanced combustion fireplaces and inserts can be an effective way to heat with wood See how wood can be an efficient, low emissions (including Greenhouse gases) sustainable heat source, while providing comfort and pleasure
?? What are you looking for???
The airtight woodstoves of the 1970 s and 80 s s were major sources of air pollutants, due to poor combustion!
How Wood Burns Typically there is a flame over some but not all of the wood. Remote from the flame, the volatiles are boiled out of the wood and leave the combustion zone as smoke without getting completely burned, or even ignited, resulting in high emissions and creosote.
Emission regulations for woodstoves forced development of new technologies
Initially, to meet Emission Regulations, woodstove manufacturers utilized oxidation catalysts to clean the exhaust and meet emissions regulations Field trials, including an extensive one in Vermont, showed that catalyst were not truly effective after a relatively short period, due to catalyst decay/poisoning, bypassing, warping, etc..
A better solution, first taken up by Canadian manufacturers and then almost universally across North America, was...
Advanced Combustion Design
Advanced Combustion Efficient Clean- Burning Woodstoves
Intense Primary and Secondary Combustion with primary air wide open
Secondary combustion of the volatiles, with the primary air source almost closed
Woodstove Emissions Particulates, g/h 25 20 15 10 Dirty Woodstove Advanced Combustion Woodstove 5 0
Air Requirements of Woodstoves & Fossil Fuel-Fired Furnaces/Boilers Air Changes/hour 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 Conv Gas/Oil Furnace/Boiler Hi-Effic Gas/Oil Furnace/Boiler Adv Combust Woodstove
Wood Pellets
High Efficiency Wood Pellet Fireplace
Woodstove & Pellet Stove Emissions 25 20 15 10 5 Dirty "Airtight" Woodstove Today's Advanced Combustion Stove Well-Designed Pellet Stove 0
Characteristics of Good Wood Pellet Stove Tested to pseudo EPA 1990 low emissions potential is realized high efficiency due to low excess air (80%+) Wide firing range (modulation 6:1 or better) with good EA control over range Air wash for fire viewing Small diameter flue (3-4 ) Can be side-walled (& DV d) with care
Central Heating with Wood Furnaces & Boilers
Wood Furnace Issues (1) For wood furnaces, combustion systems are not nearly as advanced as for woodstoves Emissions and creosote formation very high, and overall seasonal efficiency is typically low ( often<50%) Duct clearances greater that oil/gas furnace
Wood Furnaces Issues (2) Furnaces cycle in response to thermostats (>2000 times per heating season), while a woodstove keeps going and going. Furnaces, instead of shutting off completely when demand has been satisfied, turn down the combustion air supply and the combustion goes into a stewing mode, with high creosote-type emissions Each time there is a demand change, there is a high pulse of emissions until the combustion becomes stabilized
Wood Boilers Many wood boilers have relatively sophisticated combustion systems They have the potential for fairly low emissions and higher efficiency
Downdraft Combustion
Wood Boiler Issues Boilers cycle in response to aquastats and thermostats (2000-20 000 times per heating season) There are the same or even greater issues of stewing and demand change as for furnaces However, if a large, well-insulated hot water storage tank is used in conjuction with the boiler, the appliance can be run hot for long periods to recharge the tank, and the tank can actually supply the house heat demand, completely decoupling the boiler operation from the thermostat
Wood Boiler Performance Draft Issue For a downdraft combustion system to perform well, it must have stable, good draft. This requires a properly-sized, well-insulated, inside chimney, and with minimal house depressurization An induced draft fan may also be necessary (although there may be deposit/corrosion problems)
Alternative Pellet-Fired Systems for Boilers
New hope for clean, efficient, central wood furnace
What about Outdoor Boilers?
Issues With Outdoor Boilers No global requirement for safety nor performance certification Most use primitive combustion technologies, with very high pollutant emissions and low efficiencies A few are have combustion systems similar to the good wood boilers previously described Because they may sit low in relation to the building they are heating, they may have draft problems due to downwash, or to stack chilling Best coupled to inside hot water storage Casing and distribution losses outside (& inside) the house envelope can lower final efficiencies
Potentially Clean-Burning Outdoor Boiler Design
What about other bio-fuels? High ash wood pellets Other biomass pellets: corn, straw, Biomass-derived liquids: Bio-diesel (mix with No.2) Bio-oils Alcohols...
What about fireplaces?
A conventional woodburning fireplace has twice the pollutant emissions of the old, dirty woodstoves, and 10x the emissions of today s advanced combustion woodstoves
Fireplace & Woodstove g/h Emissions 50 45 40 35 30 25 20 15 10 5 0 Dirty Airtight Woodstove Fireplace Today's Advanced Combustion Woodstove
How much air does a fireplace want? A conventional woodburning fireplace has 20x the air requirements of an advanced combustion woodstove or a high efficiency furnace
Air Requirements of Furnaces & Fireplaces AC/h 1.4 1.2 1 0.8 0.6 0.4 0.2 0 HiEffic Furnace Conv Furnace Conventional Fireplace
A conventional woodburning fireplace is extremely inefficient, burning at extremely high excess air (>1.4 air changes per hour) If all the heat it generated found its way into the home (not likely if it s on an outside wall) as the enclosed cavity can radiate to the outdoors as well), the maximum efficiency would be ~25%. Considering the huge amount of house air that is actually swallowed up by the fireplace, on very cold days the efficiency can actually be less than zero!
Low Maximum Efficiencies of Conventional Fireplaces Maximum Efficiency, % 30 25 20 15 10 5 0 1500% EA Outside Air Room Air On very cold days, efficiency can drop BELOW 0% due to cold outside air!
A conventional woodburning fireplace creates serious indoor air quality problems, under three distinct periods of its operation.
With a roaring fire, the conventional fireplace is a large, powerful exhaust device (400-800 cfm)
IAQ Problems due to Fireplace 1 Start-up - Fireplace spills particulates, hydrocarbons, PAH s, combustion gases until draft well established 2 Fireplace roaring - high draft can cause combustion gas spillage from other appliances 3 Fireplace tail-end smoulder - weak draft can spill toxic carbon monoxide into house
Conventional woodburning fireplaces are incompatible with most North American homes!!
Still, what is the most desired combustion system in North American homes?
A Fireplace!
Fireplace Solution Outside Combustion Air Large size hole/duct needed to handle roaring fire ( 80-120 in 2 ) similar to flue size, with only small pressure drop allowed Under certain draft conditions, air supply tube can become flue with resultant fire hazard
Supplying Conventional Fireplace with Outside Air + Danger!!! + Neutral Pressure Plane WIND + + - - With intake on leeward side of - house, large outside air connection may become a better vent path than the fireplace chimney, with disastrous results
Artificial Firelogs
Fireplace Solutions? Artificial Firelogs lower emissions sig (5 x reduction) constant low burn rate eliminates backdrafting problems no sig heat supply with min. efficiency gains beyond reduction of heated air loss only good for sporadic FP usage for decorative purposes only, to minimize indoor and outdoor environmental impact
Non-Solutions for Fireplaces Fancy grate Tight-fitting glass doors Outside combustion air Artificial firelogs None of the above make fireplace efficiency acceptable and most make zero-to-marginal improvements to IAQ
What to do?
Advanced Combustion Woodburning FIREPLACE
Advanced Combustion Woodburning Fireplace
Fireplace & Woodstove Emissions 50 45 40 35 30 25 20 15 10 5 0 Conventional Fireplace Dirty Woodstove Advanced Combustion Fireplace
Air Requirements of Furnaces & Fireplaces 1.4 1.2 1 0.8 0.6 0.4 0.2 0 Conventional Fireplace Conv Gas/Oil Furnace Hi-Effic Gas/Oil Furnace Adv Combust Woodstove
Efficiencies of Advanced Combustion Fireplace & Stoves vs Conventional Fireplace 80 70 60 50 40 30 20 10 Room Air Outside Air 0 Adv Stove Adv Fireplace Conventional Fireplace
Characteristics of Efficient, Safe, Advanced Wood Fireplace Tested to EPA 1990 Preheated prim & sec air Ceramic glass door Insulated comb. chamber & baffle Air wash for door Good circulating fan Insulated outer casing Extremely attractive fire
What about existing fireplaces?
Advanced Combustion Fireplace Insert
Advanced Combustion Insert for Existing Fireplace
Masonry Heaters
Masonry (Fireplace) Heater Concept common in Europe Large mass with convoluted flue path to extract heat Short periods of high burn to generate heat, which is stored in masonry for slow later release May or may not be clean burning and efficient, depending on system - simple combustion air flow changes nearly doubled efficiency
Masonry Heater
Characteristics of Good Masonry Heater Should follow MHA guidelines for design and installation Installed inside house envelope, usually in centre with most/all sides exposed Dampered outside air supply Operates at high burn for relatively short period, potentially with good excess air, storing heat for slow release into house
However, with evening burn, uncontrolled release of heat into house overnight may counter potential savings of thermostat cutback
What combustion systems offer the greatest potential for renewable energy in new and existing homes? The Advanced Combustion woodburning appliances of today!
If harvesting is done properly, forests can be self-sustaining. Wood heating can be considered a truly renewable resource and greenhouse gas-neutral!
For an advanced combustion woodstove or fireplace to be truly efficient & effective, it must be properly sized & located!
Location Location Location
If not properly sized & located, efficient woodstoves & fireplaces can Be ineffective in supplying heat to house Result in significant local overheating Cause sig. client dissatisfaction
Advanced Combustion woodstoves & fireplaces are primarily a localized space heaters, releasing significant amounts of heat by radiation
To be effective, and to prevent overheating, the woodstove or fireplace should be properly sized, and located in a major living area which sees other parts of the house
Good Fireplace Layout
Where to Install: On inside wall, or On outside wall, but inside house envelope, or If unavoidable, on outside wall, but having significant insulation on fireplace outer casing
Fireplace/Furnace: Integrated Gas Fireplace with Zoned Air Ducting
On-Going Advances in Fireplace Technology Inc. input modulation with good EA Minimal-to-zero pilot energy Minimized air requirement Powered exhaust, no dilution (sealed or not) Ducted systems with modulation/zoning
Potential for Next Generation Fireplace
As houses are getting lower and lower heat demands, people will become increasingly unwilling to pay $$$ for a high-efficiency furnace. Yet they will demand to have a fireplace. As it s going to be there in any case, why not have the next generation fireplace satisfy all the house energy demands, efficiently?
Condensing Fireplace-Based Integrated Space-Water Ventilating System
Next Generation Integrated Condensing Fireplace High Efficiency > 90% Local space heating - Flame radiant Central heating - fan coil or hydronic baseboard/radiant floor Tap water heating Second burner behind for summer operation Ventilation with heat recovery Pleasure of a flame
Summary Advanced Combustion Woodstoves and Fireplaces ( EPA 1990) have the potential to be clean-burning and efficient The above should be properly sized and located Pellet stoves which have been tested for emissions (<3gph) and efficiency (>78%), offer more automation Masonry heaters built according to MHA Guidelines can be suitable for some applications Wood-burning central furnaces are generally problematic Some advanced-combustion wood boilers, particularly coupled with water storage, have good potential, but require significant user understanding and monitoring All wood burning appliances require good venting systems, properly sized and located inside the house envelope Only consider an outdoor wood boilers that has been properly certified and tested for low emissions and high efficiency by a recognized agency (and then reconsider) With clean-burning equipment, wood burning can have a significant role to play in Vermont s green energy future
Review of Objectives Understand performance advantages with advanced woodstoves and pellet stoves Appreciate the technology limitations and installation/operating requirements of central wood-fired boilers and furnaces Understand why you should be wary of many outdoor boilers Appreciate air supply, venting, pollutant emissions, IAQ problems, other drawbacks with conventional fireplaces Recognize why advanced combustion fireplaces and inserts can be an effective way to heat with wood See how wood can be an efficient, low emissions (including Greenhouse gases) sustainable heat source, while providing comfort and pleasure
The potential is there. The pleasure is there. The comfort is there. The efficiency is there. The environmental benefits are there Take it!!!
Fireplace & Other Heating Publications on the net http://energy-publications. nrcan.gc.ca/index_e.cfm Heating with Wood All about Wood Fireplaces All about Gas Fireplaces
If you wish more information, or wish to discuss specific problems, please don t hesitate to contact Skip Hayden, at (613) 996 3186 or e-mail at skip.hayden@nrcan.gc.ca
IAQ Problems due to Fireplace 1 Start-up - With poor draft, fireplace spills particulates, hydrocarbons, PAH s, combustion gases until draft well established
IAQ Problems due to Fireplace 2 Fireplace roaring - high draft and huge air demands ( ~ 1.4 AC/h ) can cause combustion gas spillage from other appliances
IAQ Problems due to Fireplace 3 Fireplace tail-end smolder with almost pure charcoal - weak draft can spill toxic carbon monoxide into house
Fireplace Solutions??? Fancy grate Makes effectively no difference to efficiency Tight-fitting glass doors little efficiency gains (to 15-20%) reduces drafts and lessens tail-end spill lowers radiant heat from fire to room still high emissions & low efficiency only marginal improvement
Fireplace Solutions??? (cont) Outside Combustion Air Large size needed to handle roaring fire ( 80-120 in 2 ) similar to flue size, with only small pressure drop allowed Under certain draft conditions, air supply tube can become flue with resultant fire hazard Safer to supply air to room, perhaps around heat exchanger, but lose in efficiency Still high emissions and low efficiency Generally impractical for conventional fireplaces