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Heating with Wood:Vermont’s Renewable Resource
Skip HaydenIntegrated Energy Systems
Better Buildings by Design 2009Burlington, 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 youlooking for
??
?? ?
The airtight woodstoves of the 1970The airtight woodstoves of the 1970’’s s and 80and 80’’s were major sources ofs were major sources ofair pollutants, due to air pollutants, due to poor combustion !poor combustion !
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.
How Wood Burns
Emission regulations for woodstoves forced development
ofnew 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 . . .
AdvancedCombustion
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
0
5
10
15
20
25
Dirty Woodstove
Advanced CombustionWoodstove
Particulates, Particulates, g/hg/h
Air Requirements of Woodstoves & Fossil Fuel-Fired
Furnaces/Boilers
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Conv Gas/OilFurnace/Boiler
Hi-Effic Gas/OilFurnace/Boiler
Adv CombustWoodstove
Air Changes/hourAir Changes/hour
Wood Pellets
High EfficiencyWood Pellet
Fireplace
Woodstove & Pellet Stove Emissions
0
5
10
15
20
25
Dirty "Airtight"WoodstoveToday's AdvancedCombustion StoveWell-Designed PelletStove
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 WoodFurnaces & 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 PerformanceDraft 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 10xthe emissions of today’s advanced combustion woodstoves
Fireplace & Woodstove Emissions
05
101520253035404550
Dirty AirtightWoodstove
Fireplace
Today's AdvancedCombustionWoodstove
g/h
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
0
0.2
0.4
0.6
0.8
1
1.2
1.4
HiEffic Furnace
Conv Furnace
ConventionalFireplace
AC/h
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
05
1015202530
1500% EA
Outside AirRoom Air
Maximum Efficiency, %Maximum Efficiency, %
On very cold days, efficiency can On very cold days, efficiency can drop drop BELOW 0%BELOW 0% due to cold due to cold outside air!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 areincompatible with
most North American homes !!
Still, what is the most desiredmost desiredcombustion system
in North American homes ?
AFireplace !
Fireplace SolutionOutside Combustion Air
• Large size hole/duct needed to handle “roaring”fire – ( 80 - 120 in2 ) 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
• With intake on leeward side of house, large outside air connection may become a “better” vent path than the fireplace chimney, with disastrous results
+
+
WIND
Neutral Pressure Plane
+
-
+-
-
Danger !!!
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 ?
AdvancedCombustion
WoodburningFIREPLACE
Advanced Combustion
WoodburningFireplace
Fireplace & Woodstove Emissions
05
101520253035404550
Conventional Fireplace
Dirty Woodstove
Advanced CombustionFireplace
Air Requirements of Furnaces & Fireplaces
0
0.2
0.4
0.6
0.8
1
1.2
1.4
ConventionalFireplaceConv Gas/OilFurnaceHi-Effic Gas/OilFurnaceAdv CombustWoodstove
Efficiencies of Advanced Combustion Fireplace & Stoves vs
Conventional Fireplace
01020304050607080
Adv Stove AdvFireplace
ConventionalFireplace
Room AirOutside Air
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
MasonryHeaters
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 ofGood 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 energyin 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 combustionwoodstove or fireplace
to be trulyefficient & effective,
it must beproperly sized & located !
LocationLocation
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 fireplaceshould be properly sized,
and locatedin 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 Fireplacewith
Zoned Air Ducting
On-Going Advances inFireplace 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 IntegratedCondensing 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 HeatingPublicationson 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 3186or e-mail at
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 in2 ) 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