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

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Dirty Woodstove

Advanced CombustionWoodstove

Particulates, Particulates, g/hg/h

Air Requirements of Woodstoves & Fossil Fuel-Fired

Furnaces/Boilers

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0.05

0.1

0.15

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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

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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

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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

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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

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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

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Conventional Fireplace

Dirty Woodstove

Advanced CombustionFireplace

Air Requirements of Furnaces & Fireplaces

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ConventionalFireplaceConv Gas/OilFurnaceHi-Effic Gas/OilFurnaceAdv CombustWoodstove

Efficiencies of Advanced Combustion Fireplace & Stoves vs

Conventional Fireplace

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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

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 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