Heating with Wood

Skip HaydenAdvanced Combustion Technologies

Better Buildings by Design 2008Burlington, VT, February 2008

Heating with Wood

Part 1

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 outdoor boilers• Appreciate air supply, venting, IAQ problems, drawbacks

with conventional fireplaces• Recognize why advanced combustion fireplaces and

inserts can allow the fireplace to return to Vermont homes• See how wood can be an efficient, low emissions (including

Greenhouse gases) heat source, while providing comfort and even 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

At first, 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 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

Presently there are NO emissions regulations for wood burning central heating equipment !

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 quite low (<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 ….

• Instead of shutting off completely when demand has been satisfied, with furnaces, the air is closed off 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• The potential for fairly low emissions

and higher efficiency is there

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

• Most advanced wood boilers use a downdraft or sidedraftcombustion system. However, to perform well, this requires stable, good draft, possibly using an ID fan, ideally with a properly sized inside chimney, and with minimal house depressurization

European Power Pellet Burner for Central Heating Systems

New hope for clean, efficient, central wood boiler or furnace

What about 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

• Best coupled to inside hot water storage• Casing and distribution losses outside the house

envelope lower efficiencies• Not recommended at this time !

What about fireplaces ?

A conventional woodburning fireplace has twice the emissions of the old,

“dirty” woodstoves, and ten times the emissions of today’s advanced

combustion woodstoves

Fireplace & Woodstove Emissions

05

101520253035404550

Dirty AirtightWoodstove

Fireplace

Today's AdvancedCombustionWoodstove

A conventiional woodburing fireplace has twenty times 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

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 as the enclosed cavity can radiate to the outdoors as well), the maximum efficiency would be ~20%. Considering the huge amount of house air that is actually swallowed up by the fireplace, under 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 BELOW 0% due to cold drop BELOW 0% due to cold outside air!outside air!

A conventional woodburning fireplace creates serious indoor air quality problems, under three distinct periods of its operation.

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

With a roaring fire, the conventional fireplace is a

large, powerful exhaust device(400-800 cfm)

IAQ Problems due to Fireplace

3 Fireplace tail-end “smolder” with almost pure charcoal - weak draft can spill toxic carbon monoxide into house

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 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, sim to flue size, with LITTLE pressure drop allowed

• Under certain draft conditions, 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

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 !!!

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 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 ?

What about existing fireplaces ?