Developed with funding from the U.S. Department of Energy and U.S. Environmental Protection Agencyby the Southface Energy Institute, for more information contact Southface Energy Institute, 241 Pine St., Atlanta, GA 30308, 404/872-3549, www.southface.org

Combustion Equipment Safety

1.13.99

Combustion appliances are efficientFuel-fired combustion appliances are often the mostefficient and cost-ef fective way to produce heat.Combustion appliances today commonly use natural gas,propane, oil, kerosene, and wood as their fuel. Examplesinclude:

❏ furnaces ❏ ovens❏ space heaters ❏ fireplaces❏ water heaters ❏ wood stoves❏ ranges ❏ gas clothes dryers

Although combustion appliances have been used formany years and have been installed in millions ofhomes, careful installation is still required to ensuretheir safe and ef ficient operation, especially in today’smore energy-efficient homes.

Problems posed by combustion equipmentDuring the combustion process, the appliances burn fuelusing oxygen from the air and release exhaust gasessuch as carbon dioxide, water vapor, nitrogen oxides,and carbon monoxide. While most combustion appliancesoperate properly and create no ill-health ef fects,occasionally a situation develops which can causeserious medical problems, and sometimes death. Thisoccurs primarily from carbon monoxide poisoning when:

❏ Combustion appliances malfunction and exhaust highlevels of carbon monoxide either directly into thehome, into the ductwork, or into leaky exhaust flues;

❏ A negative pressure difference is created in a home thatcauses an appliance to backdraft – pull combustiongases down the exhaust flue and into the home.

What about fuel-burning stoves and ovens?There is growing evidence that gas-fired stoves andovens create high levels of carbon monoxide in homes.

There are five precautions which should help preventproblems from stoves and ovens:

❏ Install a carbon monoxide detector in the living areanear the kitchen

❏ Always install a kitchen exhaust fan that is vented tothe outside and operate the fan whenever cooking

❏ Have the stove or oven, as with all major appliances,serviced regularly (every 2 years is recommended)

❏ Do not use a stove as a space heater

❏ Buy a stove with pilotless ignition

Are unvented gas space heaters safe?Most energy and health exper ts advise against unventedcombustion appliances. Even when operating properly,these units produce unhealthy exhaust gases such asnitrogen oxides and excess water vapor. If these unitsare present, do not weatherize the home and adviseoccupants to crack windows open to ensure ventilationwhenever the units are operating.

Balance the air pressure in the house and provide spotventilation for non-vented combustion appliances, suchas gas stoves.

Negative house pressures affectcombustion appliancesPressure differences that backdraft an appliance limit thesupply of oxygen to the burner, which cause it to produceincreased levels of carbon monoxide. While clothes dryersand vent fans may cause negative pressures, a majorcause is due to imbalances created by heating andcooling systems with duct leakage or poorly designedreturn ductwork. Forced-air heating and cooling systemsshould be balanced — the amount of air delivered throughthe supply ducts should be equal to that drawn throughthe return ducts. If the two volumes of air are unequal,pressure imbalances will occur in the home.

Leaks in the supply side of a duct system can create anegative house pressure by pushing less air into thehouse and pulling more air in through the return side -this increases air infiltration from the outside and thepossible backdrafting of flues and exhaust vents becausethe house is attempting to equalize the pressure bypulling air in from other sources. Another shortcomingof leaky supply ducts is a general loss of efficiency andincreased energy costs.

Supply

Return

Positive pressures may be a problem tooDuctwork systems that are leaky on the return side havetheir own set of problems: the house becomespressurized, generating air leakage out of the buildingenvelope. Air from unconditioned sources, such as acrawlspace, is pulled in through the leaky return systeminto the living space - air that may be contaminated withradon, mold, pesticides, or toxic chemicals. Positivepressures in cold climates may drive moisture throughwalls and cause deterioration. Hot humid air is pulledinto the ducts in summer - cold air is drawn into theducts in winter. If leaky return ducts are located nearcombustion appliances, such as in an equipment closetor crawlspace, the negative pressure could cause fluesand chimneys to backdraft. Carbon monoxide hazards

Carbon monoxide (CO) is a colorless, tasteless, odorlessgas that is poisonous. There are hundreds of deaths inthe United States each year because of CO poisoning.

Effects of CO exposureCO (ppm) Time Symptoms

35 8 hours Maximum exposure allowed by OSHA inthe workplace over an eight hour period.

200 2-3 hours Mild headache, fatigue, nausea anddizziness.

400 1-2 hours Serious headache- other symptomsintensify. Life threatening after 3 hours.

800 45 minutes Dizziness, nausea and convulsions.Unconscious within 2 hours. Death within2-3 hours.

1600 20 minutes Headache, dizziness and nausea. Deathwithin 1 hour.

Install a carbon monoxide detectorCarbon monoxide detectors arehighly recommended in homes withfuel-burning appliances. Thedetectors signal homeowners viaan audible alarm when CO levelsreach potentially dangerous levels.Some models have digital readoutsof current CO levels, which areuseful to the homeowner tomonitor household air quality,

while some less-expensive models indicate varying levelsof CO with dif fering alarms. CO detectors are either aplug-in variety or can be hard-wired and should beinstalled in rooms with direct connection to combustionappliances, such as kitchens with fuel-burning stovesand ovens, areas near combustion closets for fuelburning heating systems, and rooms with fuel-burningspace heaters. A low wall socket location is preferredfor plug-in models since CO is heavier than air.

Foam bafflesdamper sound

Air passes through baffle

Wall cavity

A transfer grille provides pressure equalization betweenrooms

Special issues with pressure differencesPressure differences can also result in homes with tightductwork if the home only has one or two returns. Wheninterior doors are closed it may be dif ficult for the air inthese rooms to circulate back to a central return duct.The pressure in the closed-of f rooms increases, and thepressure in rooms open to the return decreases.

Rooms with one supply and no return should have:

❏ a 1-inch gap under the door connecting to an area witha return.

Rooms with two or more supplies should use one of thefollowing:

❏ a transfer grille installed either in the door or throughthe wall between the room and the area with a return(a wall-mounted transfer grille must be totally sealedfrom the rest of the wall cavity) and it may contain asound baf fle to lessen noise transmission betweenrooms; or

❏ a jumper duct that connects the two rooms; or

❏ a separate return.

Pressure dif ferences in the conditioned space of a homecan af fect indoor air quality and energy use.

Combustion closet design issuesToo often, combustion equipment closets are connectedto the occupied rooms of the home via air leaks orlouvered doors – this can be dangerous and is notrecommended. Carbon monoxide, and other pollutantspresent in the combustion closet, can leak into the restof the home and create dangerous conditions for thehomeowners. Also, using room air for combustion robsefficiency since that air must be replaced by infiltration.The combustion closet should be air sealed and isolatedfrom the rest of the house and ventilated to the outside.

Combustion equipment closet featuresInsulate the interior and exterior walls of the combustioncloset, finish the walls and ceiling with drywall, seal allholes and leaks between the closet and the rest of thehouse, and install a non-louvered, weather-stripped door. Inaddition, provide outside air for combustion by installing two

ducts in the closet that are sized for the specificcombustion appliances. One duct should extend to within 12inches of the floor of the closet, while the other extends towithin 12 inches of the ceiling (check with local codes). Theducts should extend to the outside, to a ventilated crawlspace, or to a ventilated attic.

The sizes of the outside air ducts depend on localmechanical and building codes. Typical sizing methodsfor appliances in a combustion closet require a givennumber of square inches of ductwork per 1,000 Btu/hourof fuel used by the appliances. Consult with localmechanical engineers to design the combustion air ducts.

Creating a combustion equipment closet ser ves toseparate the combustion appliance from the living spaceof the house. With a guaranteed source of combustionair in the mechanical room isolated from the living area,the home will be better protected against backdraftingand carbon monoxide problems.

Return grillewith filter

Seal

Door closes against solid threshold

Sealed, ducted return (if the cavity is used as a plenum, carefully seal this area since it is a return duct)

Seal penetrations and underbottom plate of walls. Air from

house

Solid door(non-louvered)must seal tightly against weatherstrippingat top, bottom, and sides

Weatherstripping

Screened high and lowcombustion air ductsinto equipment closet

Drywall

Seal around fluewith sheet metal

Supply plenum sealed with mastic and insulated

Insulated wallsbetween equipmentcloset and home

Warning:Do not storeanything in this closet.Keep door shut.

WARNING

A shelf-system installation with a closed combustion closet provides a compact and safe arrangement for anatmospheric gas furnace.

Shelf-system combustion closet

Solid (non-louvered) door with weatherstripping

Door closes against solid threshold

Insulatedwater heater

Bottom platesealed

Insulatedwalls

Seal gas and plumbing penetrations through walls

Vented exhaust pipethrough roof

Screened high and low combustion air ducts into closet - extend 6" above insulation in attic

LowHigh

A closed combustion closet is, “a little piece of theoutside located in the middle of the home”. Fresh air forcombustion is provided from sources not connected withthe living space.

Air quality in airtight homesWhile any home may encounter pressureimbalances, homes that are more airtightgenerate pressure dif ferences more easily.However, a few simple design measures canprevent these problems and still permit thevir tues of a more air tight home.

Airtight homes…

❏ Reduce indoor air problems because they preventpollutants from entering through the attic orcrawlspace (or even moist, unfiltered air fromoutside)

❏ Provide intentional ventilation for the occupants

❏ Save energy by reducing the cost normallyrequired to heat, cool, and control moisture inoutside air leaking to the inside by infiltration

❏ Require intentional air for combustion appliances

See Air Sealing Energy Bulletin for more information

Humidity, pollen

Insulation fibers, dust

Mold, mildew, radon

Formaldehydes, chemicals

Furnaces and water heatersCommon combustion appliances, such as furnaces andwater heaters, require air for combustion and to ventexhaust gases. Most of these appliances are non-directvent units–they use air surrounding the appliance forcombustion, and are more af fected by pressuredif ferences in the house. Others, known as direct ventfurnaces, bring combustion air directly into the burnervia sealed inlets connected to the outside.

Direct-vent combustion appliances can be installed withinthe conditioned area of a home since they do not rely oninside air for safe operation. Non-direct vent combustionappliances must receive adequate outside air forcombustion and exhaust venting. The primary concernwith non-direct vent units is that a malfunctioning heatermay allow flue gases to enter the area around the furnaceand be pulled into the living space.

Most new furnaces have forced draft exhaust systems,meaning a blower always propels exhaust gases out theflue to the outdoors, and are unlikely to backdraft.Atmospheric furnaces and water heaters do not have aforced draft fan. Atmospheric furnaces are less commontoday due to federal ef ficiency requirements but thisventing method is still common in many water heaters.

Atmospheric furnaces and water heaters should beisolated from the conditioned space. Those units locatedin well ventilated crawl spaces, basements and atticsusually have plenty of combustion air and encounter noproblem venting exhaust gases to the outside.

Combustion appliances located inside the house shouldbe carefully installed in a combustion closet to ensureproper, efficient operation and prevent indoor air qualityproblems. If there are concerns that poor equipment orpressure imbalances may cause backdrafting or otherindoor air quality problems, consult with a qualifiedcontractor.