Preventing Indoor Carbon Monoxide Poisoning

Nalliah Thayabharan

With the increased winter chill comes a rise in the danger from deadly carbon monoxide gas - "CO". The odourless colourless tasteless
two molecule Carbon Monoxide gas is a by-product of incomplete combustion and combustion without proper ventilation. It accounts for
greater mortality and morbidity than all other poisonings combined.  It is a sly and silent killer.  Carbon Monoxide gas can quickly build up
to dangerous levels and knock you out before you realize what is happening.

Carbon Monoxide enters blood system through lungs and displaces oxygen. Carbon Monoxide quickly binds with hemoglobin with an
affinity 200 ~250 times greater than that of Oxygen to form Carboxyhemoglobin resulting decrease in arterial oxygen content. Fetal
hemoglobin has much higher affinity for Carbon Monoxide than adult hemoglobin; thus a fetus may be more susceptible to toxic effects
than mother. Pregnant women with only moderate Carbon Monoxide poisoning have had devastating fetal outcomes.  Carbon Monoxide
is also an abortifacient and a teratogen, resulting in physical deformities and psychomotor disabilities.  Intracellular uptake of Carbon
Monoxide is a mechanism for neurologic damage and cognitive defects, particularly in memory and learning, and movement disorders
that may not appear for days following the initial poisoning.  The brain and heart are very sensitive to Carbon Monoxide poisoning; other
organs are also affected.

Awareness of the symptoms of Carbon Monoxide can lead to early intervention and prevent needless deaths.  The symptoms of Carbon
Monoxide poisoning are headache, dizziness and nausea.   

In 2003 in Ontario, 9 people were seriously injured and 4 died from Carbon Monoxide poisoning.  From 1999 to 2003, at least 17 people
have died from Carbon Monoxide poisoning in Ontario.  From 1998 to 2001, The Ontario Fire Marshall investigated 47,278 Carbon
Monoxide incidence, though the vast majority  were not serious cases.  

Incomplete fuel combustion and a vent system that does not adequately remove the exhaust  are the two basic failures to all combustion
appliance Carbon Monoxide problems and very often these two failures are interrelated. For example, during backdrafting all the
products of combustion spill into the room housing the appliance leads to the appliance to breathes its own fumes, which causes
incomplete combustion and Carbon Monoxide is produced.

Faulty Home heating systems, propane, kerosene or gas heaters, car exhaust including plugged tail pipes, barbecues and blocked
chimneys can cause a build up of Carbon Monoxide.  

Preventing indoor Carbon Monoxide problems is not impossible.  Proper installation and regular maintenance of  combustion appliances
by trained and qualified heating contractors reduces the possibility of Carbon Monoxide emissions and venting failures.  Installing reliable
Carbon Monoxide detectors is the simplest way to detect the presence of Carbon Monoxide.  But only 40% of homes have Carbon
Monoxide detectors.  And of the homes that have the detectors, only about 25% check their batteries regularly.  Carbon Monoxide
detectors should be installed near heating system and in sleeping areas.   Properly installed detectors monitor Carbon Monoxide levels
over time and are designed to sound an alarm before an average, healthy adult would experience symptoms of poisoning.

Carbon Monoxide  detectors do have limitations.  Some may not provide adequate warning if Carbon Monoxide increases to very high
levels.  Infants, the elderly,and people with heart or breathing problems are at increased risk for low level poisoning and may experience
symptoms before an alarm sounds.  Thus, a Carbon Monoxide detector is not a substitute for proper use and regular maintenance and
inspection of all potential sources of Carbon Monoxide.  

Many Home owners report Carbon Monoxide detector going off, but  find their contractor unable to diagnose the cause since Carbon
Monoxide sources elude the contractors.  Identifying the source of Carbon Monoxide is not simple.  Many contractors misdiagnose the
problems. For example they may claim the problem was Carbon Monoxide detector though Carbon Monoxide problem was evident or
say the problem was caused by freak occurrence in the weather and would not happen again.
The causes of Carbon Monoxide are varied as unvented appliances, use of gas cookers for heating, portable space heaters, hibachi and
charcoal cookers,  heat exchanger failures, lack of combustion air, overfiring, depressurization of the combustion appliance zone causing
back drafting, vent failures or vehicle running in the attached garage. Any of these might set off a Carbon Monoxide detector, but
conditions may have changed by the time the contractor arrives at the house to identify the source.  For example, back drafting of furnace
might set off a Carbon Monoxide alarm but if a window is opened, the pressure in the combustion zone will change.  This could reverse
the backdrafting and change the reading on the Carbon Monoxide detectors.  Down drafting of combustion appliances, running vehicles
in the garage, depressurization from kitchen fans, bathroom fans, dryer, central vacuum & attic vent fans, wind and charcoal grills are
usually intermittent problems and easy to overlook during investigation.

Combustion products usually go up the vent, but certain conditions may cause them to spill at the draft diverter or out the burner ports.  
Buoyant (lifting)forces that cause combustion products to vent result from temperature differences.  The buoyant forces are small and
easily over powered by other, more powerful forces.  The buoyant force is usually around 5Pa, is only 1/6 oz in a 4in diameter round vent
pipe. Wind forces, exhaust fans and the furnace blower are all more powerful and can reverse the flow in the vents.  Combustion
appliances also compete for air.  For example, when a fireplace and furnace are both operating they both need combustion air. If the
house is tight, they will compete with each other for combustion air they need.

Technicians often hold a match near the draft hood to check for proper draft, and this method is suggested by some manufacturers.  
However holding a match is not good way to check, because the match is a hot source, and the smoke will tend to rise even if it is not
being pulled up into the draft hood.  Instead, technicians should use a smoke stick of neutral density, such as those used to check for
infiltration during house leakage tests.

Vent failures are often sporadic, intermittent and difficult to reproduce.  Because pressure differences are small, it takes only a small
change to cause warm gases to vent incorrectly.  Often a down draft will occur when house is closed.  Simply opening the front door once
can change the pressures, allowing warm air to go up the vent again.  

A properly adjusted gas furnace or water heater produces hardly any Carbon Monoxide. When too much gas is supplied to the burner,
sufficient oxygen does not flow to the burner and Carbon Monoxide is produced.  I find overfiring to be the more common cause of
Carbon Monoxide problems in a Home than heat exchanger failure.  Overfiring often occurs in conjunction with intermittent vent failure and
is therefore difficult to diagnose unless proper procedures and equipment are used.  Overfired units typically still burn with a blue flame.  
Too often, without proper test instruments, the heating contractor will assume that the unit is burning clean.  

The difference between a properly adjusted furnace and a overfired furnace are extreme.  An overfired furnace can produce more than
4500ppm Carbon Monoxide in the vent.  A simple reduction in gas pressure, which takes two minutes to perform, can bring Carbon
Monoxide levels back to below 20ppm.

How do excess gas pressures occur?  Sometimes, the gas regulator can fail, or in some cases the installing contractor does not perform
the initial gas pressure adjustment.  In yet other cases, the heating contractor or homeowner, desiring more heat, may increase the gas
flow by changing the gas pressure adjustment.  Overfiring may also caused by improper orifices, which can result when units are changed
from propane to natural gas or from natural gas to propane.  Manufacturers typically desire no overfiring beyond a tolerance of +2%.  Yet I
find units 25% overfired.  Because overfiring can not be diagnosed by flame colour, it is vital that all heating contractors perform the
following steps:
1/        Check the rate of gas flow to the burner, and check for overfiring by clocking the meter(In other words using the test dial on the gas
meter to verify that the appliance is using the proper amount of gas)
2/        Check manifold gas pressure using an accurate manometer.
3/        Ensure gas orifices are correct
4/        Measure Carbon Monoxide concentrations in the combustion products.

Holes or cracks in the heat exchanger which keeps the furnace exhaust from mixing with house air, can cause Carbon Monoxide
problems.  Holes and cracks can allow exhaust to enter the duct system and be distributed throughout the house and also allow air from
the blower to enter the burner chamber and disrupt burner operation, increasing the amount of Carbon Monoxide produced.  Airflow
through large holes or cracks can cause combustion products spillage at burner ports - a very dangerous situation.  High efficiency
furnaces also have  a secondary heat exchanger, which removes heat from exhaust gases causing them to condensate.

Some of the causes of premature heat exchanger failure are:
1/        Incorrect temperature rise - The temperature can be either too high or too low caused by incorrect blower speed selection,
restricted or insufficient duct work, dirty filters or missing filters. The correct range for temperature rise is on the information plate of each
2/        Contaminated indoor air - Contaminants like chlorine fumes from the nearby laundry machines, paint thinners and aerosol sprays
are hard on high efficiency furnaces as they create acids in condensation, which attributes to corrosion. Use sealed combustion to
prevent corrosion from contaminants.
3/        Incorrect gas flow rate - Many units are overfired, running rich and hot.  Overfired units do not have sufficient air for complete
combustion.  Cracking of heat exchanger, sooting and Carbon Monoxide can result from overfiring.
4/        Oversizing - This causes rapid on / off cycling without sufficient time to heat the furnace exchanger or vent.  Condensation forms
and does not get evaporated out. The "wet time" is excessive.

Carbon Monoxide poisoning is the leading cause of poisoning deaths in North America.  To often people think that CO poisoning can not
happen to them, since they live in a drafty older house or have a new furnace.  This is not true as Carbon Monoxide poisoning can occur
in older loose houses or newer tight houses and can be caused by new furnaces as well as old ones.

Although this information reflects building science expert's knowledge, it is provided for general information purposes only. Any reliance or action taken based on the information, materials and
techniques described are responsibility of the user. Readers are advised to consult appropriate Professional resources to determine what is safe and suitable in their particular case. Expert Building
Inspections Limited assumes no responsibility for any consequence arising from use of the information, materials and techniques described.

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