Posted by: SherlockHomesSY | 29 Oct 2010

Solution Basics: Combustion Safety

Back Drafting Gas Appliances

Fans Can Cause Gas Appliances to Backdraft


The best way to avoid combustion safety issues is to not have combustion appliances.  However, gas, oil, and propane furnaces and water heaters are here to stay.

Statistically, 1/3 of all US homes have combustion safety issues.  The reason we don’t hear about it much is because our houses are air leaky enough to where it apparently doesn’t make a difference. Before improving your house, these issues have to be resolved first.

What is Combustion Safety?

Combustion safety is the proper installation of appliances, fireplaces, and garages while exhausting smoke from the house effectively under worst-case routine conditions.

Worst case is normally . . .
– All fans running in the house
– All the doors and windows closed
– Doors to rooms without return registers closed along
– Mechanical room doors closed.

What is Combustion?

When natural gas or propane burns completely, the result is carbon dioxide, steam, and nitrous oxides.

CH4 + O2 -> CO2 + 2H20 + NOX

It burns around 550° F.  This hot gas rises.  It rises faster in the winter than it does in the summer.

These new gases have to be exhausted before the temperature of the smoke reaches the dew point—where the water and carbon dioxide combine to make carbonic acid. (Carbonic acid is what we use to make soda water.)  When the acid forms, it rolls back down the metal pipes to corrode out the vent and the appliance.

Unfortunately, gas doesn’t always burn completely, so carbon monoxide is produced too.  If this gas doesn’t get out, the result can be carbon monoxide poisoning.

Unvented Appliances

A major byproduct of combustion is water.  Unvented appliances, such as gas stoves and ovens, space heaters, and fireplaces release huge amount of moisture into the house.

Another major byproduct of combustion is carbon dioxide, which replaces oxygen.  Some unvented appliances have an oxygen sensor that shuts off  the appliance when the percentage of oxygen drops below 18%.  Normally air is 21% by volume.

Where gas stoves and ovens are in use, rangehoods exhausting directly outdoors should be used.

Otherwise, all other unvented appliances either need to vented outdoors or their use discontinued.  High moisture generation, oxygen replacement, nitrous oxide formation, and the possible production of carbon monoxide are enough to discourage their use.

Provide Adequate Combustion Air

For complete combustion, enough fresh air must be provided.

From an energy perspective, conditioned air should not be used.  What’s the point of using air money was spent to heat to support combustion?  Combustion appliances need to be isolated from living spaces.

Water heaters and furnaces are often located in attics, crawlspaces, and garages to get them outside the conditioned living area.  Unfortunately, from an energy perspective, useful heat is lost—forever.

For any appliances in the living space, sealed combustion closets should be built with outdoor combustion air provided to it.  There are a number of building-code acceptable ways to do it.

High Efficiency Appliances

An alternative to providing combustion closets is replacing water heaters and furnaces with high efficiency appliances.  These appliances can be kept inside the house (a huge energy benefit) while supply air is fed directly to the appliance while exhausting cool smoke directly outdoors.

Proper Pressure

It only takes 3 Pascals (Pa) of pressure to backdraft a water heater, 4 Pa to backdraft a fireplace, and 5 Pa to backdraft a furnace.  A water heater can have flame rollout at 8 Pa.  How much is a Pascal? Technically, its 1/250 of 1 inch water column of pressure.  One inch water column is what’s produced when a toilet is flushed.  In this business, a Pa is so small we call it a gnat fart.

Do you get it? It doesn’t take much to backdraft a water heater, fireplace, or furnace.  A fan, or collection of fans can do it.  When a house is air sealed, depressurizing a building is much easier to do.

When appliances experience depressurization, steps must be taken to ensure the combustion appliance zone the appliance is in does not become depressurized under routine worst-case conditions.

Attached Garages

Our vehicles produce smoke too, but the byproducts are more than carbon dioxide and water.  Vehicle smoke is deadly in a variety of ways.

Ideally, garages should be completely separate from the house.  Where they are attached, the wall separating the house from the garage needs to be sealed from the foundation to the roofline.  Most of the time, the roofline is sealed by gypsum board (drywall) ceiling.

CO Monitors

When there is an attached garage or gas appliances, it’s critically important to distribute carbon monoxide monitors throughout the house.

Gas Leaks

Part of combustion safety is ensuring there are no gas leaks in the gas distribution lines.  I’m shocked at the number of houses where I’ve found gas leaks at the gas meter.  Another vulnerable place is where the appliance is connected to the gas lines.

In Conclusion

Before any work is done to your house, combustion safety needs to be diagnosed and any issues corrected to avoid deadly consequences caused by marginally performing gas appliances.

Most home performance contractors and consultants are required to be certified by the Building Performance Institute (BPI) to do a combustion safety analysis.

Unfortunately, most of the certified contractors and consultants are not trained in how to determine if combustion appliances are installed correctly.

Take referrals to licensed and qualified professionals seriously.  For your family’s sake, make corrections to combustion safety as highest among all other priorities.


Diagnosing combustion safety issues can be found on my home performance website.

The requirements and procedures for completing a combustion safety analysis is found on BPI’s website.

Home Energy magazine has an article, Combustion Safety Checks: How Not to Kill Your Clients.

Next time, a related subject: Pressure Balance.

If you have an attached garage or combustion appliances, please heed our warning . . .

WARNING! Do not implement any of these solutions without considering the impact on IAQ, moisture control, & heat transfer. A combination of solutions are usually needed to improve health, building durability, comfort, and energy efficiency. Ignoring this warning may lead to disease, deterioration, high energy bills or worse!


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