Carbon monoxide issues only pertain to gas appliances.  (Wrong!)

Gas Water Heater and Furnace on Same Flue

Issues with carbon monoxide usually arise because of problems with venting or combustion air. A perfectly good furnace or water heater may produce carbon monoxide when the flue is partially or fully blocked.

This may be hard to understand, but pay attention!

Carbon monoxide problems are not CAUSED by gas appliances.  Gas appliances will produce CO, but it is almost always caused by something external to the appliance.

The distinction is hard to grasp, but important. 

An appliance (stove, water heater, or furnace) that has NOT had a mechanical or electrical failure would not normally produce significant amounts of CO.

In all but the rarest of cases, an appliance WITH a malfunction will NOT produce CO, at least in enough quantity to cause a problem. 

A furnace may not start or starts with a rumble, a water heater pilot may keep going out or the water heater may not produce enough hot water. These are all symptoms of a component failure. The equipment may not operate, but it doesn’t produce carbon monoxide.

Insufficient combustion air and insufficient venting are the two main items that determine if CO will be created and if it will become dangerous for the occupants.

Flue back draft issues can cause carbon monoxide.

Click the image for a larger picture. Note that anything that can depressurize your home can also cause the flues for your furnace and water heater to stop working and potentially back draft.

Look for problems with:

  • Insufficient combustion air
  • Lack of flue draft
  • Blocked or leaking or disconnected flue
  • Crushed or blocked flue cap
  • Open or leaking return-air duct work
  • Open blower compartment panels
  • Exhaust fans or dryers that cause the flue to back draft
  • Improperly connected water heater tees
  • Improperly sized flues or connectors

Don’t forget all the “other” sources of carbon monoxide contamination;

  • Engine driven back-up generators inside the structure 
  • Engine driven power sprayers inside the structure 
  • Engine driven air compressors used inside a structure still under construction 
  • Construction or temporary heaters 
  • Charcoal or gas grills being used indoors for temporary heat or cooking 
  • Excessive use of candles 
  • Non-vented, gas-fired space heaters 
  • Non-vented gas fireplaces 
  • Automobile exhaust (can permeate a structure even when idling in the driveway.) 

Keep in mind that carbon monoxide is a “skinny gas”.  It permeates anything.  It can go through the smallest cracks, gets past doors with seals on them and seems to migrate right through walls.  (I don’t know if it really can go through walls, but it gets around faster than any other gas I’ve tracked.)