Chimneys are responsible for venting the byproducts of combustion and allowing combustion air to be drawn in or blown in when the heating equipment has a blower to assist introducing combustion air into the flame.

When a chimney or metal vent system is blocked or not functioning properly for some other reason, there a risk of combustion by products not exhausting the home.

Compounding the situation, should inadequate venting reduce the amount of combustion air needed, carbon monoxide will probably be produced which is then dumped into the home.

Check to make certain your service/installation company has a draft gauge to test a vent system for proper operation.

The photo above shows the Bacharach mechanical draft gauge testing a gas fired hot water tank through a hole drilled in the vent system.  Most of the newer electronic combustion analyzers are also capable of measuring draft digitally.

An "extinguished match" test used to be a common way to see if the smoke was drawn up the stack, but this is simply not sufficient.  Observing smoke being drawn into a vent system is not an accurate way to test a vent system.

Keep in mind, properly drafting vent systems are producing very small amounts of negative pressure to draw the byproducts of combustion out of the home.

For example, a gas fired hot water tank's acceptable draft reading is .02 to .04 water column inches.  Imagine how little pressure it takes to draw water from a glass up into a straw 2 to 4 hundredths of an inch.  That's not much.

Chimney draft is constantly changing depending on temperatures (inside or outside), barometric pressures, wind conditions, etc.  Keep your eyes open for indications that occasionally the vent system is failing.

Some of the things you can look for are:

Rust or corrosion on the vent system or heating equipment.

In the above photo, note how the initial run of vent pipe on top of the boiler is rusted.  Also, the cabinet above the burners shows signs of deterioration.  Both indications that this unit is not venting properly all the time.

Missing bricks around the top of the chimney or discoloration/deterioration of the mortar.

In this photo (looking down from the top of the brick chimney) you can see several bricks which have fallen down, partially obstructing the chimney.  Also note the deterioration of mortar.  Burning gas or oil produces a tremendous amount of moisture which is very acidic.  If the chimney is not venting properly, the moisture in the flue gases will condense on the brick/mortar and eventually cause damage.

Here you can see that when the cleanout cap at the base of the chimney was removed, a tremendous amount of debris is evident.  This is an indication that the chimney is deteriorating.

For more technical information on draft, click here