Chimneys are intended to vent fuel combustion product to the outside. Combustion product fumes consist of a mixture of hot gases some of which are toxic. The most notorious combustion product component is carbon monoxide which is odorless, poisonous, and deadly. Therefore, the integrity of the connector pipe, thimble connection, chimney flue liner, and chimney walls must be maintained to prevent leakage of fumes.

One of the products of combustion is water vapor. In the past, on less efficient oil or gas burning boilers and furnaces, flue gas temperature was higher than it is today. At firing, the chimney flue interior would heat up relatively quickly and water vapor condensing on the flue interior would evaporate. However, with today's more efficient boilers and furnaces with lower flue gas temperatures, condensation accumulates and remains in the flue longer because there is not enough heat being lost up the chimney to raise the flue temperature above the dew point. This is especially true on chimneys with three walls exposed outside.

A properly installed flue liner is all the more important on today's relatively higher efficiency heating appliances. If the flue liner is thermally isolated from the chimney brick masonry with an air gap, then the flue liner has a greater chance of heating up and clearing the condensation. Without any liner, the large thermal mass of the brick masonry walls is slow to warm up. Condensation accumulation will accelerate deterioration from the inside out. Brick spalling and mortar erosion are often found in unlined chimney flues. Tile liners can also crack and spall as a result of increased condensation.

By the way, a connected and operating heating appliance is not necessary for chimney flue condensation to occur. During the winter, while inspecting an unoccupied seasonal cottage over an enclosed crawl space, I once found an inch deep layer of frost condensation on the interior of a fireplace chimney flue. The chimney was exposed on the north side and there was no damper in the fireplace flue.

Why does condensation inside an unlined flue have the potential to damage the chimney? First, the cycle of freezing and thawing of water in the bricks and mortar joints is very destructive. Second, compounds in the flue gas are very corrosive.

Rain falling down into lined and unlined chimneys can add to the problem. That is why conditions at the top of the chimney are very important. For example, on a lined chimney, the chimney cap mortar is intended to seal the gap between the liner and the chimney walls. If the cap is cracked or loose, water will leak through and accumulate in the gap. Even if he cannot climb on the roof to inspect the chimney cap, your home inspector should look for the tell tale efflorescence on the chimney exterior that would indicate that water has leaked into the gap. Efflorescence on the chimney exterior can also be caused by leakage of the flue gas into the space between the liner and the chimney wall.

Fireplace and wood stove chimney flues have to be cleaned regularly because, over a period of weeks or months, the fumes condense on the chimney interior causing soot and creosote to accumulate. In uncleaned chimney flues, I have found large loose sheets of creosote that have broken loose from the chimney walls and fallen down and lodged on the thimble, obstructing the flue and reducing the draft to a dangerously low level. Rain water and condensation water that loosened up the creosote continues to soak into the sometimes deep accumulation of creosote at the bottom of the flue. The black liquid that leaks out the base of the chimney is very corrosive and not only rusts out the cleanout door but can weaken bricks and mortar at the base.

Here is one thing I have observed on some wood burning chimneys. Every time the wood stove is fired, hot flue gases enter the flue and the tile liners heat up and expand faster than the more massive brick walls of the chimney. What happens is that the expanding tile lifts up on the top of the chimney and introduces vertical tension in the chimney walls. Eventually, a mortar joint opens up all the way around six or eight courses down from the top. I have seen horizontal cracks like this as much as 1/4 inch wide. That is probably why some chimney masons use a flexible sealant between the chimney cap and the top tile. I have found that this expansion cracking does not seem to occur when refractory cement liners are used in place of fired clay tile liners but that would require further investigation.

Your home inspector should recommend that chimney flues be cleaned.

Usually, the chimney is visible outside, in the attic, and in the basement but is concealed behind walls inside rooms in the house. Every available surface of the chimney should be investigated. Your home inspector is checking for openings through which smoke and fumes could leak out of the chimney. Look for missing, cracked, or soft bricks. Sometimes, where mortar or parts of bricks are missing, there are holes through into the flue. Chimneys in older houses are often unlined. Occasionally, even in an attic, you will find a loosely fitting tin plate over a former stove hookup. Cast iron thimble covers are often concealed behind walls. All unused former flue connections should be filled in with solid masonry.

Here is an example of why old unused thimbles should be closed up with solid masonry. One evening during my youth, my family was operating a wood stove connected into a fireplace. Suddenly, we could hear a deep roaring sound throughout the house. In my room on the second floor, I could feel heat radiating from the direction of a cast iron flue cover. As I watched, the latex paint on the cast iron cover turned light brown as the cast iron reached a red incandescent glow and the paint burned off. We all rushed outside and saw sparks flying straight up from the chimney top. This all happened very quickly. The next day, after everything had cooled off, I removed the thimble cover to see what was inside. I found that the thimble opening extended horizontally about six inches to the interior of the chimney flue. I surmised that the cover got red hot because it was in contact with burning creosote and cinders. Shortly after that, my family removed the wood stove from the fireplace.

In older houses, your home inspector should check to make sure that floor beams and joists are not let into the chimney walls. In some cases, this practice exposes combustible wood to the hot flue gases. This condition should be corrected by a building contractor and brick masonry contractor.

Chimney flues can become obstructed not by creosote but in other ways particularly while not in use. I have found broken pieces of fireclay tile liner and bricks obstructing fireplace flues. Sometimes, after a house has been vacant for a year or more, birds will build a nest somewhere in the chimney flue. subsequently, the nest falls toward the bottom of the flue and obstructs the flue connection.

Ideally, there should be a flue cleanout access door in the base of the chimney one foot below the lowest flue connection. Your home inspector will open this door and place a small mirror in the bottom of the flue. He is looking for daylight coming down from the top of the chimney. Because of the corrosive action of creosote, soot, and cinders, it is not unusual for the cast iron cleanout door to be rusted shut. I have found old rusted doors falling off the hinges. It is also not unusual to find that the whole bottom of the chimney flue is packed completely full with creosote and soot and cannot be inspected until it has been cleaned out.

All chimney flues should be lined. Several chimney lining methods are available. If your home inspector finds unlined flues, you should hire a masonry contractor to make further investigations into the feasibility of installing flue liners.

The roof inspection provides an opportunity to look at the condition of the chimneys above the roof surface.

I do not recommend this for safety reasons, but some home inspectors will actually get up on the roof and look down into the chimney flues. They are trying to assess the conditions inside the chimney flue. By getting a close look from the roof surface, they are able to check for conditions not visible from the ground. For example, cracked and eroding mortar, cracked and spalling bricks, cracked and spalling flue tiles can be found. Creosote build-up can also be checked. The condition of spark arresters and temporary chimney flue covers can be investigated. If your home inspector reports that the chimneys need maintenance and upgrading, you should have the chimneys investigated further by a brick masonry contractor.

It is not unusual for chimney draft, particularly fireplace chimney draft, to be adversely affected by wind direction, surrounding topography and trees, open windows and doors, flue obstructions, flue temperature, and other factors including the design of the fireplace, smoke shelf, flue height, flue cross sectional area, and location of the chimney on the roof.

When a heating system is installed in a crawl space under an older house, the flue connector pipe sometimes enters the chimney at the very bottom. A similar condition is often found on chimneys built onto a house from outside. If that is the case, the only way to check and clean the flue is to shut down the heater and remove the pipe from the thimble. This should only be done by a licensed heating system specialist and/or chimney cleaning service.

For unusual flue connections and chimney conditions such as this, your home inspector is likely to recommned further investigations by a representative of the state fire marshal's office.