Fireplaces, Heating Stoves, Flues and Chimneys - Fireplace exhaust backing up

Our house is about 30 years old and has a wood burning fireplace with a block/brick chimney. The gas water heater in the basement also vents out the chimney. Recently we began to smell a faint amount of smoke in the basement when there was a fire burning. Yesterday it got to be very noticeable in smell, but you still can't actually see any smoke. I looked around in the basement for the source and noticed that warm air was being pushed out of the air gap between the hot water heater and its exhaust pipe. At the time the water heater was not operating and the exhaust pipe was cold. To confirm that this "breeze" wasn't caused by the water heater itself, I checked it today and there wasn't a breeze. Of course, we won't use the fireplace again until this problem is fixed, since the exhaust from the hot water heater probably wouldn't vent properly if it kicked in during a fire. We used to burn pine but stopped doing that after a chimney sweep said it caused a chimney fire hazard - we now burn only hardwoods like maple, walnut, and cottonwood.

I can't visualize what could possibly be forcing warm air down the pipe. Any ideas?

Here are some pictures of the fireplace, not sure if they will be of any help:

http://im1.shutterfly.com/procserv/47b5dc00b3127cce9030b430a2bd00000016108FZtWbho0s

In Basement Where exhaust pipe enters chimney

http://im1.shutterfly.com/procserv/47b5dc00b3127cce9030b43ea2b300000016108FZtWbho0s

Where the smell was coming from:

http://im1.shutterfly.com/procserv/47b5dc00b3127cce9030b452a2df00000016108FZtWbho0s

I forgot to mention that there is NO smell coming from the fireplace or in any other room of the house other than the area in the basement around the water heater.

Are you saying that the hot water tank and the wood burning stove share the same flue? It's fine if the chimney has two liners of which one is dedicated for the wood burning appliance but if they share a flue then you have a problem and a potentially dangerous situation.

If they do share the same flue things like running a dryer (if vented to the outdoors) can cause the type of behavior you're describing. What is the exact configuration of your gas appliances / wood burning appliances with respect to chimney flues?

Scott

I am pretty sure there are two separate flues, because there two separate stacks coming out the top of the chimney, as well as two access doors at the base of the chimney in the basement. We have a gas burning furnace which is the main source of heat and does not vent through the chimney. The only appliance which uses the chimney is the hot water heater. The fireplace is only for recreational use and has been since the house was built 30 years ago. We average about 1 fire per week during the cold months for the past 2 years. This problem has only started this year, and has gotten more noticeable in only the past couple weeks. Thanks for your help.

Given what you've described thus far it sounds as though smoke must be exiting the wood burning flue, and then reentering your house through the flue used for the hot water tank.

As you burn wood the hot air that exits your house via the chimney must be replaced somehow. Any opening/gap is a candidate for replacing this air including chimney's. If you're exhausing air through some mechanical means such as a bathroom/stove fan or a dryer at the same time that you're burning a fire then the volume of air that must be replaced increases. If your house is tight (sealed very well) then the likelyhood that replacement air is coming from a chimney increases.

Unfortunately I don't have any real solution for you. It seems to me that you need to provide an alternative source of replacement air for your fireplace. Maybe some of the forum experts can chime in with some alternatives.

Scott

Thanks, that theory makes a lot of sense. We replaced about half of our windows last year and they are a lot less drafty. There was nothing else exhausting outside while the fireplace was in use. I will try another fire with a window cracked to see if that solves the problem.

What still puzzles me is why the "breeze" I was feeling was so warm. Is it possible for a crack to develop between the two flues, allowing the hot smoke to be sucked into the other flue while still within the chimney?