Why does my chimney backdraft when I run the central air conditioner and the clothes dryer?

Question

When my furnace fan is running (central air) and I start the clothes dryer (located in basement), or the kitchen vent, the fireplace (main level of ranch home) backdrafts and the home begins to smell smokey. This occurs when there is no fire and the flue damper is closed. The fireplace has a vent. I have also added a flex tubing running from the outside into the basement to provide "make-up" air for the furnace in the basement. Nothing seems to work - any advice would be greaty appreciated.

Answer 1

Some of the posters have hit on it. You need more make-up air in the house.
Vent fans, dryers, range vents and furnace/AC all create a negative pressure in your house when in use. To make up for that, air is drawn in wherever possible.
The answer is to add a make-up air vent which leads into the return air trunk of your heating and cooling ductwork. What it is: depending on the size of the home, probably a 6" duct that has a vent to the outside of the home (or 2 6" ducts in a larger home). There is a baffle inside the duct that only opens when a negative air pressure situation arises. It can be done as a DIY project if you have any "tin knocking" experience. Otherwise any local heating and cooling company can do it for you in about an hour or two.
You'll find this a better solution than opening doors, windows or flues.

Answer 2

The A/C is a closed system, which means it has an air-return, which is usually centrally located and may be near the thermostat. The ductwork for this runs to the rooms in a house and is supposed to return to the air-handler to be further cooled and sent back through the house. If there is a blockage in one of the rooms, such as the door is closed and there are no other registers to allow the air to return to the main unit, and also there is a window open or some other outlet for the air being pushed into the room by the A/C blower, then the blower, by way of the "return air" duct is having to draw air from the chimney (damper notwithstanding) to replace the air forced from the otherwise-sealed room. This could be the area where the dryer is located, or some other room with an outlet. Smoke test some areas for an indication of where the air is going.

Answer 3

When you run your clothes dryer or kitchen vent it sucks air from the house and it creates negative air pressure inside the house. If the air pressure inside the house is lower than the air pressure outside, air can travel backwards down the flue and and suck the smoke back into your house. To solve the problem, open a window before you use you dryer or kitchen vent. The air will now come back through the window instead of the fireplace.

Answer 4

The central heat and A/C pulls air from the whole house circulating it throughout the house this by itself isn't so much the problem but the dryer vent and stove exhaust fans push air out of the house thus causing a low pressure situation that needs make-up air . You may just need a little more make-up vent or contact a reputable fireplace dealer for an updated sealing system . or chimney sweep.
When the firplace is used there needs to be some sort of fresh air comming into the house to make-up for what goes out the flue. also to keep the carbon monoxide under control. contact a reputable fireplace dealer

Answer 5

Im guessing that the Clothes dryer and the kitchen vent are drawing out more air than your flex tube is letting in from the basement.

If you're in a small home, that is tightly sealed, the problems are exagerrated.

I'm guessing you have Tyvek around your house and vapor barriers. Also, your home is never drafty.

When your vents kick in(while your furnace is on) they start drawing more air than your house and flex tube allow in.....so it goes to the fireplace. With your flue closed, the air is drawing at a higher velocity(because its travelling through a tighter space)...thereby creating a more turbulant air flow(thus, stirring up smoke in the firebox)

Try opening your flue and then turning on your furnace and vent(Im guessing you'll get the air flow, but less smoke.)

Your house has to have some air leakage(for the obvious reasons you've mentioned) The only solution is to let more air leakage into your house(from a better source than the fireplace)

Maybe rip out some weatherproofing, crack a window, or scab in another tube upstairs.

Answer 6

more air is leaving the house via broken/missing/leaking ductwork than is coming back into the returns so that air is now coming back into the HVAC system via the only open hole-the chimney.

you have leaking supply ductwork.
you can either find all the leaks and seal perfectly the supply part of the system.
OR
you can put in a make-up air duct with damper that goes to some outside air source.
you will be losing efficiency by constantly heating/cooling new outside air but you will no longer be making up the negative pressure by chimney smoke/air.

Answer 7

AC pushes air into a room,, dryers try to force air out of the tub via a tube to the outside! you may have a hole in the vent tube which sucks the air from the room and pushes it out the dryers vent!! the dryer then is causing a vaccum in the roomwhich is being fed by the fireplace back drafting

Answer 8

make a bigger "make-up" air duct. it shows that the one u r having is still not sufficient for the job (ie air being sucked out of the house is faster than being brought in). or u can put a suction fan on the make-up airduct to bring in the make-up air faster. this way u can still maintain the air-duct size.

in any normal house, when there is a negative pressure in the house, air is suppose to come in from under the door and through window seals, etc. yours comes in through the damper in the chimney... so go n get the #&$%^$^% damper replaced/fixed.

cheers......

Answer 9

one has to calculate the air flow for each stuffs. it is due to the vacuum created in the room due to the other fans. you can prevent it by a damper on the fireplace fan. the flex ducting is not making up enought air. if this duct damper is not working, find a motor for the main fan with a break. there is motors with break, when the powersupply cuts, the break engages.

Answer 10

Fireplace Backdraft