Poor Draw in Fireplace


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Old 10-20-13, 05:43 PM
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Poor Draw in Fireplace

Hey Guys,

Just moved into a new house and I'm realizing that the draw on the fireplace is less than ideal. The fireplace is on an exterior wall of a room with cathedral ceiling. The chimney has two stacks, one for the fireplace and the other for the furnace exhaust. There is a cap on the liner as it exits the crown. The chimney is about 4 feet above the roofline. The firebox has a cleanout leading to a chamber in the chimney in the basement.

During my first two fires, I've noticed some smoke escaping the firebox and entering the room. This persists even after the chimney is warm. The house smells like smoke the day after the fire. I should also mention that the damper is fully open.

There is not much remedy of I open a window in the room.

The chimney was recently cleaned when we purchased the house.

Any suggestions? Should I remove the cap?

All help is appreciated.
 
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Old 10-21-13, 05:13 AM
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It's hard to say what's wrong without seeing your fireplace and/or knowing the dimensions of everything or knowing anything about the cap. Fireplaces are a very old and well understood technology and many books have been written about fireplace design and what it takes to draw properly without wasting too much heat. I would measure the fireplace & chimney and start researching to find out if it's a potentially good design or a bad one. Some building codes provide minimal direction for how to construct a fireplaces so there is a lot of room for "creativity" by the builder and sometimes they are built more for aesthetics than function.
 
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Old 10-21-13, 05:15 PM
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Thanks for the response. I will take some measurements tomorrow and post back with some photos. My initial thought is the firebox is too large for the flue. When I close the glass doors with the bottom vent open things seem to draw much better. It's probably an sir velocity issue.

I'm sure you guys will have more to add...

Thanks.

Bryan
 
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Old 10-22-13, 07:43 AM
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Years ago I picked up a 100+ year old book on fireplace design. I was taken by how scientific fireplace design was over 100 years ago. The book was full of diagrams and formulas for various designs. All sorts of things impacted whether a fireplace would draw; firebox size, height to width to depth ratios, flue size & length... A lot goes into it. I suppose back when your cooking & heating was done with a fire you worked hard to get it right. These days fireplaces are little more than selling points to a realtor.
 
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Old 10-22-13, 09:52 AM
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Hi Bryan,
I live in cold country and have had a fireplace or wood stove most of my life, but only recently have I learned how one really works. But before I risk confusing you, let's look for other problems. Are you sure the chimney is not obstructed? Is the cleanout door in the basement closed and reasonable air tight? I assume it is a masonry chimney, how large (diameter)?

Now the how. Chimneys do not draft as we have been led to understand. Warm air and cold air (all fluids) are subject to gravity and being pulled down. Cold air is heavier than warm air so when they compete for space, the cold air moves to the bottom and pushes the warm air up.

During heating season your house becomes slightly pressurized with the cold outside air pushing in the lower leaks (or windows) and displacing the warm air inside forcing it up and out the upper leaks. Introduce a fireplace/chimney and its location between the high exfiltration and the low infiltration becomes important. Leave the damper open on a cold day and you will probably experience a cold draft coming in from that outside chimney. Note, if opening a window is going to help, it should be the lowest window in the house.

Pictures will help, but part of the problem can be the front of the fireplace. Does it hold the warm air in the chamber above the fire and below the (hopefully) lighter column of warm air in the chimney? Or, does the cold air pushing on the fire cavity result is some going up while some spills out to the front?

I'll wait for pictures, but winter has just started and two things will change. The chimney will get colder, but your furnace will run more often, which may at times help to keep those gasses hot and improve the flow up and out.

Bud
 
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Old 10-25-13, 08:41 AM
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Hey Guys,

Thanks for the answers thus far. I have some measurements and I think I may have narrowed things down a bit.

First, the opening of the firebox is 42x27.5=1155 inch square.
The flue as measured while standing on the ridge was 8.5x12.5 outside dimension and 6.75x11 inside dimension. Again, I was on a steep roof and didn't take the time to be super precise. So, I calculate a working area for the flue to be about 74 inch square.

Opening to flue ratio = 1155/74=~15. Seems too high, or that my flue is undersized for the size of the firebox?

The other measurements I noted were a total of 17.5 feet from the bottom of the firebox to the top of the flue.

I have just about 2 feet of chimney above the ridgeline, but the flue is off center and is about 3.5 feet above the adjacent roofline.

I do have a cleanout in the basement, and when I looked at it yesterday I did notice that it was not very tightly secured and could probably use some caulking around the edges. I'll look into doing this today.

No obstructions. Recently swept. The damper is fully open though its ability to open fully is limited by the control arm. I may pull the cotter pin and see if I can get it to open up a bit more if not hindered by the control arm.

I don't feel cold air falling through the chimney into the firebox when there is no fire, and in fact the air in the firebox feels close to ambient temp inside the house (I assume the ambers are totally extinguished from last nights fire).

All this said, what do you all think?

Thanks,

Bryan
 
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Old 10-25-13, 09:40 AM
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One sorta easy test to see if the flue is too small for the firebox opening is to install a steel smoke guard at the top of the opening. The guard closes off part of the face making the opening smaller. Since it hangs down from the top of the opening it also has a dam effect to help prevent smoke from rolling out. Glass doors or some other front for the fireplace is another option. Even if the doors are open when in use the frame usually reduces the open area of the face a good bit.

How old is your house? Is it tightly sealed & energy efficient? If the house is sealed well there may not be enough air entering the house to allow smoke to go up the chimney. It's pretty easy to test by opening a door or window a few inches and see if the draw improves. Don't open a window right next to the fireplace though. You don't want to create room drafts that could affect the fire.

I assume your fireplace has a damper. Have you seen it? Does it open all the way? If the damper restricts the size of the flue or if it does not open all the way this can restrict the flow up the chimney and cause a draw problem.

How deep is your fire box (front to back)? If the fireplace is too shallow it can cause problems. In my town the mills built worker villages and most have very shallow Rumford style fireboxes intended to house iron grates for coal. Lighting a wood fire in one almost guarantees filling the room with smoke.
 
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Old 10-27-13, 10:47 AM
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Working chimney

A chimney must be no shorter then 16 feet from the highest opening to the top of the chimney.
It works best if the chimney area is less than 8 x 8 inches.
If there is a source of make up air that is at least the same area as the chimney and preferably below or behind the fire.
For best results the chimney should be positioned in the center of the house, should exit at the top of the roof, and project 3 feet above the ridge. A warm chimney that is well insulated will work better then a cold chimney.
When lighting the fire, keep in mind that the warm air has to lift a plug of cold air that is 16 feet or more in height out of the top of the chimney and it has to warm the chimney before it will start to draw correctly. It will probably help to have a window open while the fire is starting, this will avoid smoke entering the room.
 
 

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