Smokey Fireplace

[jma24]

I had been thinking to turn this fireplace into a high-efficiency unit, but it’s looking too hard and too expensive - will have to wait for another time.

So, I decided to try to get this fireplace to work correctly. Last time we used it, it smoked up the whole house and the fire brigade came out. Advise is very much appreciate.

It is an older Pre-fab unit, probably from the 70s or 80s, a small fireplace, 29.5” wide.

The first thing I did was to install a chimney cap (there wasn’t one). I chose a Vacu-Stack unit, and installed it. It seems to have solved the downdraft problem I had, which is great.



However I’m not out of the woods, the fireplace still leaks some smoke, rolling up the front top lip.

I’m burning it now, it’s hot, burning at 800 degrees, burning dry pin oak, dampers are open. Should be pretty ideal conditions.



I thought there was an OAK, but it looks like it’s actually just a hole from the outside into my basement ceiling, probably the remnants of a basement clothes dryer vent.

I’m guessing this is a problem that the previous owners had, because the HVAC return vents in this room are blocked.



Still, I am getting some smoke roll off the top of the firebox and into the room.



I don’t think it’s a negative pressure issue - cracking a door helps solve the problem when the fire is starting but not once the chimney is hot.

When you look at the front of the fireplace, the firebox is actually below the top of the arch. Could it be that installing a simple Smoke Guard would solve my problem? I was thinking of a 4” sheet metal between the masonry and the firebox, to square off the opening.

 

[begreen]

Assuming the flue is clean, it sounds like this may be negative pressure. Check upstairs for any air leaks to outdoors. Recessed ceiling light cans, leaky attic door, attic fan ceiling vent, leaky windows, exhaust fans, etc. are all possible causes.

 

[jma24]

Assuming the flue is clean, it sounds like this may be negative pressure. Check upstairs for any air leaks to outdoors. Recessed ceiling light cans, leaky attic door, attic fan ceiling vent, leaky windows, exhaust fans, etc. are all possible causes.

Thanks for the reply. Interesting. When I open the door, air does flood in fast.

I did a lot of work sealing the top floor ceiling to the attic to improve insulation. There is also a whole house fan which whilst turned off, isn’t sealed. And the hole in the basement where the dryer used to be.

It won’t do any harm to go around upstairs with a tube of mastic and a smoke stick, and also seal the whole house fan for winter. This would also save money on heating, so win-win.

In the meantime I installed a temporary ghetto smoke guard and it has solved the problem. Curiously it is very sooty around the right side of the fireplace. Not so much around the left.

 

[jma24]

Assuming the flue is clean, it sounds like this may be negative pressure. Check upstairs for any air leaks to outdoors. Recessed ceiling light cans, leaky attic door, attic fan ceiling vent, leaky windows, exhaust fans, etc. are all possible causes.

You may be right. I sealed the whole house fan today, which was the main cause of leaks. The house is noticeably less drafty now, it’s quite dramatic.

I also tested for pressure using an incense stick. There are 3 interior doors to this room.

Door 1: From the 2nd floor, via steep stairs. Strong positive pressure, like a waterfall of cold air.
Door 2: To the basement, via steep stairs. Strong negative pressure.
Door 3: To the main house, same floor. Strong negative pressure.
Chimney: Strong downdraft when cold.

Also note there is a main staircase in the main house. The house seems to cycle air up the main staircase and down the smaller staircase by Door 1.

Now I’ve closed doors 2 and 3 and left door 1 open. After a few minutes I get a roaring fire, over 900 degrees instead of 700 yesterday.

Tomorrow I will test removing my ghetto smoke guard. Maybe it is not necessary?


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[Simonkenton]

I know a little bit about fireplaces because I built the fireplace pictured at left.

There is a 1 to 10 rule used when building a fireplace. The opening of the fireplace must be about 10 times greater, than the area of the flue.

If your fireplace opening is 30 x 30 inches, 900 inches total, then the flue must be about 90 inches in area. A flue that was 10 by 10 inches on the inside would be just right because if the flue is a little too large, that is fine.

I often see this rule violated and the result is, a smoky and unusable fireplace.

So what is the size of your fireplace opening? I see your flue is a pipe, what is the area of the pipe? You need your Geometry lessons from high school, pi R squared.

My daughter in law spent big bucks to get her 1960 fireplace rebuilt including a metal, round flue pipe. Spent big bucks and the damn thing smoked. I went up on the roof and measured the diameter of the flue. She was doing about a 1 to 12 or a 1 to 13. Flue was too small.
Weird to me because, this guy was highly recommended. He did this all the time and he routinely rebuilt fireplaces that smoked up the living room.

Interestingly, the original fireplace from 1960 followed the 1 to 10 rule just about to the square inch.

So I told Monica to reduce the size of the fireplace opening by 5 inches. She went to a wood stove store in Atlanta and they had a decorative metal piece that was designed to do just that, she installed that at the top of the fireplace opening and the thing quit smoking.

 

[jma24]

There is a 1 to 10 rule used when building a fireplace. The opening of the fireplace must be about 10 times greater, than the area of the flue.

If your fireplace opening is 30 x 30 inches, 900 inches total, then the flue must be about 90 inches in area. A flue that was 10 by 10 inches on the inside would be just right because if the flue is a little too large, that is fine.

Great question. The flue is 8” diameter, so approx 50sqin.

The opening is 29.5” wide and 17-21” high, so approx 560sqin.

My ghetto smoke guard reduces the height to 17in even, or close to 500sqin.

Is it possible that this small difference is the cause?


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[begreen]

As you are discovering there can be a multitude of causes, not just one. You're on the right track. Sealing things up will waste less heat all the time and it will help with the fire.

 

[jma24]

As you are discovering there can be a multitude of causes, not just one. You're on the right track. Sealing things up will waste less heat all the time and it will help with the fire.

Thanks for the encouragement. It’s 27 degrees outside. The thermostat was at 70 and changed to 65 an hour ago.

Before today it would be 65 by now and the furnace would be firing. It’s 68, and the house is silent. Insulating the fan has made an incredible difference.

I was told over and over that this fireplace would never draft. Mostly by “experts” trying to sell me a new one.

A huge thank you to the folks on this forum who replied and the others who wrote threads I read.


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[Simonkenton]

You were breaking the 1 to 10 rule and were about 1 to 11. So yes I think it will make a difference.