Flue size?

[GAH]

We just had a masonry fireplace built as part of a screened in porch. We used it this season for the first time and have been getting smoke back into the porch and build up on the face of the fireplace. The box is 39(w)x29(h)x25(d). We had three fireplace companies come out to clean and they all said that the flue is too small. Looks like it is about 12.5x8 but with the tiles the flue opening is 11x6.5.  Is the flue too small? Would a stainless steel liner help instead of tiles? Should we have the contractor rebuild from the box up? Any other suggestions? Thanks!!!!

[Irondragon Forge ClayWorks]

Welcome from the Ozark Mountains.

I would say the flue is too small and a liner would only make it smaller. A flue for a solid fuel forge is totally different than one for a fireplace but the larger the flue opening the better the draft will be. Not knowing where in the world you are located, it will be hard to give advice, but here I would have the contractor rebuild it to a minimum of 21X12 flue with a weather head at the top that wouldn't restrict the flow. Our free standing fireplace has a 12 in. round flue & chimney and it draws very good, never a hint of smoke in the room.

I can't control the wind, all I can do is adjust my sail’s.
Semper Paratus

 

[The Jeff]

Acording to Google, rule of thumb your flue opening should be no less then 1/8 the area of the opening of your fireplace.

Opening  39'' x 29'' -     3.25' x 2.417'= 7.855'.    1/8 of 7.855' is .981'.

Flue 11'' x 6.5'' -   .917' x .54' = .495

You need a bigger flue.  Like an 8.5'' x 18''  or 12'' x 12'' inside diameter

Also height makes a difference on draft. I don't know what your height is but it needs to be atleast 3' above the roof. Look up 3-2-10 rule.

 

[George N. M.]

Also, does your fireplace have a smoke shelf?  If you don't know what it is, it is a horizontal shelf in the back of the fireplace at the start of the chimney.  As the hot gases go up the chmney a vortex (roller) forms on the smoke shelf and increases the draft.  Years ago I had a girlfriend whose fireplace was not drawing.  I crawled up into it (fun job) and discovered that the smoke shelf had a build up of ash and dirt on it.  I cleaned it out (another fun project) and the fireplace drew much better.

It sounds to me that the contractor is going to have to pull out the existing chimney and rebuild it with an appropriately sized chimney compar4d to the fire box.

As mentioned previously the chimney needs to be higher (probably 3-4') than the highest point of the roof so that wind across the roof will not blow down into the chimney and dreate a back draft.

"By hammer and hand all arts do stand."

[Frosty]

If you live in the USA there are building codes, fireplaces fall under Federal regulations. If the contractor didn't get it right he may not be certified to do the work. Which would maybe mean having it removed and built by someone who knows what they're doing. Then again you may be getting odd air eddies you can't even feel on the porch. 

Getting stoves to draw properly can be tricky but fireplaces can be a real mother bear.

Frosty The Lucky.

[The Jeff]

Like Frosty said, you could be having issue with eddies. Depending on if your chimeny is on the leeward side and how wind catches your roof etc.

Elevation also affects chimney draft, higher elevations require taller chimneys.