Replies

1. 
   David_Thomas | Jan 29, 2002 10:44pm | #1

   *
   Chimneys can be too narrow, too wide, or too short. They can not be too tall.

   Too narrow is a constriction (like too small a water pipe) and if the draft is not sufficient to overcome the back pressure of those frictional losses, then not all the air goes up the chimney. i.e. smoke fills the room.

   If the chimney is too large in diameter, the hot air can "get lost" in that large volume and not get the flue gases hot enough to create good draft. This is uncommon.

   Chimneys can easily be too short. It is the height of the chimney (and the temperature - flue gases versus outside air temperasures) that create draft. Draft is the difference in pressure between the chimney and the room. Draft is the driving force for all the fire's gases plus some room air to be drawn into the chimney. Not enough draft means not all the fire's gases will go up the chimney.

   Too short a chimney would be my best guess if the problem fireplace are in a one-story portion of the house or on the top floor.

   Additional note: Draft, being derived from a temperature difference, also depends on the outside air temperature. Problem fireplaces smoke the most on days in the 50s or 60s (degrees F). The same fire in the same fireplace on a 10F day would be fine. And why have a fire on a 60F day anyway?

   Quick, temporary and ugly fix: Tape some aluminum foil across the top of the problem fireplaces, lowering the top of the opening by 4 to 8 inches. This can help a marginal fireplace while you consider better fixes. Also can help for those fires beign lit during warm weather.

   Homework assignment (a bit of a science project): Extend the chimneys temporarily with some metal ductwork of the same diameter. 3 or 4 more feet can make the difference. Keep notes on when there is and is not a problem. Especially outside air temperaures and wind direction and approximate velocity. Which segues to:

   If the chimneys terminate too close to the peak of the roof, the draft can be reduced by eddies of air caused by the wind causing downdrafts in the lee of the peak (downwind side).

   The no-brainer is to install all chimneys to a height above any roof line. But if desperate to save a few bucks and willing to take a chance on poor draft during windy weather, there are some guidelines like 1 foot below the peak for every 10 feet (I think) of horizonital distance from the peak. This is too avoid wind-caused downdraft problems. Such a chimney on shallow-pitch single story ranch may still be too short.

   Did anyone combine the flues of multiple fireplaces? If so, repost with that info. That is a whole other can of worms. -David

1. 
   Cloud_Hidden | Jan 30, 2002 02:24am | #2

   *2' above the level of the roof 10' away from the chimney.

1. 
   tom_trout | Jan 30, 2002 07:37am | #3

   *On this smoking fireplaces situation. I think the problem is in that all the dampers for the fireplaces need to be closed in order to burn one. There is lack of draft because there is too much open flue, which causes loss of pressure. Tom Trout

1. 
   Bob_Walker | Jan 30, 2002 06:08pm | #4

   *The 2' / 10' is a rule of thumb.I'll bet the house has a nice simple roof line, right?Actually, a fireplace in a wooded area is likely to be problematic, and a complicated roof line can wreak havoc with fireplace drafting.As noted above, experiment with firebox opening size and flue height, but don't expect to be able to cure them all. (Question, is someone who builds a very large house with 10 fireplaces willing to hear and accept that some of the fireplace problems might not be curable?)

1. 
   Cloud_Hidden | Jan 30, 2002 09:00pm | #5

   *>The 2' / 10' is a rule of thumb.Bob, it's in my code book as minimum requirement. Is that different than being a rule of thumb?Jim

2. 
   David_Thomas | Jan 31, 2002 12:38am | #6

   *Jim and Bob: Thanks for the correction. I couldn't remember if it was 1' drop / 10' horizontal or 1' / 5' (which it is). But taller is always better.I too was wondering about 10 fireplaces in one house. And if flues were combined, then Tom is right - close those other dampers. Also, hang signs on the dampers to indicate that they are closed. No fireplace smokes up a room faster than one with a closed damper. -David

3. 
   Bob_Walker | Jan 31, 2002 04:47am | #7

   *H-CloudYou're right, it is a code requirement: IMHO it should be considered a minimum.I put that very badly in my firsdt post

4. 
   jcallahan | Jan 31, 2002 05:02am | #8

   *Could it be possible that the house is too "tight"? I've heard of of a case where the windows in a house had to be cracked in order for the fireplace to draw. Do these fireplaces use outside air? Just a thought.

5. 
   Joe_Ashcraft | Jan 31, 2002 09:39am | #9

   *Tom, that is a situation that I never thought about, I will address that with the homeowners. Thanks Joe

6. 
   Joe_Ashcraft | Jan 31, 2002 09:41am | #10

   *Bob, Those are very good points. The roofline is fairly simple with a main ridge and some dormers. The owners are looking to fix the problem so they can at least use some of the fireplaces. Thanks Joe

7. 
   Joe_Ashcraft | Jan 31, 2002 09:43am | #11

   *David, Thanks for the info. That is an excellent idea about the signs. Thanks for the useful information. Joe

8. 
   Joe_Ashcraft | Jan 31, 2002 09:44am | #12

   *The house is very tight, but the fireplaces have their own fresh air intake, but they may not be big enough to accommodate the size of the fireplaces. Thanks Joe

9. 
   calvin_ | Feb 01, 2002 04:27am | #13

   *Joe, make a note to come back with the answer to the problem so we can all learn from it, OK? TIA

10. 
    Joe_Ashcraft | Feb 01, 2002 08:38am | #14

    *Calvin, Yes I will make a note and hopefully learn something. Joe

11. 
    tom_trout | Feb 02, 2002 12:24pm | #15

    *Joe, Close all outside air (make up combustion air) and close all dampers to all fireplaces, then pick the fireplace most likely to burn. The one with the longest flue and I hope it is at least 18 feet tall and open its damper and not the outside makeup air and build a top down burning style fire. Fuel on the bottom and kindling and paper on top and it burns like a candle. Make sure there is no negative pressure in the house, Exhaust fans, dryers or any openings and make sure that the home heating system fan is off. Also it must be a temp. difference between the inside and outside and the greater the better. Without building a fire there should be a draft in this selected fireplace which you can feel. The warm air in the house is being forced out due to the difference of pressure between the outside and inside, the taller the house the greater the pressure the higher you go. With this draft established build a fire and play with the makeup air, probibily no difference. Rumfords are problematic when adding too much fuel to too cold a fire, so put enough wood in for a good burn. This is one way to test each fireplace. While this one is burning open up other fireplaces and check them for draft, once noticed close them until all have been inspected. Hopefully, all fireplaces have their own flue and dampers. Happy burning comes from using dry wood only.

12. 
    jonmarden | Feb 02, 2002 06:37pm | #16

    *is this by chance a tudour style home? did they put decorative clay chimney pots on top? once came across a similar situation with 3 rumfords in one house...the clay pots had choked down the flue dramatically and were having the same effect....I've built about a dozen or more rumfords to date and never had a smoking issue arise

13. 
    Tim_Rice | Feb 06, 2002 03:56pm | #17

    *Bob,You said "Actually, a fireplace in a wooded area is likely to be problematic..."How so? I live on a heavily wooded lot, have burned 1-1/2 to 2 cords of wood a winter for the last 4 years. The only times I have problems is the occasional downdraft in high winds and with a dying fire in warm (>35) weather. The house has 3 FPs, only one gets used.

14. 
    David_Thomas | Feb 06, 2002 08:55pm | #18

    *Tim: I'll rephrase Bob's thought as, "a MARGAINAL fireplace in a wooded area is MORE likely to be problematic". Sounds like you've got properly sized flues and good height on your chimneys. And/or the distance and height to the trees is such that the house is not in the downdraft of the forest edge.From computer air modelling I have done, downdraft and eddie effect can potentially extend a distance 5 times the height of the obstacle. The downdraft can reduce chimney effectiveness (probably only noticeable at a much closer distance) and the eddie can entrain smoke in the downwind wake of the obstacle. So having the BBQ in the lee of the house can be worse than if it were in free air.When campfire smoke keeps shifting, you are seeing eddy effects. The air flow within the eddy is turbulent, chaotic, and keeps shifting. Consider that the windsock at the airport never whips around like the campfire smoke in a forest does. -David

15. 
    Luka_ | Feb 06, 2002 08:59pm | #19

    *i When campfire smoke keeps shifting, you are seeing eddy effects.I usualy just tell eddy to stand somewheres else.

16. 
    Bob_Walker | Feb 07, 2002 03:25am | #20

    *David said it a lot more clearly than I and with much more authority!

17. 
    Thomas_Quinn | Feb 20, 2002 01:13am | #21

    *Smoking Fireplace - Our house is located on a steep slope about 40' below the crest of a high hill. Tree cover is limited and the wind is often strong. We put a fireplace in the kitchen. It has a 28" firebox mounted on a 14" pedestal that stands against an outside wall. The chimney is a metal pipe that extends about 12' above the roof. The down draft from the chimney, forcing smoke into the room, has been a constant problem. We've tried enclosing the firebox with glass and adjusting the air vents. Nothing works. We tried installing a gas burning unit and it was a disaster. Same problem and the soot was filthy, requiring scrubbing and re painting the room. We looked at increasing the length of pipe but were told we needed another 15' and that looks pretty bad. So we're now considering demo-ing the existing fireplace and building a large, in the wall, brick, firebox and chimney that will extend to the required height. How do we know this will work and how do we protect ourselves from spending a lot more money and ending up with a fireplace that still won't draw smoke correctly?

18. 
    Bob_Walker | Feb 20, 2002 08:14am | #22

    *"How do we know this will work and how do we protect ourselves from spending a lot more money and ending up with a fireplace that still won't draw smoke correctly?"Easy, find a mason who will guarantee that it'kk work.When you find one who'll give that guarantee, cross off of your eligible list.+++++++++++++++++++++++++++++++++The next statement is true.The previous statement is false.

19. 
    FredB | Feb 20, 2002 08:47pm | #23

    *Well.....How did you fix this? My unstated guess was that the answer was either too short chimneys, or insufficient combustion air to the malfunctioning fireplaces. Didn't chime in because y'all were covering very nicely thank you.But What was the actual reason?