Why does smoke come in the house from a wood burning stove when starting it from coals?

I argue with my mom a little about some smoke comming in the house from a wood burning stove that sits away from the wall, and the pipe is connected on all sides, but for some reason smoke comes in rather than directly up the chimney. Why does this happen when the doors are open?

The coals are left over from the fire bank of the previous heating fire (which my mom doesn't believe is always in the back also).

Two reasons.

1). You may have creosote buildup in your chimney. That's not good. Creosote can cause very active chimney fires and destroy your chimney or even burn down your house. Every chimney needs to be cleaned every Fall. And should be checked a couple times every use season. You REALLY don't want creosote near any fire source.

2). The much simpler reason is that cold air is heavier than hot air. It takes a lot of "pushing" by the hot air to force the cold air up and out of the chimney. -Once it does, the chimney will warm up and draw the smoke up the chimney (assuming the chimney is clean). But at first the rising hot air of a small fire will have trouble pushing out the cold air. The remedy for cold chimneys is to burn something more highly combustible (paper/cardboard) at first in order to warm the chimney. But beware, flaming paper can sometimes be drawn up the chimney by the rising hot air. AGAIN, not good if you have creosote buildup.

P.S. By the way, if you do have a chimney fire one of the best ways to put it out is to (really quickly, run) soak paper in water, and throw the whole wet wad of paper on the fire. It'll make instant steam and put the fire out. If you just throw water on the fire, it may or may not make steam and you could extinguish the stove fire but still have a chimney fire. Plus, it's just a darn mess if you throw water into a stove or fireplace. Wet paper (or a well kept and handy) fire extinguisher will much better do the trick.

Here's a relevant thread - Smoke coming out woodstove door

I'd guess reading through that will probably answer your question. If not please elaborate.

Which way the smoke goes is all about airflow.  Where is the air flowing FROM and what is in the way of where it's flowing TO?  (I suspect many people forget the first part)

Depending on the age and style of the chimney, banking your fire is probably causing excess build-up which disrupts airflow.  Get it swept to test if this is the issue.  

I know one household that banks their fire and they have to get it swept twice a year (once in the summer and once just after the Holidays) as it creates so much build-up.  For our home, we have smaller, hotter fires and heat up objects around the woodstove (stones, extra bricks) as well as the air with heat-driven fans so we keep the heat in the house and don't have to have the fire on so often.  This way there's only a white powder left in the fire box at the end of the burn and the sweep always complains that our chimney is too clean, even if we wait a few years between sweeping.  

Another issue you mentioned is you closed up your house.  Most fireplaces are designed (yes even modern ones) to have some air intake like a gap under the door.  That way the air comes from somewhere to flow OUT the chimney.  If there's no where for the air to come FROM,  it will try to enter via the chimney - thus create back flow.  When we had our woodstoves upgraded, the airflow test showed the insulation was too good and we had to install a special hole in the wall to open up when we light the fire.  It's also important to reduce the dangers from carbon monoxide.  The nice thing about getting a custom hole instead of relying on the draft from doors and windows, is you can control where the air flows so it doesn't cool down the people.  

In the 3 room rock cabin my great uncle lived in, they opened the door (middle of winter - the Cold part of Ontario) for 5 to 10 min when starting the wood cooker so the air can flow in and then closed the door once the cooker was going strong.  They said this was because the flue was broken and there wasn't anyone who fixed 150-year-old stoves anymore.  So they opened the door to help the airflow.  

If it's an older chimney, there may be a flue that needs adjusting at the start (and closing again once the fire is going - think of it like a choke on a gas powered tool)

And of course, there's always airlock.  As described above, this is when the heavy, denser, cold air prevents the hot air from rising.  However, this seems least likely if you are still getting heat off the old fire.  To test this.  Crumple up one bit of paper and toss it in.  Light it,  Let it burn.  Repeat until the chimney is heated and the smoke is going in the right direction.

I lived in a house with a really steep roof pitch to help slide the snow off, and the chimney came out of the peak of the roof. Looking back I'd guess it had 40-50ft of chimney. It always back drafted horribly when starting it from scratch. What I figured out was the large volume of cold air in the chimney was falling down as the warm air from the fire started to rise up. My solution was to run an electric space heater in front of the stove door for about 10-15 minutes before starting the fire, and it completely solved the back drafting issue. Unless I was being impatient, and didn't give the space heater enough time to clear the cold air mass.

When you say you're starting from coals; are you starting from orange coals, or black coals? It would be strange for it to back draft if you're starting from orange coals. If it back drafts with orange coals I'd make extra sure I kept my carbon monoxide detector up to date, because they do expire.

I've never lived in a house tight enough to cause back draft problems, but I can see how a lack of airflow seeping in would cause a vacuum inside the house that would suck air down the chimney. I don't know how you'd get a space heater to work with a top loading wood stove, but it should be pretty easy to try a space heater with a side loading wood stove.

If you have a torch nearby and your stovepipe isn't surrounded by bricks or stone or whatever, you can run a flame along the naked stove-pipe to heat it up, re-starting the thermal siphon (warm air traveling upward) in the chimney.

If built-up creosote is a concern, I don't know how this method would affect things for sure but it seems like there may be less chance of a chimney fire started than if you primed the stovepipe with a small fire.

Cold air in the chimney stack can certainly be an issue. I start my morning fire from left over coals and had no problem until the recent below zero weather.
The stove smoked more than normal even once it was hot. That was not normal for the Fisher Mama Bear.
 I found out that overnight my spark guard on top of the flue had plugged up with ice and soot.  Once I removed the spark guard the stove was back to working just great. With 6" of powder snow on the ground and the cold temps it won't be needed until next fall.

Welcome to Permies, John Builderguy!

Good catch on the spark arrestor. These are really important, but they can indeed clog up. A lot of people don't even know they're in the chimney.

That's a lot of creosote though. Did you shine a spotlight down the chimney to check if it needs a sweep for safety?  

I did not look down into the flue but it needs to be done as soon as the roof is dry. Right now the wet metal is just like a sliding board and I also need to get a flue brush before I go back up there. Ultimately I need to insulate the 3' of chimney that projects above the roof. That will help keep the flue gases hot until they totally exit the structure.

Stephen is on to it,i was going to suggest a hair dryer to heat the pipe but a small propane torch will do it as well,heat that pipe and get the air flowing upward before putting anything that will create smoke in the stove.You can always check the air flow with a small flame such as a cigarette lighter or match in front of the air holes on the front of the stove,if the flame gets sucked into the stove put the wood or paper to it.On the other hand Johns suggestion should be the first thing to look at,if your not sure get up there and check it out to see if there is an obstruction.Have you got  a small drone or access to one from a  friend? Use that instead of a ladder!

John Builderguy wrote:I did not look down into the flue but it needs to be done as soon as the roof is dry. Right now the wet metal is just like a sliding board and I also need to get a flue brush before I go back up there. Ultimately I need to insulate the 3' of chimney that projects above the roof. That will help keep the flue gases hot until they totally exit the structure.

LOL I've done a controlled slide off metal roofs so I get it! Didn't care for it either.

That said, I don't know your setup, but porting a wood stove (hot) into a masonry chimney (cool) can build up some creosote fuel that might bite you in the hindquarters. I've seen a couple of close calls. Anyway, it's worth keeping an eye on.