Chimney Draft Vs Atmospheric Conditions

So, I have a little time to kill. I thought I would open up a discussion on how atmospheric conditions impact how well the chimney drafts or can keep a draft. This is a tangent off a different thread that I thought could use it's own. We can all agree (I reckon) that outside air temperature is the biggest influencing factor. When it's cold out the draft works good, when it's warm out - not so much.. What about humidity?

My buddy Scott got me thinkin when he said dry air is heavier than wet air (I hope you don't mind I mentioned you Scott, you have my regard as being one of the 'smart' fellas around here). I'm not real fluent in the gas laws department but I like science and am slowly catching on lol.

Avogadro's law states that equal volumes of gases at the same temperature and pressure contain an equal number of molecules. Nitrogen and oxygen make up a bulk of our atmosphere. Water vapor is part of that also. What's interesting about water vapor is that a molecule of H2O has a molecular weight of 18.02. N2 (atmospheric nitrogen) is 28.0134 and O2 (atmospheric oxygen) is 31.9988. So, when calculating the weight of a cubic foot of humid air and comparing it to a cubic foot of dry air (temp and pressure the same), the humid air will actually be lighter.... It's lighter because the water vapor displaces some of the nitrogen and oxygen molecules (Avogadro's law). Who'd a thought? I know, right?

So, how does this affect our chimneys? The components of our coal exhaust are mainly nitrogen (78%), carbon dioxide and unused oxygen.. Turns out that the molecular weight of CO2 is 44.01 which is heavier than all of them. A minor component Sulfur Dioxide (Sulphur Dioxide) is 64.06. After looking at these molecular weights its amazing our chimneys draft at all lol. But, our chimneys work since the air in them is lighter than the outside air because of temperature difference.

So here's were my speculation comes in. If we introduce an atmosphere that is humid (and lighter) then the bar is raised, when it comes to making the gases in the chimney lighter than the outside air to create draft. If we introduce an atmosphere that is dry and heavier, then the bar is lowered when it comes to making the gases in the chimney lighter so that they can rise. My conclusion on this is that a humid atmosphere actually favors chimney drafting .

Thoughts and comments welcome.

I am a sick man with a coal stove...

Edit -

My conclusion on this is that a humid atmosphere actually favors chimney drafting .

It's the other way around...

i noticed you reaching for answers in the other thread

All I know is my Manometer reads its lowest, which here is hardly ever under .05, right before a storm and just at the beginning of the storm, and more so if the wind is light of course. My Chubby Jr can drop down to .01-.02, and my other stove .03-.04. Those are normal readings for many here but I am usually in the .05-.06 range even with baro's set at lowest possible marks and I like how my stoves run at those settings.

I have a draft here any day of the year, which many here dont. No A/C in summer and usually have wind, both may help with that. So what are my observations worth?

Not scientific by any means, just observations, which I trust as much or more.

Think pressure differential Lee. The bigger the differential the better the chimney drafts. As atmospheric pressure drops, the density of the chimney gasses gets closer to ambient pressure for the day. Hence, the chimney develops less draft.

Remember when we used to go back and forth discussing temperatures?

Temperatures drive density and density (pressure) drives draft.

On a cold high pressure day, a hot air ballon with climb faster for a given bag density than an a cold, wet, low pressure day.

Make sense?

Lightning wrote:My conclusion on this is that a humid atmosphere actually favors chimney drafting .

I have this backwards don't I.. I can't believe I screwed that up lol

The dryer heavier air favors chimney drafting...

Yes, dryer seems more responsive (better draft).

As some have said, myself included, the stove seems more sluggish to respond on cold, very damp, cloudy days. On dry days at the same temperature it is much more responsive. Why so slow when it's cold and damp outside, I'm not sure of the "cause".

With 12 years of 9 months a year using the same stove - and not just for heating, but making many stove temperature changes during the day for cooking/baking, - I defiantly see a pattern. Some of those really damp days when it's starting to turn to fog near the freezing point, is like running the stove when it's over 75F outside.

What I want to start doing is check the stove's responsiveness to my barometer and see if there's any correlation.

Paul

You will absolutely see a correlation Paul.

scalabro wrote:Think pressure differential Lee. The bigger the differential the better the chimney drafts. As atmospheric pressure drops, the density of the chimney gasses gets closer to ambient pressure for the day. Hence, the chimney develops less draft.

This really makes good sense logically and barometric pressure was also something I was hoping to discuss. What about the pressure in the stove room? I assume It would peak and trough with the ambient pressure outside since the house isn't air tight.

scalabro wrote: As atmospheric pressure drops, the density of the chimney gasses gets closer to ambient pressure for the day. Hence, the chimney develops less draft.

This part I'm struggling with. Wouldn't the density of the chimney gases ebb and flow with the ambient pressure (density) outside to maintain the same differential?

So how would that work for a stove at sea level compared to a stove at 5000 feet where the pressure is substantially less?

warminmn wrote:All I know is my Manometer reads its lowest, which here is hardly ever under .05, right before a storm and just at the beginning of the storm,

Yes this jives with the humidity factor (after I discovered I had it backwards lol) since humidity makes the air lighter which ups the demand for the chimney to make it's gases even more lighter so they'll draft. I haven't noticed it very much since I have my baro set pretty low all the time. I was just searching for "the light of truth" (as franco b would say) in the science of it.

It also jives with the low pressure area effect that Scott talked about since storms and/or rainy weather are usually associated with low pressure.

Sunny Boy wrote:What I want to start doing is check the stove's responsiveness to my barometer and see if there's any correlation.

This is a great idea if you can get a different barometric pressure reading at the same AOT, humidity and winds along with the same stove circumstances to make an unbiased observation with. Not saying it can't be done, but ya know, all those variables going on again...

scalabro wrote:On a cold high pressure (less humid?) day, a hot air ballon with climb faster for a given bag density than an a cold, wet, low pressure day.

Are high pressure areas always associated with less humidity?

I'm not a meteorologist but I would say that yes, simply because the higher pressure air (Dry) weighs more and it's hard for water to condense in dry air. I think the driest air on earth occurs on Antarctica.

Will Sherrick is an aficionado of all thing meteorological, maybe he will chime in.

scalabro wrote:I'm not a meteorologist but I would say that yes, simply because the higher pressure air (Dry) weighs more and it's hard for water to condense in dry air. I think the driest air on earth occurs on Antarctica.

Will Sherrick is an aficionado of all thing meteorological, maybe he will chime in.

Few people know that,..... it's so dry all year-round that Antarctica actually qualifies as a desert.

Paul

Cool thanks yeah I was just trying to separate the humidity effect from the barometric pressure effect. The trouble is that they are so closely associated with each other, it makes it hard to prove which one has more or less influence on chimney draft ability.

Like in your example of the balloon, humidity has a huge effect both ways. The dry higher pressure atmosphere is heavy. The air in the balloon is also extremely humid because of the hydrocarbon propellant used to heat the balloon which adds to it being even lighter.

Hypothetical question. If we were to put a whole house with a running stove in a big box, then lowered the atmospheric pressure in the box by 1 psi, what would the chimney draft do?

Any thoughts to an answer are welcome.

Lightning wrote:Hypothetical question. If we were to put a whole house with a running stove in a big box, then lowered the atmospheric pressure in the box by 1 psi, what would the chimney draft do?

Any thoughts to an answer are welcome.

Is the box airtight?