Cat stove chimney question

[EatenByLimestone]

One of the wonderful things about cat stoves is the slow steady burn you can get. How low you can go seems to be related to keeping the chimney warm enough to keep drafting and keeping the cat warm enough to stay engaged.

Would these stoves be able perform better in this role with a smaller diameter chimney? Say a 4" pipe? The chimney would stay warmer as there's less volume, and more heat should be kept in the stove.

The first thing I ask when I ponder a question like this is if the idea has merit, why aren't there any companies doing it? My first thought I thought is availability. We've all seen the marine stove guys having an issue finding pipe. I dont think pellet pipe is rated for wood stoves, but the equipment exists to make the pipe, if there was a reason to make it, a company would step up to do it. The other thing I can see, is fly ash, creosote, etc could impact performance and safety much faster. The volume of a small pipe gets remarkably smaller with 1/4" of ash on the walls. A counter to this is the stack velocity is much higher so you really shouldn't have much accumulation. Would there be trouble getting the cat hot enough with the smaller stack to get lightoff?

So what says the brain trust?

 

[bholler]

One of the wonderful things about cat stoves is the slow steady burn you can get. How low you can go seems to be related to keeping the chimney warm enough to keep drafting and keeping the cat warm enough to stay engaged.

Would these stoves be able perform better in this role with a smaller diameter chimney? Say a 4" pipe? The chimney would stay warmer as there's less volume, and more heat should be kept in the stove.

The first thing I ask when I ponder a question like this is if the idea has merit, why aren't there any companies doing it? My first thought I thought is availability. We've all seen the marine stove guys having an issue finding pipe. I dont think pellet pipe is rated for wood stoves, but the equipment exists to make the pipe, if there was a reason to make it, a company would step up to do it. The other thing I can see, is fly ash, creosote, etc could impact performance and safety much faster. The volume of a small pipe gets remarkably smaller with 1/4" of ash on the walls. A counter to this is the stack velocity is much higher so you really shouldn't have much accumulation. Would there be trouble getting the cat hot enough with the smaller stack to get lightoff?

So what says the brain trust?

If all you were ever doing it may work well on 4". But for startup and running higher you just wouldn't have enough volume. Availability is not an issue there is already stove pipe and liners readily available and chimney pipe is available custom order from some manufacturers if there was a demand it would be readily available as well

 

[Ashful]

If all you were ever doing it may work well on 4". But for startup and running higher you just wouldn't have enough volume.

This. We don’t always run low, and we also need to be able to open our door for loading! Some folks with larger doors already have smoke spillage on a 6” pipe, if they don’t warm it sufficiently first... image if the pipe cross section were shrunk by 56%!

 

[Highbeam]

The king needs an 8” pipe. With a 6” pipe people report insufficient draft on overnight low burns to prevent creosote smell. That’s what I was told by bk when I inquired about the king or even the 7” flue apex furnace on the 6” flue in my shop. Running these 7-8” flue stoves on a 6” is analogous to a 6” stove on a 4” chimney.

Why does this happen? It has to be low draft strength at low flue temperatures. The flue diameter is like the ram size on a splitter. For a given pressure, the larger diameter pipe pulls harder at all temperatures.

I still don’t understand why the king needs an 8” stack. It’s just 4.5 cubes. I don’t think it’s because of volume requirements at high temps, I think it’s about draft at low temps. The larger noncat wood furnaces only need a 6”.

Good question. The above ramblings are my theories only.

 

[Ashful]

I spot a problem with your theory, though. Since most know very basic circuit equations (eg. V = i * R) better than fluid dynamics, let’s think of the height of your chimney as the potential (voltage), and the diameter of the pipe as your conductivity (1/R). At very low air flow (very low current), as we would have at a low burn rate, the resistance (1/ pipe diameter) doesn’t matter much. You will still have the same static pressure.

It is only when flow is sufficient to produce noticeable pressure drop, that pipe diameter comes into play. Noticeable is the operative word here, we are working on very small pressure differentials (~0.05” water column). The way we increase flow is to open our air inlet, or open our loading door.

Those of us running a barometer (eg. Magnehelic) and key damper can see evidence of this. Adjusting the key damper at a high burn rate (or with the loading door open) will have an enormous effect on pressure reading, and much less so at a lower burn rate.

 

[EatenByLimestone]

This. We don’t always run low, and we also need to be able to open our door for loading! Some folks with larger doors already have smoke spillage on a 6” pipe, if they don’t warm it sufficiently first... image if the pipe cross section were shrunk by 56%!

See, here's where I get hung up a bit. With a 6" pipe, it sometimes takes a bit to sufficiently warm the air to get the draft moving. The volume of a small flue would get warmed up faster. It should flow just fine once up to temp. And should keep flowing long after the air in a 6 or 8" pipe stagnates and stops pulling.


What makes 6", or 8" the magic flue diameters? I know there is an optimum flue size opening for a fireplace of a given size. This makes sense as you have to be able to pull the exhaust out of the fireplace.

I'm not sure how this equates to woodstove design as our fires are more controlled. Except for the times we reload, our air intakes are pretty small.


I guess a question that might need to be asked, is where does a company that designs a woodstove start? We often state the chimney is the engine of the woodstove. Does a company start with the premise of "We want to design a woodstove that will run well off a 6" chimney." Or do they make a pretty box and throw a 6" collar on the top?

 

[Ashful]

See, here's where I get hung up a bit. With a 6" pipe, it sometimes takes a bit to sufficiently warm the air to get the draft moving. The volume of a small flue would get warmed up faster. It should flow just fine once up to temp. And should keep flowing long after the air in a 6 or 8" pipe stagnates and stops pulling.

Remember, because outside air is colder than inside air during stove season, the pressure differential from the top of your chimney to the bottom is much greater outdoors, than it is indoors. This has the effect of forcing air from outside into your home at the lowest floors, and drawing it out at the upper floors, and this is the very effect that makes your chimney work. In fact, this is why they call it the stack effect.

The amount of air your chimney can move is a product (as in multiplication) of this static pressure and your chimney cross section. Just like circuit theory i = V*G.

The warming of your chimney serves to amplify this effect, due to increasing the differential between inside and outside, but it is not the cause of it. A chimney should still draft without added heat, unless you have a specific problem that makes you prone to draft reversal effects.

 

[Highbeam]

Deep. Good explanation ash.

 

[Highbeam]

The warming of your chimney serves to amplify this effect, due to increasing the differential between inside and outside, but it is not the cause of it. A chimney should still draft without added heat, unless you have a specific problem that makes you prone to draft reversal effects.

Unless it's the same temperature inside as outside. Admittedly unlikely in the winter but in that case the density and weight of the air column is the same inside and outside of the chimney and no draft would occur. I get this in the shop sometimes, need to spike the chimney column with some heat to lower density and power the draft.