Duct fan vs. natural convection...does this make sense?

[mgh-pa]

I was experimenting with better ways to move heat to my main floor from the basement below where the woodstove is. On the opposite side of the basement from the woodstove is our stairwell, and a year ago (after checking with my insurance agent, and local codes officials), I cut a 16" x 10" opening in the floor and installed a vent above and little over from the stoves location. This seemed to do alright in moving air (thermometer reads 92 degrees when stove is cooking pretty good, but normally in the mid to upper 80s). I decided to installed a 500cfm duct fan into the vent to see if I could move the air a little better. Long story short, after experimenting a little while, I don't see ANY improvement in air flow or temp increases in the main floor when compared to natural convection. Is this a product of possibly having a insufficient fan, or is it just...well...normal?

 

[mgh-pa]

*Sigh* I learned (on here) the opinions on this subject of cutting a hole through the floor is varied. If there is a damper with a fusible link, there is no issue (IMHO). The vent is cut, in place, and works. I have no worries or concerns with it in place as it is. I don't need to be told that this is not acceptable per codes (because both my insurance agent and the local codes office said I was good to go...if they're wrong, I suppose so be it).

As for the movement of air, your analogy makes sense to an extent. The stairwell leading down to the basement on the opposite side of the stove/vent would and does act as the cold air return, correct? A thermometer placed near the base of these stairs confirms this in my home (always cooler than at the vent side). I assume, you're saying that unless the fan/duct system is continuous, trying to force that volume of air through the convection cycle in such a large space is futile? That's what I was wondering.

 

[mgh-pa]

part of the problem is multifold positive pressure wants to gravitate to negative pressure, your hole creates another passage for that to happen since it is not a closed loop system
colder air at floor surface also wants to descend and will follow the path of least resistance that could mean being the vent hole too. It makes mechanical sense to remove the cold air to allow light warmer air to fill that space
I bet your system would work much better placing a small box fan on the floor at the stairway opening and forcing removing the cold air towards the heat scource

Makes sense. Good suggestion on trying the box fan. I may try that out and see how it works.

 

[LLigetfa]

First off, I would like to see how the fan was installed into the cavity. A duct fan works best in an actual duct where the flow is kept laminar. In the open, turbulence will diminish its capacity and some of the flow could short circuit.

The second issue would be with moving too much air too fast. It will dilute the temperature of the air and reduce the natural convection forces.

So, to answer your question, I think you have too much fan in the given situation.

I use a 650 CFM duct fan installed into a sealed housing that cannot short circuit. It draws stratified cold air off the floor of my crawlspace and blows it up through the floor above, directly across the firebox. The return air path complements the natural convection pattern flowing down the stairs. The hearthroom will always be a bit warmer because of the radiant heat but the airflow does convert much of it to convective heat.

 

[Hogwildz]

Did you try reversing the fan so that it pushes the cooler air from upstairs down to the stove area?