Another moving heated air thread....

corey21 said:

Does it matter if the fan is blowing on the stove from a distance?

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Mine is in the hallway outside the stove room, a good 20' from the stove itself.

My floor plan has me thinking about this issue.
No central air handler, but i ducted off the peak of a cathedral
- horizontal run about 8' then down about 40" - for circulation with a duct booster.

Always hard to say how much air will be moved.
Hope it works for you.
If not, investigate a timer for the lowest/quietest speed on your air handler.

TradEddie, sounds like you are finding out what I did, pulling the cold air from the opposite end of the house from your stove and dump it into your stove room at a slower pace than your furnace blower. (to many cfm's) That is where the booster fan comes into play, also why a box fan works best on low speed, your not moving the heat to fast or trying to push it through duct work.The negative pressure you create by pulling cold air out at a slow pace will let the warm air travel on its own alot better. A decicated duct system would be great, a pain in the butt or to much expense for most I will agree, but for those of us that like to hands on make things work the best possible, and sometimes learn the hard way is all part of it.
Also (like Egclassic said, unblock the returns to use the furnace, you will ware out the limit switches and shut it down, same effect as a really dirty filter. Todd 2

hilbiliarkiboi said:

My floor plan has me thinking about this issue.
No central air handler, but i ducted off the peak of a cathedral
- horizontal run about 8' then down about 40" - for circulation with a duct booster.

Always hard to say how much air will be moved.
Hope it works for you.
If not, investigate a timer for the lowest/quietest speed on your air handler.

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Cathedral ceilings have always been a problem Hilbi, Ive seen alot of duct work ideas tried on new construction and the age old ceiling fans still seem to do as good in that app. They do have multi speed fans for furnaces/air handlers that run super slow but wow are they proud of them $$$ they are also in the new HVAC equipment but you,ll pay for it also. I tried a real small slow blower in my furnace and the air was still to cold coming out or the registers. The trick is to pull the cold air out from a distance and dump it at the heat source, at low cfm's.

Wishlist,your idea will work just fine, that is the set up that i have and it works great.pushing the cold air from the back bedrooms to the stove with a small energy consuming fan froces the warm air to the back bedrooms well.Plus the noise is not even noticable.I tied into the supply ducts pulling from the back two bedrooms with a y since the supplies are on the floor(cold air) and the warm air on top just naturally gravitates back there. Now, if I need it to happen quickly,I place a box fan on the floor pointing towards the stove room(living room) for about 30 min then switch to the in-line duct fan to maitain. I have four girls in this house, they are very critical and all are warm.

Wish and MoPar- I'm sort of in the same boat. I'm trying to move my cold air back to the stove, but I'm unsure about size of duct and fan cfm. What are your setups? How many sq ft? Etc?

The return air goes down the stairwell to the basement. In the winter time I replace the solid door to the basement at the bottom of the stairs with a louvered door. It would probably work better if I cut return vents through the floor at each end of the house....maybe I'll get a round tuit one of these days.

egclassic said:

You have to be careful blocking off return air vents and supplies as well. Furnaces are designed for a certain load factor (to breath) and by blocking returns and supplies you can damage your system. I am not an HVAC tech, but I have asked our techs at work about blocking returns and they all told me absolutely do not.There is a reason there are not diverters on the return grills. I do not think my system was designed properly because the largest returns are closet to the furnace and the returns at the back of the house hardly draw at all.
The only time I use my A/C fan is when I overfire my stove and get it too hot in the stove room. (I usually try not to do that, but hey, accidents happen) One of those large returns is directly across from my stove, and I'll agree with you about 70* blowing air feels cold.

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I have a manometer on the return duct, so I know I'm not starving the fan, just diverting the suction to the rooms I want air drawn from. There lots of problems with my central air, since it was a retrofit, but lack of return grilles is not one.

TE

FanMan said:

Actually the difference in density between the cold and heated air is insignificant in this case. However, you'll have less heat loss in the ducts if you're moving cold air through them instead of heated air.

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Have to disagree with you there. The difference in the cool and heated air is enough to make a big difference. The proof is when we find the cool rooms warming up significantly. In short, it works, no matter what the theory.

Backwoods Savage said:

Have to disagree with you there. The difference in the cool and heated air is enough to make a big difference. The proof is when we find the cool rooms warming up significantly. In short, it works, no matter what the theory.

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Never said it doesn't work, just that the density difference isn't enough to be a major factor in why it works.

FanMan said:

Never said it doesn't work, just that the density difference isn't enough to be a major factor in why it works.

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One reason it may be easier to move the cool air and increase the flow overall is because you're moving it over the flat floor. The warm air has to go through doorways which are lower than the ceilings in the rooms and obstruct the flow. A slight difference in density is probably enough to help the process along.

FanMan said:

Never said it doesn't work, just that the density difference isn't enough to be a major factor in why it works.

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I am not an HVAC guy, and I am new to burning, but I have a pretty extensive engineering education and training, so I may be able to provide some insight here as this is just fluid flow/mechanical engineering type stuff. The easiest way to move this air is going to be to take advantage of natural circulation. The natural circulation is driven by the "Thermal Driving Head" which is caused by the density difference in the two fluids (cold air and hot air). There is a nice little segment on Thermal Driving Head and Natural Circulation here (it starts on page 37):

http://www.cedengineering.com/upload/Fluid Flow.pdf

There is a 5% difference in density between 60 degree air and 85 degree air http://www.denysschen.com/catalogue/density.aspx. To put that in perspective the difference in density for water for a 100 difference in temp (50-150F) 2%. That 5% difference in density is HUGE as far as a thermal driving head is concerned. Nuclear reactors use a much smaller thermal driving head to cool natural circulation plants because as a subcooled liquid water is nearly incompressible and therefore relatively hard affect density change on, yet it still creates enough head to cool a plant. This head is nearly always driven most efficiently by moving the more dense cold air and allowing it to "sink" through the less dense warm fluid.

So you are right in that as far as "pumping" the cold air with a fan, the density is not a huge factor in how much volume you can move, it is a big factor in driving that natural circulation. I hope this helps with some of the concept errors here on why pumping the cold air works.

Well I am definitely learning here, I didn't know it was better to move the cold air to the stove room.
I have used the central furnace fan and it did. Work some, but Didn't seem to work good enough to justify the added cost of electricity so I quit that..
The cold air duct is in the hall, like 8 feet from the stove.
I do have a blower on the stove itself but, I also run, and 8 inch fan, on low, behind the stove pipe, directed toward the hallway, is there any reason I should not do that???.
I also run a ceiling fan in the stove room, so ! with the 8 inch fan and, the ceiling fan, is this overkill?

HDRock said:

I do have a blower on the stove itself but, I also run, and 8 inch fan, on low, behind the stove pipe, directed toward the hallway, is there any reason I should not do that???

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As CT_Sub stated, natural convection (warm air rises in the stove room and spill out the top of the doorway, cold air is drawn in low to replace it) is a force that you want to work with, not against. Blowing cold air low, back to the stove room, is assisting the air to move the way it already wants to go.

You may have already thought of this, but no matter where you locate the fan(s), you're still transporting hot air across the ceiling, so make sure you hyper-insulate the ceiling to reduce heat loss. That loss is greatest where temperature differentials are the highest (at the ceiling).

wishlist said:

Thought I would share a pic of the supply vent near the stove. Its on the right side on the exterior wall . Figured it may help get more of an understanding of what I'm gonna try.

This vent will be isolated from the rest of the supply vents. The fan will draw air from the the supply side of the forced air furnace which the house has 3 supply vents , 1 in the hall around the corner and 2 more in the bedrooms. Hopefully I can get a fair amount of cool air with the 8" fan. I'll be sure and post pics and do a little " not so scientific" experimenting.

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I just don't see this accomplishing what you're wanting to do. Maybe it will make the room that the stove is located in a little warmer, but it won't help move hot air elsewhere.

Distributing heat via air you have to think in circles, as in circulation. You're pulling colder air out of a room when the objective is to circulate heated air out of it.

Hi wishlist, it's similar but not the same. You're removing cold air from the same room as the heat source and moving it to some distant place that is outside of the loop. (Some of that is the air cooled by the windows above the vent in your photo...) Where is it ending up? And what happens to it from there?
That's different than enhancing a circulation loop that already exists.

Imagine your room as a pool of water, and your woodstove pumping out hot water that immediately rises up to the top, forcing cooler water down. The hottest water is always up against the ceiling. If the door openings are, say, 3 feet below the ceiling, then you have the hottest water pooling in that room to that level and never leaving. That's the real problem. Sucking out the cold water from the bottom doesn't necessarily cause that hottest water to move into adjacent rooms; more than anything, it makes the average temperature in that room higher.

I used to mess around with fans and all that stuff. Then I put in a single ceiling-level vent and hyper-insulated the ceiling. Voila. The convection loop into the other side of the house formed by itself, no fans needed.

I applied the same principle to circulating the output from my solar air heaters, making sure their loop didn't conflict with the woodstove's. Silent. Reliable. No moving parts. Works great.

What starts, and maintains, an effective loop are high temperature differentials through direct, low-resistance pathways. Allow the hottest air pooling at the top to move out and the rest will fall into place automatically.

Some houses used to be designed with louvered vents above the doorways for exactly this purpose. I think they had a specific name given to them but I can't recall it at the moment.

As I convert my house from oil forced hot air to propane, I put a vent in the wall near ceiling level above the the gas stove; a pair of pancake fans directly below between the same pair of studs draws hot air from the room with the stove down and blows it at floor level into the adjacent room. Hasn't been cold enough yet to evaluate how well it works.

In my cabin which has a cathedral ceiling in the living room (where the wood stove is), there's a ceiling fan which I run in reverse (blowing up) when using the stove. It helps a lot in warming the room as it circulates the air so hot air doesn't collect near the ceiling. There are also openings at ceiling level between the living room and the bedrooms; air pushed by the ceiling fan in the living room tends to flow into the bedrooms where it cools and flows back through the doors into the living room (I run ceiling fans in the bedrooms in normal (down) mode until it's time to go to bed.

Wish, no disagreement here, I'm not arguing whether it "helps" or not, I hope it does. What I try to do is understand the forces at play, how they naturally work, and then find ways to work with them if possible. How we describe a problem determines how we try to solve it. Good luck to you.

Precaud, Could you explain "I used to mess around with fans and all that stuff. Then I put in a single ceiling-level vent and hyper-insulated the ceiling. Voila. The convection loop into the other side of the house formed by itself, no fans needed." with a little more detail? I have a ranch home and working in the attic is pretty easy. I am guessing I could run some ductwork like you mentioned and hyper insulate.