Wood Stove Heat Transfer

[HawkinsHollow]

This is an addendum to my Heat Activated Wood Stove Fans post earlier. My goal is to move heat from my main living room where the stove is installed towards the original house. The new living room is 12x25 with 12 foot vaulted ceiling. It steps down to normal 8' ceilings as it goes into the kitchen towards the original house. My idea is to use the space above the cabinets as a duct to suck air out of the top third of the living room ans shoot it towards the hallway leading towards the thermostat and intake of the original houses HVAC.

My idea is to seal up above that cabinet to make a duct of sorts with an intake coming from the red sqiggly box in the picture and pushing air out the left side of the cabinet. You can buy quite thin axial "squirrel cage" fans. The plan was to put one in that left side sucking the air in from the heated room and blowing it towards the thermostat and intake at the end of that hallway. The fan I am looking at moves 23 cfm (on high I'm assuming.) I would probably keep it on medium. The way I figure it if I can move 18 cfm of 80 degree air and push it towards the original side of the house it would get warmer over there. Right now I have the thermostat set on 71. Sometimes I see it registering temps of 72 or 73. There are fans with higher CFM, maybe I should find one that moves more air. I just don't really want to hear it. They say these fans are only about 38 decibels hich is considered silent for an appliance. Is this going to make a difference? I feel liek it has to make some difference. I am just looking for a few degree increase, nothing dramatic.

Is this going to make a difference? I feel like it has to make some difference. I am just looking for a few degree increase, nothing dramatic. Sorry this is a bit all over the place, if you have any questions to clarify just ask.

 

[ponytug]

Well, we did something similar in one house. My $0.02 would be to put that hot air intake higher on the vertical wall with the high ceiling warm room (hotter air), and then duct it to lower down near the floor in the adjacent space.

Depending on your HVAC set up, if you have ducts, and can run the fan (blower) at a low speed, then a high mounted return may also help, BUT from experience a breeze (draft) of less than really warm air can be uncomfortable. Your body notices the heat loss from the moving air, and the perception tends to be one of "cool" not "warm". YMMV.

If it were me, I think that I would start with a fan in the warm room blowing toward the cold room. It might not be comfortable to walk through, but after a few hours or a day, it would give you an idea of what is possible and how much air you want/need to move.

All the best,

Peter

 

[ericm979]

We did something like that in the new house. The house has a great room with the stove, main hallway and then a 90 degree turn to the hallway to the primary bedroom. That's a long ways for warm air to flow from the stove.

We had the HVAC contractor put in a system to pull air from near the high point in the great room, 8' across a conditioned space and exit into the bedroom.

It's not all that effective that I can tell. It's loud enough in the bedroom (from air flowing out the grate) that I turn it off at night. But there's a mini split head in there so if it does warm the room the mini split will just run less. The air in the great room is only a couple degrees warmer so that might be why its not noticeable.

 

[MossRoad]

This is an addendum to my Heat Activated Wood Stove Fans post earlier. My goal is to move heat from my main living room where the stove is installed towards the original house. The new living room is 12x25 with 12 foot vaulted ceiling. It steps down to normal 8' ceilings as it goes into the kitchen towards the original house. My idea is to use the space above the cabinets as a duct to suck air out of the top third of the living room ans shoot it towards the hallway leading towards the thermostat and intake of the original houses HVAC.
View attachment 4836246
My idea is to seal up above that cabinet to make a duct of sorts with an intake coming from the red sqiggly box in the picture and pushing air out the left side of the cabinet. You can buy quite thin axial "squirrel cage" fans. The plan was to put one in that left side sucking the air in from the heated room and blowing it towards the thermostat and intake at the end of that hallway. The fan I am looking at moves 23 cfm (on high I'm assuming.) I would probably keep it on medium. The way I figure it if I can move 18 cfm of 80 degree air and push it towards the original side of the house it would get warmer over there. Right now I have the thermostat set on 71. Sometimes I see it registering temps of 72 or 73. There are fans with higher CFM, maybe I should find one that moves more air. I just don't really want to hear it. They say these fans are only about 38 decibels hich is considered silent for an appliance. Is this going to make a difference? I feel liek it has to make some difference. I am just looking for a few degree increase, nothing dramatic.

Is this going to make a difference? I feel like it has to make some difference. I am just looking for a few degree increase, nothing dramatic. Sorry this is a bit all over the place, if you have any questions to clarify just ask.

23cfm sees awfully slow. Think about it. How far are you planning to move that air?

If you had a 12x12 duct that was 23 feet long, it would take you 60 seconds to walk to the end of it. 11.5 feet would take 30 seconds. 6 feet would take about 15 seconds. It doesn't seem you'd get much flow out of that, and the air might cool off considerably before it gets to the other end.

I have a 250cfm inline high temp duct booster fan to move air from my basement stove room up to our living room through about 20' of 8" insulated duct. It doesn't put out much more than a normal furnace duct.

That seems like a pretty small space for a duct run. What are the dimensions?

 

[Oaktree]

I did something like that at my previous house. Had a couple rooms that tended to be on the cool side, ran ducts above the ceiling and down inside of walls, put fans (quietest ones I could find) in the ducts. It helped somewhat, but not as much as I would have liked. Might have worked better if I'd put a hood above the woodstove...planned to, but never got around to it.

 

[MossRoad]

I did something like that at my previous house. Had a couple rooms that tended to be on the cool side, ran ducts above the ceiling and down inside of walls, put fans (quietest ones I could find) in the ducts. It helped somewhat, but not as much as I would have liked. Might have worked better if I'd put a hood above the woodstove...planned to, but never got around to it.

I set the 8" metal duct right on the top of the stove. I can adjust the angle to bring in more or less heat. Have to be careful not to melt the in-line duct booster fan. The one I have is rated for high temperatures. If it was in a nice, finished room, I'd make a nice, finished hood. But it's not, so I won't.

 

[ponytug]

Fan noise is something that I personally find quite annoying, and I will do a lot to reduce it. For the same CFM rating the noise levels can and do vary tremendously, and aren't necessarily that much more expensive.

@HawkinsHollow I do think that @MossRoad's points on total air movement are well taken. There isn't much mass in air, so it takes a fair bit of air moved to move significant amounts of heat. That's why I would try with a cheap fan first so you can have an idea.

All the best, Peter

 

[BnRidge]

CFM = BTU/(ΔT*1.08)

BTU to CFM Calculator

A typical desktop heater will put out 1500w, or about 5000BTU.

Plug in the numbers, using 80deg - 70deg = 10deg, result is 462cfm.
Experiment with it to come up with 23cfm. About 25btu and 1 deg difference is close.

 

[jyoutz]

Fans to move wood stove heat act to distribute and try to equalize air temperature around the house. You wouldn’t necessarily feel warm air coming out of the vents, but if air is moving, it’s equalizing temperatures throughout the house.