Modifying my add on wood burner

[RATHRBCUTN]

Ok guys I am thinking about cutting out the top of my add on furnace to make the air chamber bigger, I have heard from people around my way that this is a good way to get more heat through the ducts. What do you guys think, have any of you done this, if so can you post a pic, and maybe explain a little bit about what all I have to do. I seems way to easy to be good.

 

[laynes69]

What kind of wood furnace. If you do it, then up the airflow around the firebox. I have 3 8" ducts coming from the top of my wood furnace. Soon I will change that out to an 8x20, and put an 8x20 into the bottom instead of 3 8". If you do that, run it in series with your furnace, its a night and day difference.

 

[JeffHK454]

What kind of wood furnace. If you do it, then up the airflow around the firebox. I have 3 8" ducts coming from the top of my wood furnace. Soon I will change that out to an 8x20, and put an 8x20 into the bottom instead of 3 8". If you do that, run it in series with your furnace, its a night and day difference.

Aren't three 8" round ducts about the same area as one 8"x20' rec. duct?

I actually switched from a 14"x14" sq. to a single 14" round and improved the velocity through my duct work.

Are we talking just enlarging the opening or increasing the size of the air chamber above the inner fire box?

More heat from the same wood is always an interesting proposition!

 

[laynes69]

3 8" ducts equals about 600 cfms. An 8x20 equals about 900 cfms. I have over a 1000 cfms of return, trying to force through 600 cfms on the woodfurnace. Increasing the hole on the top of the woodfurnace, also cutting at the base of the jacket for the air to enter around the firebox.

 

[AOD]

Sounds like you need to fabricate a plenum, just like on top of a gas furnace. You can cut away the top of the jacket, leaving about a 1" lip around the edge of the hole to screw the plenum to. The taller the plenum the better, your takeoff ducts should be at least 4" below the top of the plenum, so that it can build some static pressure. I would use 10" ducts if possible.

 

[JeffHK454]

My output side looks like this...

 

[RATHRBCUTN]

It's an old stove, the name on it says Johnson Energy, I couldn't find anything on the web for them making wood burners. The top measures 32" by 24" with two 8" ducts coming out the top. I am cutting out the entire top leaving 2" all the way around. The chamber is going to be 24" tall with the 2" duct coming out of the top of it and running right into the duct work for the hose. I will take some before and after pics. I was told by the local chimney cleaning guy this would really help.
We moved to the hose about 3 years ago and was told that the stove hadn't been used in 20 years or so, if that tells you how old it is.
The other project I am doing is working on the blower, as only one of the fans was hoked up, because the other doesn't work, and the combustion blower wasn't hooked up either. So hopefully before winter really hits I will have all of this done, and not burn through so much wood this year.

 

[JeffHK454]

How does this improve heating performance?

You would think having a huge void above the firebox would hurt performance...

 

[laynes69]

I wouldn't open mine up that much, but if you burn it hot it would probuce enough heat. I use my lp furnace blower, so I push 1200+ cfms of air through mine. By only leaving a couple inch gap, it won't allow for alot of air to mix around the jacket. But then again, it may work well for gravity, who knows.

 

[AOD]

The plenum is not so much about getting more heat as it is about moving more air effectively. A properly designed plenum will allow the blower to build enough static pressure to move the warm air down the ducts and overcome resistance. If you have an outlet thats too small, a bigger fan will only move so much air through it, and it will be louder as velocity increases. Remember, BTU's = CFM x Temp rise x 1.08. CFM is the biggest number in the equation, so the more effectively you can move air the more effectively you can move heat.