Outdoor heat exchange stove

[Caveman]

Hello, new to this site, and it looks great!

I just built a large wood stove and then built a small building around it. The plan is that I will send the heat produced by the stove, which is then captured by the room, into my shop. Well I built it and have it up and running. I initially just had a piece of 6" triple wall at the top of the building coming into my shop, with another piece of 6" down near the floor to take air from the shop into the little building. The air coming out of the pipe was flowing ( slowly) just by convection and was 330 degrees fahrenheit. After about 6 hours my shop was only 5 degrees warmer. Thinking it was a flow issue, I replaced the 6" with a piece about 24" in diameter. After a few hours with that setup, still no change. I think I need a fan to encourage the air flow. So my question is, does anyone know of a duct type fan (maybe around 12") That can take that kind of heat?

 

[Slow1]

Hmm... Not sure that I buy into your design as optimal (or even safe) However to help answer your question -

IF the 'building' is well sealed then I would suggest you put the fan in the cold air return and pump cold air into that space to then be heated and returned out through the hot vent. This has several advantages including not having to heat your fan and the basic value of moving cold/dense air vs trying to pull hotter air back.

Other things to consider: How well insulated is tihs "small building" - you are likely to lose a lot of heat throughout the whole system. Also I'd insulate the heck out of the air ducts leading to/from the building.

From a safety point of view - consider a fire in that little building - what do you have in place to mitigate that (flowing into the duct work to the shop) etc? At a minimum I'd have some sort of alarm system in place not to mention perhaps building the small building out of non-combustable materials.

As you are moving air out of the area of combustion I'd also be sure to put a CO alarm near where the hot air enters the shop...

 

[Caveman]

The design is not optimal, because not all the heat can be captured as it would be if the stove was inside. As far as safety is concerned, I believe it is safer. No flames in the shop where there are combustibles nearby. Also gives me more space in the shop.

I tried blowing air into the building and the flow seemed to be minimally effective. From my experience negative pressure systems seem to work better than positive, but of course every situation works differently. I did have a fan sitting on a small shelf to help pull the hot air out, but it overheated and shut off.

At 330 degrees, I think heat loss is not the problem, I think its an air flow issue.

The front of the stove (door, combustion air intake) is barely sticking out of the building. Unless the smoke stack fails, the fire cannot get in the building. Imagine a regular wood stove with its butt stabbed into the little room. That's kind of what I have...

 

[mellow]

Interesting setup, do you have some pics you could post? Could you convert it to a wetback system and use a car radiator in the shop to disperse the heat that way?

 

[firefighterjake]

A lot of the heat I get in my house is radiant heat vs. convection heat . . . depending on the stove type you may be "losing" a lot of the heat from the radiant heat.

Hard to say if your set up is any safer or not . . . I will say though that my father had an OWB and he mentioned how safe it was to not have the OWB in his house. Only problem is he built a combustible building around the OWB and it was either a hot cinder or faulty electrical connection that led to his OWB/shed catching on fire. Probably would have been OK even then . . . except that he didn't want to get too wet or snow covered every time he had to go outside to load the stove so he placed the OWB/shed only a few feet from his house . . . He lives in a new house now . . . courtesy of All State.

 

[Caveman]

The shed is built entirely out of steel. And here is some pics...

 

[YkDave]

i can see how that would be highly inefficient, but like you say if you have 300f+ temperatures at the duct, your current issue would be airflow

If you were blasting away with the stove going nuclear and had low air temp at the duct, then you would obviously be losing all your heat out the stack or the shack.

i would agree with using a fan on the lower cold air duct and pushing the cold air into the stove shack, but the shaft would have to be pretty well sealed or you would be drawing negative pressure in your shop causing cold drafts and blowing the hot air out th eleaks of the stove shack.

a fan on the hot side is probably your easiest bet at this time. but its not going to be the cheapest, unless your air temp is dropping to at least 100f once you get air movement. Thats typically the max temp limit of a direct drive duct fan motor. Your safest and most viable option would be a belt drive (external motor) duct fan, most standard ones are good to 200f with high temp models up to ~400f

You'd also obviously want to make sure your cold air return is sized larger or youll be pressurizing the shop and drawing negative pressure in the shack which is not cool on many different levels (potential to draw smoke/ect out of the stove/pipe as well as blowing hot air out of the shop)

 

[Caveman]

You have some good points that help confirm what I've been mulling over in my mind as I try to tweak this system. I really believe it can work. My current plan is to make sure the building is totally sealed up and try the fan on the shop air return again.

If this doesn't work, know any good brands for a high temp fan like you were describing?

 

[Bigfoot]

your plan looks awesome. i was dreaming up something like this last winter. a thought is to try using a cheepy fan the size you need just to see if the idea will even work, if so then upgrade then.......maybe you can build a lean to around the stove if it doesent work & remove part of the existing wall. that stove probably cranks

 

[altmartion]

maybe make the "ducting" longer, put some holes or small outlets and cap the end. somehow get a small furnace fan In the return side. this should build static pressure in the ductwork and should move the air. it would take some adjusting but, if it works. it would be set up like a forced air furnace system, just on a smaller scale. finding a fan small enough may me the hardest part. do you think it would work? or am I as crazy as my wife say's?

 

[Caveman]

The stove absolutely cranks, just need to figure out how to get the air flowing good.

Altmartion, that's what I'm thinking, and I think it will work when I get the right balance of duct size and fan power...

 

[altmartion]

what is the air temp at the unit?

 

[Caveman]

what is the air temp at the unit?

Don't know, I never measured that.

 

[altmartion]

The stove absolutely cranks, just need to figure out how to get the air flowing good.

Altmartion, that's what I'm thinking, and I think it will work when I get the right balance of duct size and fan power...

It just may take time. do you think it stay at 300 degree if you were to move the air at 500cfm? I am trying to get a close idea of ductwork to start at.

 

[YkDave]

You have some good points that help confirm what I've been mulling over in my mind as I try to tweak this system. I really believe it can work. My current plan is to make sure the building is totally sealed up and try the fan on the shop air return again.

If this doesn't work, know any good brands for a high temp fan like you were describing?

I do, I've got the literature at work for them. But I'm not sure if your into spending 2k on one of those suckers!

 

[rwhite]

Cant see how big your shop is (looks huge) or if it insulated but that design doesn't seem like it will work.

1. The stove shed is uninsulated so there goes 95% of your heat just warming up the pasture.
2. Radiant systems have hundred of feet of pipe transfering heat to an absorbing substance. Your transfering heat from 8' of pipe to air.


Bottom line is you either heat the concrete floor with radiant heat, install a hundred feet of stove pipe and hope you can pull hot air through it, or move the stove inside the shop.

 

[altmartion]

if a shell was built around the stove and was hooked to proper ducting it could work. finding the correct cfm will be the hardest. it will take time but it does have potential. I think anyway.

 

[Sprinter]

A lot of the heat I get in my house is radiant heat vs. convection heat . . . depending on the stove type you may be "losing" a lot of the heat from the radiant heat.

I agree. I think you're losing a lot more heat than you think just from radiant energy lost to the outside. A stove of that type is putting out a majority of it's heat as radiant. Very inefficient to be outdoors in a metal shed. Maybe if the stove house were very well insulated and you had a good, safe circulation system with well insulated duct. But there still would be a lot of safety issues I'd be concerned about.

What size is the shop? We have a lot of members here who have small, efficient stoves in their shop that don't take up much space and don't burn much wood, either.

 

[Caveman]

Shop is 40' X 60' and very well insulated. Even in single digit temperatures it doesn't get below 40 with no heat source. So if I can just get it a little warmer I'll be happy. I don't need it to be 80 in there.

YkDave, yeah never mind. Too expensive!

 

[altmartion]

Shop is 40' X 60' and very well insulated. Even in single digit temperatures it doesn't get below 40 with no heat source. So if I can just get it a little warmer I'll be happy. I don't need it to be 80 in there.

YkDave, yeah never mind. Too expensive!

I think if you tighten up the shell or make a new tight one and work with static pressure and cfm you will make it warm. you probably should put in fire dampers and an alarm of some kind.

 

[begreen]

What about moving everything into the building and running it as a barrel stove with proper clearances?