Using a Bell with a conventional woodstove - could it work?

After the great success of my 5" BB Rocket Bell/Water heater (thanks to all the help on this forum) I'm looking to improve the efficiency of some of our other wood-fired appliances.

I have a fairly expensive Steel box stove room heater in a room in our property. I really don't want to modify the stove (in case I have to sell it on at some stage...!) but I would like to capture and store some of the huge amount of heat that is currently pouring out through the flue to the outside world.

Is it possible to connect the 6" (150mm) exhaust from the stove to a brick bell? I envisage rear flueing from the stove into the bell and fitting a sort of highly insulated heat riser inside the bell with enough space to allow the gasses to stratify. The existing Stainless steel insulated chimney flue would then vent the exhaust from low down near the floor of the bell.

Would it work without stalling the draught?

Thanks for any help.

We had a similar idea and - instead of going with brick - added an extra chamber above the stove, in the flu, with baffles. It is a steel box, and the weight is just supported by the stove/pipe. It doubles the surface area of the stove in the room, before it disappears into the chimney, and seems to make a big difference to how effectively the stove warms the room. We have the advantage of a very tall chimney with plenty of draw.

Thanks Michael.

The existing chimney on our stove installation is a lightweight, insulated stainless liner and currently draws very well. I suppose if I built a brick bell it may be worth reducing the ISA slightly to compensate for the lower gas output temperature of the stove as compared to a batch box.

Thanks again for the reply.

One issue is that conventional stoves operate very differently to batch rocket stoves, with mass storage. They throw a lot of heat out quickly. I think the idea of adding a brick storage to a stove as described will be problematic - they prefer totally different operating modes.

The steel box idea complements the conventional stove well, because it also dumps heat directly.

It's a good point Michael and one of the reasons I posted the question.

I'm thinking if I want to experiment along this route, as long as I incorporate some form of bell/chimney bypass into the construction I could always use the stove in 'bypass mode' all the time if the draught or performance of the stove was compromised.

Thanks again.

There has been some people who built a brick bell behind a normal box stove. It works, provided you have a bypass installed. There are also some examples ot a stove with a barrel on top or behind, preferably fed low and exhausted low. The stoves in question need to be run hotter to charge the bell which isn't a disadvantage in itself because it will burn cleaner as opposed to tuning it down to a smolder.

And last but not least: absolutely no need for a riser in such a bell, steel or otherwise.

Thanks once again Peter for your learned opinion. Having no need for a riser in the bell will make things much easier on the construction side.

Would it be worth reducing the ISA of the bell in my proposed application? (the stove output flue is 150mm - 6")

John Harrison wrote:Would it be worth reducing the ISA of the bell in my proposed application? (the stove output flue is 150mm - 6")

The original stove is extracting heat during the burn, so the bell should be accordingly smaller. How much smaller is anybody's guess, depending greatly whether there're vermiculite boards inside the firebox or not.

In my opinion you could start with half of the recommended ISA for a batchrocket and build it in such a way that you could build it higher later, topping it up as it were.

Thanks Peter - I'll give that a try.

The stove has a vermiculite board insulated firebox and double glazed door. I've replaced the heavy steel baffle with a thin Stainless version which doesn't rob as much heat from the fire so the firebox is relatively lightweight and insulative.

In that case, you probably can get away with 2/3 of the normal ISA value to start with which comes down to 3.5 m² or 37.7 ft². Remember, the pipe feeding into the bell should be slightly higher than the exhaust. The mass of such a bell is largely irrelevant regarding heat extraction, the more mass the longer heat retention. The other way around, less mass means quicker and higher temperatures going into the room but it won't last until morning.

Thanks again Peter.

I'm looking for quite a large amount of mass in the bell as the stove itself gives off a lot of heat quite quickly, but the room soon cools down once the fire goes out. The stove is rated at around 8kw maximum output.

For the bell I was planning to use an internal skin of old clay bricks and fire bricks for the high heat areas and then cladding the exterior surface with some sections of slate, so as to match the inglenook fireplace the stove currently stands in. Is it best to have an expansion joint between the 'inner' brick bell skin and the slate or can I simply attach the slate to the bricks with mortar?

Thanks again for all the help.

I personally don't think wood stoves are clean enough for use with a bell. They just don't get hot enough and have to short of a burn time for their lower temps. All those complex wood gas compounds don't get broken down all the way and will recondense in the bell. That is one reason why wood stoves must have a warm flue all the way out or they get build up. Its the wood stove dirty little secret.

You could completely insulate the wood stove for higher combustion heat, then insulated the inside of the pipe running to the bell for a longer burn time and then maybe it would work. Congrats, you just made another batch box with a nice looking outer metal shell.

If I remember correctly, creosote has a 90% decomposition rate at 1400F for .5 seconds. That does not include burn(oxidation) time. That is just what is needed to break it down so it will burn. My numbers may be off a bit but you get the point.

I'm done with metal box wood stoves, they are obsolete in my opinion.

Remember the 3 T's of efficient combustion;
Time
Temperature
Turbulence

Wood stoves are lacking in two of these.

Mike Dinsmoor wrote:All those complex wood gas compounds don't get broken down all the way and will recondense in the bell.

Mike, you're making me think there could be a health hazard there.

If the stove produced unburned woodgas, and it stored in the bell. Then you reload, and a spark hits the bell.

This could be dangerous.

Interesting points Gents - I've not started building the bell yet, maybe more research is required first...

John Harrison wrote:Interesting points Gents - I've not started building the bell yet, maybe more research is required first...

If your looking for good research, read up on thermal oxidizers and their use on emissions remediation.

Thanks Mike.

This has got me thinking...

I now understand that feeding a masonry bell from a 'standard' wood stove may cause a buildup of un-burnt wood gas (due to the relatively low stove firebox temps) that could be ignited by a spark from the stove and cause an explosion issue.

However, on first firing of our super-efficient batch box rockets, is there not a short period of time until the fire is established and the firebox gets up to heat where we generate the same un burnt 'wood gas' which is presumably stored in our masonry bells? On test my 6" 'J' tube rocket threw out flames from the top of it's heat riser and presumably my 5" batch box does too,  which would obviously be capable of igniting a build up of wood gas. Should we be worried...?

I'm going to keep an eye on this thread, because I have a steel stove that will be used in the living room (a very small one -- it's a Vermont Castings Aspen, the smallest one they make, I think).  My plan is to encase the stove with bricks underneath and on three sides, leaving only the front and top with the steel exposed to the room.  This will be partly for safety and partly for heat storage.  I'm interested in whether or not it proves safe to attach a bell to a standard wood stove, though.  If it was safe, I do have room to do that.  

I think history has shown its very safe with a bell batch box system. Same with rockets and bells. That is assuming that your build is correct and your doing good hot burns. I don't know the mechanism but it looks like build up in the bells is not a problem. Hopefully someone else can chime in with a tear down experience.

Think about this, the EPA does not regulate Masonry Heaters because they believe they burn to cleanly.

I don't want to come across like wood stoves are very horrible, they have their purpose. I personally just don't like them.