wood furnace/rocket hybrid

Hi everyone,


Recently I installed a furnace/ water heater that heats the radiators at my place, the system works great but:

1- It demmands a lot of wood

2- Creosote builds up really fast.

I was wondering if I could adapt a rocket stove to my furnace so I can get a cleaner and more efficient burn. I attached some drawings explaining how my furnace works and an idea of how could I adapt a rocket system to it (?).

Has anybody tried something like this?

Thanks and sorry for my terrible english

It looks to me like you should lower the rocket stove so the exhaust goes right into the heat exchanger rather than having to drop down. Eliminate the horizontal run too if possible.

I am trying to do the same thing, only with radiant floor heat vs radiators, Borja Diaz. I'll be lurking here to see what develops! Your english is fine, by the way! I hope the more experienced posters here will chime in, as my limited knowledge says you're on the correct path… Bob, I think he's got it close to correct, in a "round-about way"- I believe your thinking is "conventional" vs RMH, which means he's already got what you recommended, if I read your post correctly. He's trying to bring his conventional flame path into the new RMH flame path to conserve fuel...
I've been looking at a lot of the posts here, and I'd recommend you both read as many as you can, I believe most of our "newbie" questions have already been answered by others that were newbies at one time too!

Look @ this post: it's an Outdoor Boiler that is doing exactly what we are trying to do, and he has done a LOT of experimenting for us!
https://permies.com/t/23295/wood-burning-stoves/Secondary-burn-igniting

C.Ray Gill wrote:I am trying to do the same thing, only with radiant floor heat vs radiators, Borja Diaz. I'll be lurking here to see what develops! Your english is fine, by the way! I hope the more experienced posters here will chime in, as my limited knowledge says you're on the correct path… Bob, I think he's got it close to correct, in a "round-about way"- I believe your thinking is "conventional" vs RMH, which means he's already got what you recommended, if I read your post correctly. He's trying to bring his conventional flame path into the new RMH flame path to conserve fuel...
I've been looking at a lot of the posts here, and I'd recommend you both read as many as you can, I believe most of our "newbie" questions have already been answered by others that were newbies at one time too!

Look @ this post: it's an Outdoor Boiler that is doing exactly what we are trying to do, and he has done a LOT of experimenting for us!
https://permies.com/t/23295/wood-burning-stoves/Secondary-burn-igniting

He's replacing the fire inside the conventional heat exchanger with an external rocket stove (excellent idea). In this case the downdraft section serves no purpose that I can see, in a RMH setup it provides radiant heat which in this case would be better off delivered to the heat exchanger. Without the downdraft section it will light easier and draw better.

Some baffling inside the heat exchanger may be helpful, perhaps one at the exhaust to make it draw from the floor like a bell.

C.Ray Gill: Thanks for both the posts and your kindness for defining my english as "fine"

Bob is right, I am trying to get a cleaner burn using a rocket stove and use the hot fully burnt air to heat up the heat exchanger.


If I understood it right, the downdraft section´s purpose is not to help burn the fumes but to release some of that heat to the outside, correct?


The problems are that 1- My heat exchanger lies on my garage floor and it is really heavy (800lbs or more).2- The flue pipe, coper piping, safety devices etc have been already fixed so even if I could lift it I would have to call a plumber to rearrange all the system setup. 3- The flue pipe is already in an angle close to 45 degrees, if the heat exchanger was in a higher position the angle would be even lower and maybe the system would not have enough draft to move the hot air up the flue pipe.

I guess I could build a RMH with a burn chamber so high that could be connected to my heat exchager via the upper door (the wood feed) but I am afraid that it would not be really efficient since the hot air path would be very short, not giving enough time to the heat exchanger unit to absorb much heat.

What do you think?

A rocket mass heater (or any rocket based technology) is a controlled chimney fire with a strong (rockety) draft.

So what we are trying to do in that instance is create the conditions for an extremely efficient high temperature fire that won't force smoke in the wrong direction, won't inadvertently explode, and won't meltdown what you built the RMH out of.

The reason I mention all of that is because of my concern over the question about whether the downdraft is necessary. Yes. Most emphatically yes, otherwise you will not have a rocket stove at all or the performance you expect. The insulated vertical heat riser and the downdraft surround create the strong air pump action which makes your rocket stove rockety.

As the hot air (predictably) rises up your heat riser the temperature in the heat riser goes up. When the gases reach the top they tumble over the edge (this is the torus) and the turbulence allows for additional combustion to take place (this produces more heat, and more efficiency). Now, the barrel or downdraft area, as long as it is cooler than the very hot air coming up the riser will want to fall. This creates your double-pumped super strong rocket behavior. The riser (pump 1) gets things flowing up and the downdraft (pump 2) creates that falling pump which together create a sick draw and a very strong push to get the exhaust through your mass and outside.

Insulating between the riser and the downdraft helps to increase the force of both pumps by increasing temp (and draft) in the riser and increasing the temperature difference (and draft) between the downdraft and the riser. If the stove is sluggish it's often because of an issue with one or both of the pumps. Both of these pumps, the insulated riser, and the heat shedding downdraft are required in a good design because:

1) without insulation the high temps in the riser move to the downdraft and stall the second pump
2) if the downdraft enclosure (usually the barrel) cannot shed heat or does so too slowly eventually it will get hot enough to stall the second pump
3) if at any time the rocket pumps stall the pretty sideways fire becomes a dangerous smoke belching demon, so anything to keep it running smoothly is encouraged.

I would say the gains in efficiency by converting to a RMH system in the first place will more than offset the little bit of heat being lost through the barrel to keep things moving smoothly. Build the rocket burn, vent the high temp exhaust through the boiler and see if it can keep things hot enough to do what you want it to.

Borja Diaz wrote:C.Ray Gill: Thanks for both the posts and your kindness for defining my english as "fine"

Bob is right, I am trying to get a cleaner burn using a rocket stove and use the hot fully burnt air to heat up the heat exchanger.


If I understood it right, the downdraft section´s purpose is not to help burn the fumes but to release some of that heat to the outside, correct?


The problems are that 1- My heat exchanger lies on my garage floor and it is really heavy (800lbs or more).2- The flue pipe, coper piping, safety devices etc have been already fixed so even if I could lift it I would have to call a plumber to rearrange all the system setup. 3- The flue pipe is already in an angle close to 45 degrees, if the heat exchanger was in a higher position the angle would be even lower and maybe the system would not have enough draft to move the hot air up the flue pipe.

I guess I could build a RMH with a burn chamber so high that could be connected to my heat exchager via the upper door (the wood feed) but I am afraid that it would not be really efficient since the hot air path would be very short, not giving enough time to the heat exchanger unit to absorb much heat.

What do you think?

OK, I can understand it not being practical to move the heat exchanger. But the idea is not to release heat to the outside, it's to get it into the heat exchanger. Your concern about the short time in the heat exchanger is what I was addressing with my baffle suggestion, make it like a 'bell' stove. If the exhaust from the heat exchanger is drawn from the floor it should get the coolest gasses. Gasses don't carry a lot of heat because they're so light, so it doesn't take a lot of time to remove the heat. In any case it's certain to be much more efficient and cleaner.

How much height do you have to work with (the top of the door opening), and what size is the flue pipe from the heat exchanger?

Here you have some pics showing the dimensions of my boiler.

suggestions are very welcome

I LOVE the idea. Way over my head as to how.

I think the hot gasses could be slowed down by putting two pieces of sheet metal that makes the gas path longer allowing a more efficient heat exchange. I think that it would be better if I could put a sheet metal above the upper water jacket but the jackets are at different hights and there is no way I can put one there, so heating up the sides of the boiler in that section is the best I can do.


Do you think this could work?

OK, with the pics it's obvious the upper door would be pretty hard to work with compared to the air inlet, too bad. Pulling heat out of the exhaust with a barrel leaves less to go into the main heat exchanger but it may help the draft. Pushing hotter gasses down into the main heat exchanger would be ideal but harder to get to work - especially when the water is cold and absorbing more heat before the chimney. I think cob covered barrels work though. With your baffle system, keep in mind only the gas in contact with the water jackets can transfer energy. The space between your baffles is only useful at the sides. But I can't say it wouldn't work as well as my idea.

If it wasn't for all the stuff inside, you could put the riser in there - again with a baffle in the back the gas would have to flow under. I assume that stuff doesn't come out easily.

Remember, the barrel needs to shed heat and with a properly tuned rocket stove the exhaust will still be more than hot enough to get work done.

I would (step 1) build your rocket assembly out front and route the exhaust through your boiler without adding baffles or anything. When this is working as well as it can, to your satisfaction, additional adjustments could be made. I recall that the boiler is in a garage? The heat losses from the barrel heating the garage will make the space more comfortable while working there, preheat the air for combustion some (which will help) and by keeping the ambient space a bit warmer will absolutely help the performance of the boiler... so it's not that big of a loss all things considered.

For that initial step I would try to make sure that my connections to the intake on the boiler were well sealed, that the draft through the boiler was strong (so no adding obstructions or baffles yet) and that any ducting between the rocket assembly and the boiler is well insulated to maximize the temperatures getting to the boiler.

When that is done, and performance can be measured you can decide if additional modifications are needed and if any of these hypothetical problems with heat transfer really need to be addressed. Don't fall into analyses paralyses.

Borja invited me over, so my two cents:

1) That is a truly impressive amount of creosote. Have you contacted the boiler manufacturer to find out if this is normal?
I suspect your boiler is either not tuned properly, not installed properly, or you may be using wood that is too wet.
If the fire is not getting enough air, the boiler's controls or chimney might be the problem.
If the wood is too wet, that's a simple problem to fix but it takes time.

In the US we see a lot of people who don't keep their firewood dry.
They say they understand that wood must be dry, and then they pile it on the damp ground and use a tarp to "keep the rain off". The tarp keeps the moisture in, and the wood never becomes dry. Imagine sleeping under a tarp in the woods, compared with a proper tent or cabin. Imagine wrapping a cake in plastic, versus letting it sit out on the table.
Sometimes people with a damp woodpile then bring small loads of wood under cover for a few weeks before burning it. Sometimes they use heat to drive off the water. (Many house fires are started by something combustible too close to the stove, and several that I know about were started by kindling resting on or near the stove.)
But wood is not easy to dry in a few weeks. It is made of specialized, water-absorbing fibers. (Just think how trees transport water and sap up 100 feet- almost beyond the limits of physics). It soaks up any available moisture through the end-grain, and also any exposed sides. And it only slowly releases the moisture through the end-grain. If you see or hear fizzing, bubbling, or 'drooling' out of the wood as it burns, there is still too much water trapped in the wood.

Wood should be stored in a shed designed like a drying rack: lots of ventilation, good protection from rain, racks or a raised floor to keep the wood off the damp ground.
In most climates it needs to be stored about 1 year, or at least 6 to 8 months during the dry season. Only after this storage time will it be dry enough to burn clean. The only moisture remaining should be the trapped water in the resins and cellulose themselves. This resin weight will not evaporate if heated in an oven or wood kiln.
Dry wood makes a musical 'tink' instead of a dull 'thud' when tapped together. It leaps away from the hatchet when split. It is light weight, and does not lose weight if dried in a kitchen oven. (You can roughly measure moisture content by weighing your wood, then baking it on low heat for a few days and weighing it again.) Moisture meters are also helpful - dry firewood usually measures under 15% to 20% moisture content.

Every drop of water that has to be dried out by the fire absorbs a lot of heat. As it evaporates, it expands into 1600 times the volume of steam. (160000%) This steam can quickly put out the flames, robbing you of half the heat of the wood. If there is enough water, the wood cannot burn at all.

Burning wet wood, you can expect to use a lot more wood for the same energy output. Reports vary from 50% more, to double (200%) the wood, and in some cases it simply doesn't produce acceptable heat no matter how much you burn.

Even a rocket mass heater can't operate properly with wet wood.

Please excuse me if this lecture is too basic. I know that our European friends have far more experience with fuel shortages and the importance of dry fuel. But I was disappointed to discover in 2012 that even the older parts of the Americas don't have better woodsheds - they have older, more interesting piles of mushroom-growing, rotten, damp wood. Too often we blame the stove, when the operator and woodshed are much easier to improve.

For the sort of person who would rather design something interesting than build a simple woodshed, I suggest building an interesting woodshed. One idea is a portable woodshed that you can use to collect wood in the forest, then move dry wood (less weight) to the home site. Or a woodshed that works like a solar dehydrator, with sun-heated air circulation to accelerate the drying of the wood in half the time.

Before you start a new project, I'd make absolutely sure that you are getting the intended performance from your boiler that you already paid for.
All boilers have some inefficiency due to heat lost in transfer: the boiler is located in one place, and the heat must travel to another place. A central, masonry heater can be more efficient because all the 'waste' heat goes into the home where it is most useful.
But a boiler should be able to burn wood cleanly, and extract the most possible heat from the wood itself.

If the pictures you show represent the design performance of your boiler, then you may want to demand a refund.
I would have difficulty trusting the boiler, if its designers think that much creosote is normal.

Are the heat-exchange pipes located in the flame path, before the wood has completely finished burning? In a good design, the wood burns completely, and the circulation pipes are located either in the clean exhaust, or behind a wall that keeps the pipes and fire from direct contact.
Wood needs to be 1000 F (500-600 C) to burn clean. Water and most circulating fluids need to stay down in the 180-200 F range (below 100 C) to avoid steam explosions. There is no way to locate a water pipe (or tank) touching the fire without cooling the fire, and/or risking an explosion.

If the circulation is steam and not water, then steam can be maintained at higher temperatures but it is extremely dangerous to modify a steam system.
There is a book called "Dead Men Steam School" that discusses how to repair and maintain older steam systems. But I would start with your boiler's manufacturer.

It's pretty dangerous to modify any boiler without fully understanding it. Your rocket modification seems like it would be more likely to reduce the operating temperature, not increase it, so there is more danger of failure than explosion. But I would still be extremely cautious.

In my own home, I would start by getting the boiler repaired, so that it performs as designed.
If the boiler repair expert says it is already doing its job, and I am not impressed, then I might consider installing a different type of heater. For example, a rocket mass heater or Russian fireplace in the main rooms. It may be simpler to build a new heater than to successfully modify the boiler.

I could keep the boiler as a way to heat the far rooms of the house, and perhaps it would perform better with a smaller load.

...
2) If you want to think about the modification:
The rocket mass heater is a complete system, designed to balance the draft and heat a low, horizontal mass. Your boiler is a different situation, and not all the parts of the rocket mass heater may be relevant.

The boiler is also a complete system, designed to have the right draft for its type of fire, and to produce a specific temperature to heat the pipes. It is apparently not working very well, but it's important to understand the entire function. I think if you are able to understand the entire boiler, you will be able to make it work better with only adjustments, not a rocket modification.

Rocket mass heater exhaust will be cooler than the direct exposure to flames that your boiler is designed for.
Rocket exhaust is not flammable - it would serve as a fire extinguisher if you tried to run both boiler firebox and rocket firebox at the same time.
There is not going to be a good way to increase the amount of fuel going into the boiler, because it can only exhaust so much at the other end. But you can improve the fuel, or improve the firebox, to get a cleaner burn.

In the rocket mass heater, the heat riser (insulated chimney-like part of firebox) is pretty important. That's the primary driver for the clean burn. You can get a similar effect by lining some other types of wood burners with a smaller, more insulated firebox. Or you can help the clean burn by insulating around the flame path to keep things hot until all the fuel is burned. You can even make a 'heat riser' from three pieces of firewood on end, and build a small fire between them. You might be able to improve the boiler's firebox, or lay a fire inside it that improves the burn, without adding a rocket on the side.

If you are not trying to force the rocket exhaust through a long, low heat-exchanger, you may be able to skip the downdraft bell.
Shedding heat from the barrel is how we get the exhaust gases down low enough to go through a seating bench located near the floor. If you can build your rocket lower than the boiler, and don't need to lower the exhaust as it enters the boiler, you don't need the downdraft for this application.
(In a few instances where someone wanted to use masonry or cover the barrel in cob, a short bench and a tall exit chimney can compensate for the loss of the barrel's heat-shedding downdraft function. In that case it becomes more like a contraflow stove.).

Does your boiler use fans to maintain exhaust flow, or does it use only the natural draft of the chimney?
It may take a lot of tweaking to get the modified firebox and fuel load to match the boiler's intended draft.
It is far easier to make something that doesn't work, or is dangerous, than something that works better.

If you want to do a rocket pre-burner, I would consider starting with a cooking rocket style. The simpler L-tube has fewer variables. You can more easily make a door that shuts and regulate the air flow to match what's needed by the boiler. And you can make a short, insulated heat riser, then dump the flames into the boiler before they are completely cooled.

I hope this is helpful. Please let me know if any of the English is unclear. I can't translate to other languages, but I can simplify the English if needed.

I would strongly suggest, first, contact the boiler's manufacturer to diagnose the current problems (and don't tell them you want to change it).
Follow their advice - maybe there is a better air adjustment, or a better fuel loading method, or a better fuel, that will make it work right.

If they say it is working perfectly, and you want to change it, then I might ask the repair man his personal opinion of your proposed improvements. Sometimes the repair man will give you straight talk about an idea that would terrify his boss.

Yours,
Erica W

Another possibility I just noticed:

The angled connection to the existing chimney -

The boiler might be having problems due to poor air supply because the exit chimney is not pulling enough air to feed the fire.

If the boiler is supposed to rely on chimney draft, the chimney must be as designed.
Usually this means vertical, and warm, and the proper size. They do allow for sometimes making an angled connection, but that is a pretty long angled connection in my experience.

Is the masonry chimney the right size for the boiler, or lined with an insulated liner to make it the proper size?

You might try insulating the 6" pipe that connects the boiler to the chimney, and see if it cleans up the burn a bit.

-Erica

Erica,
In the circumstance where the downdraft is removed is there still a torus? In the absence of the downdraft, torus, and resulting turbulence is there lower efficiency or does the burn just continue for a longer stretch of the system?