My progress in building a wood fired boiler based on the design by Richard C. Hill

[Hobartian]

With winter not far away in the southern hemisphere I finally have the storage tank fully insulated and the heater is ready to fire!

This will be the first season that the heater will be used on a full time basis.

 

[TCaldwell]

very impressive, how many lbs wood and btu output do you expect

 

[ozzie88]

Thanks Snail for your post.

I will be encasing the burn chamber and the secondary chamber with vermiculite cement but that will be done last as once it is done it will be difficult to undo.

You raised the issue of the way I have connected the heat exchanger to the storage tank.

Originally I intended to do this with two rigid rectangular pipes that each measured about 120mm by 60 mm. I thought the sheer size of the two pipes combined would be enough for the heat to equalize by convection. However, this did not work as the movement between the heat exchanger and storage tanks due to expansion and contraction fractured the connecting joints. Presently I have the tanks joined by a 50mm stainless steel flexible hose. I test fired the boiler today using only a small amount of firewood and I observed there was a huge temperature difference between the heat exchanger and the storage tank. I recorded 88 C in the heat exchanger whilst the water in the tank was only 28 C

I think I will modify the system by installing a 50mm pipe from the top of the heat exchanger to the top of the storage tank. I will then install a pipe from the bottom of the storage tank to the bottom of the heat exchanger. That should enable a good thermosiphon loop.

In regard to adjusting the air input that will be a matter for future experimentation. From my memory of reading Professor Hill's papers, the design allows the fire to draw the amount of air that it needs.

Finally someone else who built there own boiler. I built my own and you really understand how all this works and how touchy things can be.
I looked at your photos and drawing looks like it should work. You may find that the air adjustments will be VERY fussey to get correct, just the little difference will make big change in fire. Keep the idea that you may have to work on this or change parts when building. Also I see there only one place to get air into fire? What I found out is when I added an air inlet at the end of second chamber I got the flame to burn longer and more complete, just a little to much will kill the flame. I got between 78% and 84% when I checked it Lbs. of wood and burn time etc. My boiler is an up-draft so I blow air in bottom, then in the top of bottom chamber then the second chamber then just a very little in end second chamber. When I got it all fireing right i can start with say 120* water fire up takes 6 to 12 minutes to get secondarys to fire then adj. air once and off it goes. I have peep holes so I can see the fire in chambers this is something you should think of, WOW can you see what a very little adj. of air can do to the flame, and fun to see your creation inn action. anyways.. Hope this may help,, good luck keep us posted.

 

[ozzie88]

Thanks Snail for your post.

I will be encasing the burn chamber and the secondary chamber with vermiculite cement but that will be done last as once it is done it will be difficult to undo.

You raised the issue of the way I have connected the heat exchanger to the storage tank.

Originally I intended to do this with two rigid rectangular pipes that each measured about 120mm by 60 mm. I thought the sheer size of the two pipes combined would be enough for the heat to equalize by convection. However, this did not work as the movement between the heat exchanger and storage tanks due to expansion and contraction fractured the connecting joints. Presently I have the tanks joined by a 50mm stainless steel flexible hose. I test fired the boiler today using only a small amount of firewood and I observed there was a huge temperature difference between the heat exchanger and the storage tank. I recorded 88 C in the heat exchanger whilst the water in the tank was only 28 C

I think I will modify the system by installing a 50mm pipe from the top of the heat exchanger to the top of the storage tank. I will then install a pipe from the bottom of the storage tank to the bottom of the heat exchanger. That should enable a good thermosiphon loop.

In regard to adjusting the air input that will be a matter for future experimentation. From my memory of reading Professor Hill's papers, the design allows the fire to draw the amount of air that it needs.

Finally someone else who built there own boiler. I built my own and you really understand how all this works and how touchy things can be.
I looked at your photos and drawing looks like it should work. You may find that the air adjustments will be VERY fussey to get correct, just the little difference will make big change in fire. Keep the idea that you may have to work on this or change parts when building. Also I see there only one place to get air into fire? What I found out is when I added an air inlet at the end of second chamber I got the flame to burn longer and more complete, just a little to much will kill the flame. I got between 78% and 84% when I checked it Lbs. of wood and burn time etc. My boiler is an up-draft so I blow air in bottom, then in the top of bottom chamber then the second chamber then just a very little in end second chamber. When I got it all fireing right i can start with say 120* water fire up takes 6 to 12 minutes to get secondarys to fire then adj. air once and off it goes. I have peep holes so I can see the fire in chambers this is something you should think of, WOW can you see what a very little adj. of air can do to the flame, and fun to see your creation inn action. anyways.. Hope this may help,, good luck keep us posted.

 

[slowzuki]

Just to add to the thread, the cyclone style chamber was present in the bioburner that was spun off by Woodmizer. The cyclone action helps scour the flyash etc off the cold heat exchanger walls too. Looking at their current design, I think they have moved the cyclone to after the heat exchanger? Can't quite tell.

BTW, careful of the wikipedia entries on the Jetstream etc, why? Because I wrote most of it! I gathered more information over time from speaking to other people, theres a big error about the Tempest in some versions that got copied around. I did get to speak to the surviving members of Hampton and also to the designers at Kerr that took over. I have the last prototype that Hampton produced with an increase burn chamber diameter for softwood sitting in my shop. I have no idea what to do with the thing, it was outside in PEI for who knows how many years.

 

[maple1]

Too bad Kerr didn't stick with the Jetstream - I'd likely have one now if I could buy a new one today. Just drove by the plant today, actually.

 

[Hobartian]

I have not been using the heater on a regular basis as the weather has only been cold on occasions. However, I have fired it on a number of times and it has worked perfectly.

Recently, I started the heater and added timber and when I came back some hour later I was quite alarmed to find the water in the tank at 98 C or 208 F. This is too close to boiling for my liking and I have since installed a thermostat to stop the fans at 95 C. Whilst I have two pressure relief valves installed in the tank I don't want to lose any coolant or potentially damage the tank.

I have been experimenting on the technique to start the heater. I place a few inches of small pieces of timber on the floor of the combustion chamber and then a hand full of of a dry kindling on top with a wax fire-starter. I then light the starter with only the exhaust fan running. After a few minutes I start the main inlet fan and come back about ten minutes later and load the wood chamber with heavy long pieces of timber. It is quick and easy to start.

 

[hobbyheater]

Recently, I started the heater and added timber and when I came back some hour later I was quite alarmed to find the water in the tank at 98 C or 208 F. This is too close to boiling for my liking and I have since installed a thermostat to stop the fans at 95 C.

The refractory can store a lot of heat. You may want to shut your fan off a few degrees earlier.
I run a Jetstream with 1,000 gallons storage and for example, after a 7 to 8 hour burn with the storage at 195 top and the bottom at 170, I will run the circulator for another 3 to 4 hours after the fire is out and will see a 5 to 10 degree increase at the bottom.

It looks like your boiler has a large refractory component so I have attached a picture of a Jetstream refractory liner that is around 700 lbs and a complete base at 1,000 lbs just for comparison.

 

[Eric Johnson]

Very nice looking project. Hope it works as good as it looks.

I believe Prof. Hill's basic design is the one most modern gasifiers are based on, but I could be wrong about that. I'm sure slowzuki could give us a bit more history on that--despite being a Wikipedian. He's Canadian, too, but I'm not holding that against him--too much.

 

[ISeeDeadBTUs]

Two returns from the dead in one thread . . .

And Hobart?? You get absolutly ZERO style points for that beast!!

 

[Hobartian]

Thanks for the comments.

I admit my heater does not have much style and would be difficult to sell commercially. However, it does work and can be be built at home by someone who can weld and generally construct things.

In Australia there is very little use of hydronic heating. Reverse cycle air conditioning is most frequently installed because of the relative low cost to buy and install the units. (Thanks China)
However, many are finding them very expensive to run due to the sharply rising cost of electricity.

Hobbyheater: The Jetstream refractory liners look very impressive. Their cost must be quite high but they look as though they would last.

 

[hobbyheater]

Their cost must be quite high but they look as though they would last.

The cost was very high but has proved to be worth the investment as they are very repairable. https://www.hearth.com/talk/threads/giving-a-jetstream-base-new-life.76371/
A familiar picture
I had been following your project for sometime before becoming a Hearth member. Very impressive

 

[Mark Holden]

Hobartian;
Thanks for sharing your work; I'd come across Hill's paper before, but your up to date step by step progress brings it alive.
I have the skills and [hopefully] the experience to do something similar for my place in southern Portugal.
I'd also feel more confident using insulating firebrick, having used it before to build a ceramic kiln. but I'm considering your [and hill's] method.
Your experiences in construction and firing your burner are really useful.
Do you have any idea what your combustion temperature is? What about the temperature in your flue [that would say a lot about your heat exchanger]?
It looks like all the combustion air is introduced at 1 point, with no secondary air ports; that sure makes it easier.
On the other hand, there is no pre-heat for the combustion air. I guess it doesn't matter all that much, as long as you achieve combustion temperature that's high enough.

I would eventually add solar collectors to my system, and hope to get 90% of my heat from them. I've done a lot of research on this.
I've made lots of tanks [big and small] for fuel and water in the past, but for this I'm thinking a big scrap LPG tank of around 1000 gallons. I found some not too far from here.

 

[Hobartian]

Hi Mark

It is my understanding that Professor Hill now advocates building the fire chamber from firebrick. It appears either method of construction works. It is yet to be determined whether my burn chamber will stand the test of time. I didn't spend much money on the chamber as I wanted to find out whether the heater would fulfill my needs.


I haven't measured the temperature in the combustion chamber but I know there is no build up of creosote in the heat exchanger. The stack temperature when the heater is in full operation is in the region of 400 C. The inlet air is preheated as it is delivered through a stainless steel pipe which I bent in a circular shape and is embedded in the wall of the burn chamber which reaches a very high temperature in operation.

A tank of one thousand gallons sounds fine to me. My storage is about 350 gallons and I wouldn't advocate anything smaller.

Good luck with the solar collectors. Maybe you can design your system to use both forms of heating. The wood fired boiler will heat the water in your storage tank overnight rain hail or shine as long as you feed it with timber!

 

[woodsmaster]

do you mean 400 F 400 C = 752 F. That would be sending a good amount of heat up the chiminey.

 

[Hobartian]

do you mean 400 F 400 C = 752 F. That would be sending a good amount of heat up the chimney.

I have not yet installed turbulators in the heat exchanger which should reduce stack temperatures and improve overall efficiency. On the other hand, having the tubes open makes it very easy to check them to see if they need cleaning. I know I have plenty of heat on hand and losing some up the chimney hasn't been a worry to me.

 

[Mark Holden]

400C stack temperature... it seems so wasteful! but I don't know what is "normal". I guess you can always add another heat exchanger to the stack if you want.
I'm also curious about the burn chamber; can it handle unsplit logs, or do you need several split pieces to burn?
I ask because I'd mostly cut my own wood from my pine forest. splitting wood isn't my idea of fun.
On the other hand, building a wood splitter IS my idea of fun...

my design is intended to be mainly solar, but I won't be able to build the collectors for a while. I want a very gig array, capable of heating the house while adding heat to the buffer tank during sunny winter days.
In the meantime, the woodburner would have to do all the work.

 

[Mark Holden]

An oil fired boiler in a rental property I have has the kind of thing you want;
The design is pretty similar to your heat exchanger, with 1-1/2" [as I recall] flue pipes from a burn chamber rising through a water tank. Vertical instead of horizontal.
There are lengths of twisted strip hanging in the tubes from the top. to clean them and the tubes, it's necessary to pop the top off the boiler to pull them out. I don't know if they're made from a clever material, but I assume they are. After several years of heavy use, there was no visible erosion.
You would find something like that from a company that sells heating system parts.
It would be interesting to know how it affects your system if you add them.

 

[Hobartian]

I have some reservations about the design of my wood fired boiler.

Professor Richard C. Hill mounted his heat exchanger in a position so the steel tubes were running vertically.

I chose the opposite thinking it would be easier for me to clean the tubes if they were horizontal and close to the ground. However, I did not realize the amount of fly ash that is created in the burning process and in my case this fly ash is being carried through the tubes accumulating on the floor of the tubes and at the end of the heat exchanger. When this ash accumulates the efficiency of the heater is significantly down graded but is restored when I physically remove the excess ash.

My problem is that the force of gravity is not helping me whereas in Professor Hill's design the ash has a much better chance of falling down into the clean-out chamber.

I will continue to use the heater and possibly in the off season I can work out how I can modify the design to prevent so much fly ash from being introduced into the heat exchanger.

 

[Mark Holden]

I guess now we know why the tubes were designed vertical...
I clean out the tubes in the aforementioned oil burner once every year or two, and it is a pain.
Like your situation, it's in a tight spot, and I have to wrestle the flue pipe out of the way, then work in the small space above.
So I can see why you built your as you did.
Once again, thanks for sharing your findings; it may well save someone from repeating your mistake.
"Someone" could even be me!

 

[Eric Johnson]

I'm not sure how you'd build one, but an ash-removing cyclone might help. Might be easier than re-orienting the tubes. At least one gasification boiler on the market employs one.

 

[Hobartian]

Over the past fortnight the performance of my heater deteriorated. The stack temperatures dropped to a figure hovering at around 180 to 220 C. The heating of the storage water was painfully slow and I could smell a slight pungent odour. Looking down into the fire it didn't seem to have it's usual intensity. I couldn't work out what was wrong and I started to think the problem lay in the timber that I was burning. I tried different types of timber, chopping them into smaller pieces all to no avail. Then I had the thought maybe the air inlet pump was faulty. While investigating I removed the inlet air filter. (A cheap part made in China and bought for a few dollars) The heater surged into life and the intensity of the burning was tenfold. The stack temperature quickly climbed and the temperature of the storage water increased noticeably. Apparently the filter was restricting the flow of air which severely impeded the performance of the heater. Currently, I am running the heater without an air filter until I can fit another with a much larger capacity. Of course the air does not need to be filtered for feeding into the fire but I am concerned that damage could be caused to the internal parts of the Gast Regenair air pump in using it without a filter. Fun and games!

 

[Mark Holden]

Wouldn't a simple screen do the trick? expanded over a protruding frame perhaps?
I'd think anything small enough to pass through [dust] wouldn't harm anything.
What are the specs on the air pump by the way?

 

[woodsmaster]

Wouldn't a simple screen do the trick? expanded over a protruding frame perhaps?
I'd think anything small enough to pass through [dust] wouldn't harm anything.
What are the specs on the air pump by the way?

I may be totaly wrong but I'm thinking a screen would probably plug up and or posibly melt. If you could clean it easy and the temps at the screen were low enough it may work.

 

[hobbyheater]

I I did not realize the amount of fly ash that is created in the burning process and in my case this fly ash is being carried through the tubes accumulating on the floor of the tubes
I can work out how I can modify the design to prevent so much fly ash from being introduced into the heat exchanger.

The Jetstream needs to have the bottom of the burn chamber cleaned before every firing. This is accomplished by using the the long handled scoop pictured below. I prefer to use an ash vacuum. If the burn chamber is not cleaned before each firing, it does not take long before the settling chamber gets plugged and so on up the chimney.