Advice on Sauna Pocket Rocket

Hi,

I've been following this forum for a while and was planning to build a rocket mass heater for a new sauna I'm building. But I realized that the best option was probably a pocket rocket instead. I basically want to heat the sauna fast and I don't care about the temp when I leave the sauna, so there is no reason I need thermal mass for long lasting heat..

So, I built a pocket rocket today, and here is the basic construction based on what I had around my property:

1. Old existing 8x8 ft slab of concrete from a hot tub that I took out a long while ago. This is the foundation for my sauna, which will also be approx 8x8 ft.
2. On top of the slab, I laid down 3 layers of Home Depot concrete pavers (about 1 1/2 inch thick). Mainly because I want to get rid of them and it gives me some extra height to build the building around (bad drainage means there will be lots of water on the slab when it rains).
3. On top of the concrete pavers I laid down 2 layers of old thick firebrick that I had laying around.
4. On top of the firebrick layer I put another layer of firebrick directly underneath the end of the feed tube (because my feed tube was not long enough to get a nice 3-4 inch gap between the firebrick layer in step 3 and the end of the feed tube). So basically, the fire is burning on a small pedestal of firebrick that is raised up inside the barrel.
5. I used a 30 gal barrel, open face down on the firebricks. Note this is different than typical construction because I'm burning directly on the bricks and not the bottom of the barrel. I have an 8" feed tube that is 2 feet long, and a 6" flue that is 8 feet long (I have the materials to make it 10 feet if needed).
6. I put down some silica sand from a left over paver project around the rim of the barrel on the firebrick to stop air flow from flowing through any gaps (white ring around the barrel bottom in the picture below).

The result was good. It put out a lot of heat. So much heat that I started to worry about putting it in the sauna! But after some experiments I decided I might be able to make it safe. I put a ring of firebricks around the bottom 1/4 of the barrel. The ring had about a 3" air gap between the edge of the barrel and start of the firebrick ring. This reflected the heat back into the barrel very well and then made the heat move up the barrel. The outside of the firebrick ring was not hot at all. In fact, it wasn't even warm, even after burning hot for 30min (max time I probably will be in the sauna anyhow). So, I plan to put the floor of my sauna around this firebrick ring so that my floor doesn't catch on fire. (-: Here is a picture so you can understand better what I got:



And here are my questions:
1. Is silica sand okay to get at high temperatures? It can be toxic if the dust is breathed in, but I don't see anything about high temperature off gassing or anything like that. After my first burn it seemed to do just fine. No discoloration or anything.
2. I plan to build my Sauna tall, at least 7 feet, maybe 8 feet on one side (to get a single slope for the roof). I am a little worried about the ceiling of the sauna, although it will probably be close to 5 feet away from the top of the barrel.
3. What kind of clearance or construction do I need for the flue pipe to go through the roof? That sucker gets hot and it worries me.
4. I was thinking of putting Sauna rocks (not just any rocks) inside the 3" gap between firebricks and barrel, however I don't like the idea of glowing hot metal on rocks. Is my worry warranted? I was thinking of hanging some bags of rocks wrapped in chicken wire from the top of the barrel sides, as this will avoid contact with with the extreme temperature and catch the convective heat from the glowing hot metal below. This would also give me some extra weight on the barrel so that it doesn't accidentally tip over somehow.
5. I find it hard to get a consistent heat from this pocket rocket. Sometimes it seems fine, and then all of a sudden "whoosh" and it gets super rockety and the metal starts glowing at the bottom. Is this normal from your experience? Or, do I have something to fix? I don't really like the red hot glow, I don't think it is necessary for the sauna, but it is nice to be able to see that it is that hot... sort of a thermostat for the human eye..
6. What kind of flue pipe cap should I use on top? Was thinking to use a screen type for safety, but would that add too much drag?
7. I have a lot of extra firebricks that are broken in half and have mortar on them. I was planning on using these pieces for the firebrick ring around the barrel (there is no mortar on the bricks underneath the barrel). Since it is tough to clean the mortar off, I was planning to just leave it on there. My rational is that with a 3" air gap to the barrel, the mortar will not get too hot. And I don't even know what happens when the mortar gets too hot, is it dangerous?

I'm thinking this Sauna is going to be hot, really hot. But, if I have to I can open the door and I think it should be fine. And there should be enough draft through the gap of my floor to the firebrick ring wall to be able to get good air flow through the door. Let me know if you disagree.

Anyways, thanks for any help you can provide!

Erik

?5 i have noticed that with my tiny toy one i built from soup can. i figured it was from being so small. maybe it is a trait of pockets? what size wood are you feeding it? i noticed mine did best with the thinnest pieces. sorry i cant help with your questions, just learning myself. will be interesting to fallow your project. i have been thinking about doing the same thing.

Thanks Ronald, yes the wood matters a lot. If it's <1" diameter pieces of dry cedar or douglas fir it really gets hot fast (I'm in Washington State). So, I guess I can control a lot of the heat simply by what I feed it.

Erik Anderson : I did not notice anything to change on the bottom of your pocket rocket ! That looks fine ! Actually most people build their pocket rocket with pipes
of the same diameter or the exit pipe bigger !

When you have the system all fired up and burning good there are two different ways to control the amount of heat being given off by your stove.

What happens is- you are stuffing it full of wood, and this restricts the Amount of air that can mix with the wood, at some point in the burn as Your Wood fuel gets
more used up you now have more air to mix with the fire, usually this results in an increase in the rate of burn of the fire, so you ether have to refill the feed tube
to restrict the air flow again or - let it burn down to a more manageable level

Try this (it will take practice) Start your fire with a given amount of wood to preheat your sauna, then just feed it less wood, you will quickly learn to judge the
difference in your burn by the sound of your Rocket!

You will need a large flat stone too big to fall down the 8'' hole, you will need to pinch off the air supply at the very top of your feed tube, this gives you the right
fuel / air mix, channels the air more directly to your wood fuel and very importantly, helps restrict the amount of heat you are losing from the thermo-syphon
effect of your Pocket Rocket

Do not try to use a damper in your exhaust pipe, it is an un-needed crutch that is guaranteed to turn around and cause smoke back, usually as part of the learning
process with every new person who tries to use it

Soft resin-filed wood IS harder to control, and will burn just that much faster in a Pocket Rocket But it can be done, otherwise burn the driest wood you can get,
especially at the start

For the good of the Craft ! Big AL

Thanks Allen! I'll fire it up and try to control the amount of heat like you suggested. I'm glad you mentioned the damper, cause intuitively I was thinking of doing that like my wood stove.