Back draft issues

Thanks for taking the time to read my post.
I built a rocket heater from a LPG gas tank with a proper firebox, insulated firebricks and all correct dimensions from research i did.
It was in my carport with 6" flue straight out from the bottom at 45 degrees for 1.8m then vertical for 1.8m and it runs extremely well. Have been able to hit 520 degrees Celsius on the top of tank.
I have recently installed it into my downstairs basement with the flue straight out the double brick wall and in the same orientation as i had under the carport.
First thing i noticed was that there was a constant draft inward through the rockets heater. Backwards!
Getting it lit is not too much trouble as i through a fire lighter in and blow gently to get the flow going.
It starts up fine with small sticks but as the heater warms up, I get back draft every so often and the flames are coming out the firebox! A quick blow and it goes the other way again.
Very dangerous as i cant walk away from it.

I believe its more than likely a flue length issue as i have tried to open doors to make sure its not a negative pressure issue but it did not solve it.
My vertical flue stops next to a brick wall outside with a cowl and i think its should be longer above the gutters of the house.
The flue doesn't bellow out the end it's actually pretty slow exit speed but it was the same when i had it setup outside running really well.
Is adding more flue going to help me with my problem?

I will attach a couple of pictures to show my setup.
I'm happy to answer any questions on design.

Thanks in advance

Mike.

Michael,   The dynamics of your stove changes when you bring it inside as you have clearly seen. Your words "downstairs basement" and "constant draft inward" to me sound strongly suspicious of a competing chimney between your house and your exhaust chimney (which by the way should be much higher - preferably above the roof ridge line). The wind and air currents below a covered awning sure can play some interesting tricks on you as well. Also, I have learned that its helpful to have all exterior exhaust pipe to be insulated to keep condensation down (from rusting out your chimney) and to help keep the last remaining heat inside the chimney to help with draft.

Thanks for the reply Gerry.
When you say competing chimney between house and exhaust what do you suggest I could do?
When i say basement, its actually a pool room (used to be a double garage) which has been plastered and proper sliding doors so its actually sealed and not open to drafts.
As for the flue, its stainless steel flue so rust will not be an issue. To get it above the roof ridge line i would need to add another 2-3 meters would be be guess. Would stepping it down to a 5" flue help with the draw considering the length?

If your basement is well sealed, and the house is warmer than the outdoors, air will want to rise and leak out of any cracks in the upper floor or ceiling. The only way new air can come in to make up for the leaks will be through your basement flue, as you noted.

One thing to do is make sure there are as few leaks near the top of your house as possible, reducing the inward draw in the basement. You might need a makeup air vent to allow fresh air to come in near the stove.

Another thing, as mentioned, is to make the chimney at least as tall as the ridge of your house. If it is any shorter, it might not be able to draw strongly enough to reliably overcome the "stack effect" (google it) of the house. Any wind blowing against the side of the house where the chimney ends will increase the outside pressure and try to make the chimney run backward. And insulating the exterior chimney, even if not need for condensation, will help the chimney stay warmer and increase the draft to help resist downdrafts.

By far the best way to route a chimney is straight up through the house, exiting near the ridge. This lets all the heat benefit the house, and improves the draft quality.

Wow! You really need to get that chimney above the roof line, code requires 4' above any part of the roof within 10 feet and something like 8' horizontally from from any window or door. Hot gases under the eaves of the house are not a good idea for many reasons, not the least of which is Carbon Monoxide, then there is the ignition potential of the wood roof.

Dropping the dia down is probably not going to help as it just introduces a restriction. Even the reducer you have right after leaving the stove is probably an issue, and again I believe it's a code issue as well. A double walled flue pipe will help in keeping flue temps up and therefore better draw. Also, make sure the joints of the flue pipe are well sealed. Even a little bit of leakage will destroy the draw. They do make backflow prevention dampers if you still have problems. I would also run the pipe straight up for at least a few feet before angling off at a 45.

Outside, the stove has infinite combustion air and so even minor draw works fine. Inside, the stove has to PULL air from somewhere, and depending on it's precise location in the volume of air in the house it will greatly affect the availability. You said you tried to make sure it wasn't being starved by opening doors, but depending on where the doors are it may or may not help. Code requires a fan to "pressurize" the room where the stove is if there is not a direct outside air supply at the appliance.

Thanks for the info Glenn. much appreciated.

Thanks for the post Duane. I will be sure to go up above the tiles by at least 4 feet.
That is not a reducer that you are seeing. The larger pipe is also 6 inch like the rest of the flue but is covered in a high density rockwool and then 8 inch pipe over. this is what is going through my brick wall then expanding foam around to seal.
I have no option but to go straight to a 45 from the rocker heater otherwise the flue will go up near windows.

When you say competing chimney between house and exhaust what do you suggest I could do?

A lot of good suggestions but probably the biggest is to first remove the competition by making the exhaust chimney exit above the roof line

Would stepping it down to a 5" flue help with the draw considering the length?  

I agree with Duane.... keep the CSA (Cross Sectional Area) the same

Good luck with your modifications and hope it helps to get your rockety roar back in the right direction again!

Hello to all who replied to my original post and thanks for the feedback once again.

I have an exciting update!

I have extended my flue by 3 meters so its basically in line with my roof line.
The draw is crazy now! Harder draw than i ever had when i had it outside. No need for an air inlet thats for sure.
It starts up very easily and i can put so much wood in until no more fits and it still draws and self feeds.

I'm actually thinking it may be a bit too much draw because i'm struggling to get it to the high temps that i used to get outside. I could easily get 450c - 500c and after about 30 mins when its really going hard i can just hit 350c.
I notice that with the hard draw, there is minimal coals building up whereas outside it used to build up with coals slightly. Maybe this is why i cant get the temp as high.
I also think that its burning the wood a bit quicker.

Any feedback on that would be great.

All in all, im very happy with the outcome.

I have attached a few pics once again. Last pic was taken at the end of a burn and there was a tiny bit of coals at the bottom and was still drawing!

It sure is an awesome feeling when it all works out. As for your too much draw problem, you have no thermal mass to capture and slow down the burn in the form of a bench or bell.