Ventilation via furnace chimney?

Hi. I'm new here; just made my account.

I've been looking into a rocket mass heater for a long time.
I don't have a house to put one into yet but, hopefully, I will soon.

The house I'm buying used to have a masonry chimney but it was
taken out and replace with a metal chimney. Whoever put in the
pipes really messed it up by making a downdraft before going up
the chimney, but that's neither here nor there, it will be fixed.

Would it be safe to vent a rocket mass heater through the same
chimney as a gas furnace? I was thinking that I could put
dampeners on either side of the of a(n upside down) Y joint so I
can switch between the gas furnace and the rocket stove.

Is this a good idea or is it like playing with a loaded fire gun?

Running multiple appliances on one stack is forbidden.   Also I feel that dampers are not safe for either appliance.   Even. Pilot light can burn oxygen and produce carbon monoxide.  Also Most gas appliances only use small diameter exhaust, commonly 4".   Do you know the diameter size of the exhaust stack?   I wouldn't reccommend most of what you're questioning.   Is the floor solid where the appliances are located?  Rocket mass eaters are heavy into the thousands of pounds/tons.  

There is a lot of good information here on permies.com and you should be able to find a lot of answers with a little research.   Many of these questions have been asked and answered by others in the past.  

I'm pretty sure it would be illegal as well as against code for two appliances to connect to one chimney, even if they are connected alternately and not at the same time.

I do think that it would be possible to make a switching connection that was safe and functional, but you could not do it with two commercial dampers on the legs of a Y. It would need to be something like an upper section that can swing or swivel so that it connects with one appliance or the other, but is shut off while switching so the appliances cannot both be connected or connect with each other.

You would obviously have to switch while both appliances are not burning, and ensure that the cut off appliance will not be started. This would be tricky.

Lastly, both appliances would need to be suited to the same chimney size. It might be easier to run a new chimney next to the existing one for one of the appliances.

John McDoodle wrote:Running multiple appliances on one stack is forbidden.   Also I feel that dampers are not safe for either appliance.   Even. Pilot light can burn oxygen and produce carbon monoxide.  Also Most gas appliances only use small diameter exhaust, commonly 4".   Do you know the diameter size of the exhaust stack?   I wouldn't reccommend most of what you're questioning.   Is the floor solid where the appliances are located?  Rocket mass eaters are heavy into the thousands of pounds/tons.  

There is a lot of good information here on permies.com and you should be able to find a lot of answers with a little research.   Many of these questions have been asked and answered by others in the past.  

I'm really glad I asked before doing anything. Thanks.
The furnace is in the basement. I'm not really worried about weight.
I don't know the diameter of the stack but it is probably undersized.
Thanks.