RMH - galvanized or black pipe

[dato1200]

I plan on building an 8" rocket mass heater in my home. Currently I do not have a chimney, so I need to buy around 25 feet of double walled chimney pipe.
I will also be buying around 18 feet of single walled pipe to run through the mass.

Searching the internet, it seems that people run galvanized pipe through the mass more than black pipe, and the more I think about it, I can't really remember anyone running black stove pipe through their mass.
There doesn't seem to be too much difference in price between galvanized and black pipe.

Which one is better? Does it make a difference? What pipe would you buy for the chimney and the mass?

[matt walker]

Welcome Dato. I'll give you my opinion, but I'm not sure there's any one right answer here. Most builds I've been involved in have used found/recycled materials as much as possible. It's a heck of a lot easier to find galvanized ducting used for free or cheaply than it is to find black stove flue pipe. Since it's not exposed to high temps in the mass, it's generally accepted as okay to use galvanized in there. Even if buying new, around here the galvanized can be bought in thin gauges for much cheaper than the black, which is typically 24g or heavier in my experience.

If money were no object, I'd go with the black pipe just because I find it easier to work with and its heavier gauge is reassuring when packing mud and rocks all around it. But I wouldn't agonize too much over it, around here it can be difficult to find 8" black flue pipe anymore since all the new wood stoves are 6".

As for chimney, if you can afford double wall 8" for your chimney your system will run better with an insulated chimney than without, but how important that is depends a lot on your system and how much heat you are taking out of the exhaust gasses in the mass. For most folks single wall is acceptable since flue temps will never be very hot at the exhaust, although if it's outdoors in a cool climate single wall might work against the system by creating a 'cold plug.'

[rjdudley]

[matt walker]

Just kidding Ray! I think you'll be fine. Your system is small and now that the bench is warm it will continue to have enough drive to start up and there should be plenty of heat left over after your mass to drive the system and overcome any stack cooling. If you have problems, there are things you can do cheaply and easily. Oddmar's suggestion is one, another is fiberglass insulation as pipe wrap, spend the bucks on double wall, etc. I don't think you should worry about it though Ray. Like I always say, "if it ain't broke, don't break it!" Then I usually can't leave well enough alone, so I break it anyway. But yeah, just like I learned from dad, do as I say not as I do.

[dato1200]

Matt, the galvanized stove pipe you used in your inside RMH, was that new or second hand? In your pics it looks new.

I looked around on more websites and the galvanized is certainly cheaper than the black pipe. I don't know what I was looking at before. I think I'll have to buy the galvanized single wall for the interior to run through the cob and to the wall thimble.

Good news on the chimney! I had been looking around on craigslist for a while for stove pipe, then after I started this thread I found 22 feet of triple wall for $200. Five 4' sections and one 2' section. The 8 inch interior pipe is stainless and the other 2 are galvanized.

To run the chimney, I need to get a wall thimble, bracket to hold the weight of the chimney, and a T. I can't believe the prices of the triple walled T's! I've been thinking of two alternatives:
1. just doing a single walled T for the time being
2. welding up a box to cover a single walled T, then filling the space between the T and the box with perlite.

I live in a suburban area near Philadelphia. At its coldest, it might get down to 0 F at night.

[matt walker]

Wow, you did GREAT on that chimney! As for the T, keep in mind your flue temps will be cool, much cooler than a traditional wood stove. If I were doing it I would certainly feel comfortable starting with a single wall T and monitoring it until I was comfortable with it's performance and safety. You can always upgrade from there if you feel you need to.

I had to go look at my thread to see what I put in my bench! My galvanized flue inside my benches was reclaimed HVAC pipe. Used, but fairly heavy gauge. The T's and 90's were all new, and that by itself added up to a couple hundred bucks if I remember correctly. I had good luck finding the used stuff at construction companies yards, where they had torn it out doing commercial remodels. Also, HVAC installers might have some laying around from refits.