RMH flue pipe

Hi.
I am slowly producing my bespoke design for an underfloor RMH. I'm almost there with it now, but, I am trying to work out what product people use for the flue pipe within the mass. Looking at various videos and images, it doesn't look like people use actual flue pipe (which is very expensive by the time you factor in bends, junctions for cleaning etc). So, what do people use?
Chris

Piped mass is not so popular anymore as the Bell system has become more popular.
The type of pipe that has been used might depend on other factors like…. are you going to load tons of mass over the pipe because you will then have to use a strong pipe.
There is another thread going on about using hollow concrete blocks instead of expensive pipe.

Fox James wrote:Piped mass is not so popular anymore as the Bell system has become more popular.
The type of pipe that has been used might depend on other factors like…. are you going to load tons of mass over the pipe because you will then have to use a strong pipe.
There is another thread going on about using hollow concrete blocks instead of expensive pipe.

Thanks for the reply.
I unfortunately simply don't have the space for the bell system. By using the floor as the mass, I am hoping to save space.
The 6inch pipe would be in a 10inch concrete floor, with no significant loading on the floor. From the research I have done, it seems the flue needs to be as smooth as possible, and need to incorporate clean-out access points. I'm not sure how to achieve this with hollow concrete blocks. If going with pipe, I don't know what sort of temperatures to expect and so not sure if simple spiral ducting would be up to the job. But the videos I've seen do seem to use the simple spiral duct. Am I safe to assume that by the time the gasses fall down the inside of the drum, they are cooled enough for the use of spiral ducting? Or do I need to cough up for expensive flue pipe?

Hi Chris;
I use thin wall smooth HVAC pipe in 5' lengths.   In the US that costs me $11 for 8"dia.
Thin walls in a mass can be used by surrounding them with cob and constructive placement of your large heat-holding rocks.
The spiral pipe is not recommended. It will slow down your gas flow. And with it only being 6" you want as much flow as you can get!

It depends on what you classify as spiral pipe, the corrugated, expandable and flexible pipe is not a good option but the spiral air con pipe is pretty clean inside?
Air con pipe is wound on a machine from flat galvanised sheet.
I have read that galvanised pipe can gas off some nasty stuff at certain temperatures but I dont know the details.

Past the manifold, the ducting should not get to dangerous temperatures for galvanized metal. You could always use black stovepipe for the first few feet if you have less heat transfer than usual in the barrel and manifold.

Another thought, if you are already prepared to use a 10" thickness of concrete, might be a set of half-barrel bells in the floor. If you can manage the height for say 16" total thickness, placing barrels cut in half lengthwise and butting end to end, with rows spaced a few inches apart to give solid bearing on the base, would reduce the amount of concrete you need to use and allow frictionless gas flow under much of the floor. The details would depend on your system size and floor layout. A 2" base slab for support should make the whole thing clean and simple to construct. I would even suggest that if there are corners where barrels will not fit properly, placing empty big tin cans upside down on the base would reduce the quantity of unnecessary concrete to buy.

An idea using the hollow concrete blocks might be to start with a thin (2" or so) base slab, place rows of blocks on their sides so air can flow through, with the rows spaced say a foot or two apart, cover with corrugated metal roofing, and pour a 4" reinforced concrete slab on top of that. You would get even heating from the whole floor with no extra masses of concrete.

I would recommend haunting scrapyards for the galvanized hvac ducts you need.
I'm concerned your 6" rocket won't push through very many feet through duct work as compared to the square footage of your house.
Bells offer less resistance.