Would this work

Hi All, I Live in the UK and have been thinking about building a RMH in my barn and have been thinking of alternatives to using metal pipe through the bench, which I cant find in the UK at a reasonable price.
My plan is to use either brickwork ducting or hollow concrete blocks which I have already, cemented together to create a tunnel. is this a possible solution or am I missing something



Regards
Paul

Hi Paul,

the internal chimney of the RMH acts as a pump, i.e. the flue gas is under a slightly higher pressure. If there are any cracks in the thermal mass without metal pipes the flue gas is squeezed out. You should at least buy a CO-detector with alarm function and place it close to the RMH. A properly build RMH should burn the CO to CO2 inside the burn chamber but I would not rely on this. CO (carbon oxide) is a very toxic gas and you can not smell it and not see it.

Metal pipes are airtight but concrete blocks are not. If you find a way to make them airtight, I am sure many people in this forum would be interested.

regards
Peter

Paul McCourt : Peter is totally correct, I think you already expected as much, there is no reason why you can't try this out of doors.

We do try and work at keeping the horizontal stovepipe walls inside our Thermal Bench as smooth as possible, you could paint on a layer of clay slip,
this will allow you to flow the gas faster and further through more horizontal stovepipe but, there will be a sleight trade off in heat absorption,at the
boundary between layers, made worse by any trapped air ! Hope this helps, and is timely .For the craft !

Think like fire, flow like a gas, Don't be the Marshmallow! As always, comments / questions are solicited and Welcome! PYRO - Logically Big AL

I have considered the same thing, and discussed it with a few more knowledgeable people (but not Ernie, Erica, Paul, or Allen knowledgeable). We decided:

1. If you completely coat them with cob (including the bed under them) it should seal, but you can have cracking issues in the cob because of expansion/contraction problems.

2. You need the exhaust to be cooled down a little bit before you hit the concrete, even a little cooler than you need for galvanized duct. So you need an extra piece of pip before you hit the bench.

3. They will not flow nearly as well with the rough surface. But they will probably suck the heat out faster so you just need less bench.


I was going to put this under a stock tank to heat it in the winter, but plans changed and I went to a more freeze-resistant watering system.

Try a firm called Lindab they do metal ducting. About £15 for 3m length of 150mm.

Nation wide company in the uk.

Paul mcCourt

Have you heard of the half barrel system? Cheapest easiest way to skip the flues. Tho, i've got nearly endless supply of barrels if i want!

http://donkey32.proboards.com/thread/560/half-barrel-system?page=3#page=1

http://s65.photobucket.com/user/mremine/library/NYC%20Rocket%20Stove%20Build/

http://donkey32.proboards.com/thread/833/tube-incher-bell-volume

If this can give some inspiration!

Thanks for all the comments some things I hadn't thought of there,
I will check out Lindab and the half barrel idea and maybe save the concrete building blocks for an outdoor stove
Thanks again, will let you know how I get on

Cheers
Paul.

My opinion and experience is exactly the opposite of the other opinions expressed here. In my opinion the gasses inside an RMH are at a lower pressure and leaking is not an issue in most builds. There are of course conditions where this is not the case, but under typical operating conditions a RMH will be drawing in at all leaks. This has it's own problems, but I believe most RMH's have copious leaks. Always use a CO monitor anytime you are burning indoors nonetheless.

I think your idea would work as long as you consider CSA and do your best to seal any leaks. I also believe R Scott's 3rd point is the most relevant and will be the reason it wouldn't work if there is one. Friction inside the bench will hurt for sure.

If anyone wants to learn more about the pressures, Donkey's board has good info, and here's a great starting point...

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

Matt, my green machine was blowing smoke (steam) from the bell cut, before i re tapped it with stycky al foil, as well as the crazy piping at one or two spots. Looking like when you release the steam from a presure cooker

All fixed (ish) now.

Max, yours is a special case for sure! I didn't mean to imply you shouldn't do everything you can to seal them. One certainly should seal a system in my opinion. Just wanted to offer a differing opinion of what is going on inside a running system. When starting from cold, which you are always doing since you have no mass Max, they will send gasses out of the leaks, but in my opinion that's due to a "plug" at the end of the system from a cold stack, rather than pressure being developed by the combustion core/radiator combo. As Peter, Donkey, myself, and others have noted, once warm and running the pressures are lower within the system. At any rate, upon further reflection, I think as drawn the friction on this one would be too much drag as R Scott so astutely points out, so it's a moot point anyway.

Matt Walker wrote:Max, yours is a special case for sure! I didn't mean to imply you shouldn't do everything you can to seal them. One certainly should seal a system in my opinion. Just wanted to offer a differing opinion of what is going on inside a running system. When starting from cold, which you are always doing since you have no mass Max, they will send gasses out of the leaks, but in my opinion that's due to a "plug" at the end of the system from a cold stack, rather than pressure being developed by the combustion core/radiator combo. As Peter, Donkey, myself, and others have noted, once warm and running the pressures are lower within the system. At any rate, upon further reflection, I think as drawn the friction on this one would be too much drag as R Scott so astutely points out, so it's a moot point anyway.

Well, runing a 6 incher batch rocket with 8m of tube, and a 111mm restriction before getting to the weird vertical chimney stack sure doesn't help

Matt: If you have an external (classical, vertical) chimney on your RMH the pressure inside the thermal mass is determined by two competing effects: the stack effect of the vertical chimney and the pump effect of the internal chimney (heat riser plus barrel). The stack effect tends to “suck” air through the cracks into the thermal battery and the pumping effect tends to squeeze it out. Depending on the individual construction details and wind and atmospheric pressure you will or you will not have flue gas in your living room if there are cracks in the thermal mass.

But what if there is no vertical chimney? In this case the pumping effect would always pump flue gas in your living room (again, if there are cracks in the thermal mass).

The Venturi effect, you mentioned, is as stronger as higher the speed of the involved gas is. I think the gas speed inside a thermal mass of a RMH is quite low.
Is this enough to suck air through the cracks into the thermal battery?
OK, even if this effect is low, it can work in the same direction as the stack effect of an external chimney. But I think it is too weak to overcome the pump effect in a RMH without external chimney.

regards
Peter

I totally agree with Matt and Peter. I have a RMH with a 28ft. traditional chimney with 8in. flue tile liners, 18ft. of horizontal exhaust that exit into the chimney, 6 90 degree turns. I preheat by opening an exhaust cleanout that's 6 ft. -two 90 degree turns-- from the chimney stack. I never get smoke and it always draws strong regardless of temperature or weather. However, after 5 minutes or so when I start fire in the firebox I always get at least a little smoke leaking out of the top to the barrel for about another 5 minutes, then nothing. I have 5 cleanouts which are covered by 8 inch aluminum pie plates and I can feel the suction pulling them. I agree that leaks are not good but with my rmh I think a leak would weaken the draw and not push smoke into the room. My RMH is in my shop, not my house.