Best way to build a heated bench from pavers ?

Hello,

i´ve got a bunch of questions concerning the use of concrete pavers for a RMH bench.
The bench will be situated in a rented workshop shack. Normally you think of instant heat for a workshop, but I can play music in there without disturbing anybody anytime so I like a bench that stays warm, also for the benefit of dry tools/machines and glueing wood in winter.

So conditions are that the bench shoud be removable with reasonable effort if I move out one day, and I gathered lots of concrete pavers (urbanite rocks) for the thermal mass. It is planned to be simply rectangular.

My initial plan was to construct the bench as a hollow "bell" mortared and sealed with cob (see cross-section-pic).





My worries with this are:
* Is the single seal of a cobbed gap safe enough ?
* Will I be able to resoak the cob mortar in the small but wide gaps between the pavers if I have to dismantle it ?
* And the main objective: If it´s cool and there will probably be lots of initial condensation from the exhaust, will this water puddle on the bell floor and/or soften the cob mortar and spill out ?

So I stopped to rethink the plan and came up with another solution that uses a flue run with a 180 degree-turn at the far end.
Coss-section plan looks like this:





Plan for this is to loose-stack all the pavers (except for the bricks at the top that will be cobbed in) for easy and quick assembly and dismantling. Questions:
* The pavers got a rough surface of embedded stones on one side, will the little air gaps between them harm heat conduction ?
* What will be the best infill directly around the duct ? I was thinking either to use pebbles (insulation issue ?) or to use a cob mixture that is more on the sandy side.


Any thoughts & comments which way to proceed are highly welcome !

Ralf

I would still prefer a bell bench, could be made perfrectly safe with the top pavers stacked in a running bond. I.e. the second layer overlapping the seams of the first and the third those of the second.

Thanks Peter, that´s exactly the way I planned to do if if it will be the bell bench.
My main concern with this is the condensation interacting with the cob mortar, especially at the bottom.
On the other hand, having your expert opinion and avoiding to buy new duct pipes ist tempting !

OK, it that case, the bell bench can't be the same length as a ducted bench. Due to the bell bench being entirely hollow so there's less mass to warm up. But heat up time is shorter. What do you want to build, a J-tube Evans' style or a batch box rocket? For batch boxes it's easy to calculate the bell dimensions but for J-tubes I am in the dark so to speak.

Edit: Forgot the condensation issue. Since this is mainly happening during the first half of the burn it will form a small puddle but during the rest it will evaporate again. In the drying process there's a lot of condensation going on but if you persist in keeping it hot this won't be a problem.

When the workshop isn't occupated 24/7 you need a rapidly warming up device which means it cools off during the night and you have to start over the next day. My approach would be a slightly too small heat extracting part or a bypass valve of some sort.

Cob will dry, as will your entire system, as the system is fired.  It will have condensation and give off more steam in the first few firings, but the system will not accumulate moisture--it will do the opposite--so you will not have the clay getting moistened, or water collecting anywhere, after your initial application of the cob.

Dismantling such a bench would be easy with the help of a crow bar, this cob isn't cement after all. No need to wet the cob to break it down.

Cool guys,

tanks for the replys, I appreciate it very much.
I´ll go for the bell bench, a bypass valve is already planned for that.

All the best,
Ralf