The opposite of an RMH

Ok first off I really want an RMH for heating my main Greenhouse and raised beds BUT Its snowing !! and the clay I can dig up is frozen annnnd I'm a wimp who doesn't fancy freezin his butt off messing with cob

So 2 projects I need help with please  One for now

1. Small lean to Greenhouse housing all my frost sensitive exotics for the winter isn't big  enough for an RMH anyway
I've kept it frost free with a parafin stove in the past but I dont like the stuff and its bloody expensive / I'm cheap  So this year I've tried a charcoal Angithi - or cheap home made version ........... ok I'm burning charcoal in a flower pot !! Angithi sounds better and its pretty much the same thing. Works well but the pots last a couple of weeks then crack . They dont in India but they do in England
So today built a stove with concrete edging pavoirs (they were handy - and it was freezin) This is working well so far but even choking back on the air flow (blocking up the space beneath the fire grill with another brick !! (I'm low tech - maybe even no tech  )its burning the charcoal fast - the Angithi burns slow cos no draught from below.
So whats the opposite of a "burn fast heat slow RMH" I want a "burn real slow heat gently charcoal burner" for no smoke heating this year.
The cement blocks worry me a bit as I remember a slab exploding under a fire I had a few years back - which was exciting enough when not surrounded by glass! so - Brick metal ?? (just so you know I can't weld)

Any Ideas
Help
Please 

off the cuff,

a rocket stove under 55g barrel of water may work. Ive built 30g rockets without welding, just side snips and some drilling for bolts/burn port mounts, etc. That heats heat water well enough to scrape off hog hair, 140f, no problem. good thermal mass in water.

you'd have to figure out how to deal with exhaust, its prolly not useful in the GH.

Do you have a small kitchen pot that your significant other won't miss (as in the day before New Years hearing, "Honey? Do you know where that one pot is?  I need it to make my special dessert for Mum.")?  If you can find one of those, try using that.  Since it's metal, it won't explode.

just re-read- coal.

that pot, or metal bucket, with a stack of bricks around it and a water barrel on top might be the simplest, most moderated and lasting heat sink for the time spent.

how do you feel about the coal exhaust in the GH?

If the exhaust isn't too very bad, you can pass it through a tray of soil (with plants growing in it, naturally...) to filter it somewhat. Plants tend to like CO2, especially around dawn when heating is most important.

Gosh this is a quick reply forum 

OK a pan sounds like a good idea - its ok to say it, I'm a a bit dim !! Why didn't I think of that 
I had thought of a metal bucket but galvanised steel  + heat = nasty fumes.

I had thought of heating water as a TM but was concerned about raising humidity and the associated increased risk of fungal growth etc.
I'll try putting a lid on the water container to reduce evaporation and see what happens.
The fumes from Charcoal are (once lit) pretty much just CO2 so I'm not worried about that - I'm not spending time in there its just about warming the space. I had wondered about "smokeless fuel" which (as far as I'm aware is "charcoaled" coal) I'm not sure how smokeless it is and although cheaper than charcoal its not as eco friendly obviously.

The brick stove type RMH seem, from the videos I've seen, to be built with standard brick and mortar, but I thought cement + heat wasn't a successful combo
Has anyone used dense concrete blocks for stove construction??

Thanks for the help

Roger

I wouldn't use concrete to build a firebox, use all the concrete you want to build the flue system, but you should use fire brick or a ceramic clay where the fire and immediate exhaust is.  Concrete will start to crumble when exposed to repeated high heat, in addition if your fire is smoky, concrete really likes to collect creosote.  That is one of the reasons building codes require a ceramic flue liner.

Instead of using standard cement/lime mortar for your brick core of a RMH, make a sand/clay/lime mortar that will be more heat resistant. My suggested ratio: 9:3:1 although I really see no reason why you couldn't skip the lime altogether, it would just make the clay less stable, although if you are covering the mass with a stucco or cob mixture, this wouldn't be a problem.