RMH for a small greenhouse

Im currently designing a 10x8ft greenhouse and am trying to grow year round, I live in the UK so winter can get below freezing.

To that end im designing in multiple heat gathering and reserving features, one of which is a 3ft deep gravel climate battery under the greenhouse to help keep everything warm at night.

In the winter however we dont get loads of sun and im afraid the climate battery wont warm much, so for the winter im also planning to pipe into the battery a rocket mass heater, which should provide plenty of warmth and since the mass is already their  the RMH shouldnt take up much space.

Most the the systems ive seen have been for heating medium to large buildings, but how big would i need this system to be? And how many fire bricks would i need for both the burn tunnel and the riser as they can be quite pricey here in the uk.

I should also mention i quite like the batch box design over the j-tube, i was hoping to make a version of paul van den burgs design but half the size of the wood feed to save space, would this still work the same though?

I'm not an expert, but rocket mass heaters are usually measured in terms of the diameter of the burn tube - usually 6 or 8 inches. The cross-sectional area needs to remain constant throughout the system.

Your pipe diameter will ultimately be constrained by what the barrel will accommodate.

If you look at Paul Wheaton's many videos recently posted to YouTube, you'll see the fuel feed, burn tube, heat riser, and barrel, are all basically the same size regardless of the environment it is placed in. It's the thermal mass that changes in size.

If you've not built a rocket mass heater before, read the book by Ianto Evans and Leslie Jackson, and just copy the tried-and-tested designs in there.

P.S. I live in the UK too. Are you anywhere near London? I've never built a RMH before so there's a possibility I could lend a hand as long as it doesn't involve much travel.

EDIT: Just realised my offer for help might come over a little too keen. I won't take any offense if you're not interested.

I admire your ambition and it's a clever idea but I still think this is rather challenging to do with a smaller greenhouse.  It has a very high surface to volume ratio.  It requires a surprising amount of insulation and/or heat and there is relatively little space for plants.  "Season extension" is one thing, growing year-round is another.  It may be easier to: 1) Grow more during the warm season.  2) Partner with someone to have a larger growing space, 3) have a vertical garden indoors.  (I started one on a bookshelf!) or 4) use south facing windows or make a sun-space of some sort, or make the greenhouse an attached building so you can share the heat flows with a larger structure.  5) If you make a tiny greenhouse within a greenhouse that is only 2 or 3 feet on a side, you could keep it warm with a 50-100 watt heat source like light-bulbs and/or a 500-1000 lb compost pile.  6) Consider putting it more in the ground (earthberm/Wofati).  7) If you're willing to grow cold-tolerant plants (like spinach, arugula and broccoli), then you can establish them in the autumn and slowly grow them into the cold season, and the heating requirements are much much less, this can be done even with no additional heat at all.  8) (edited): If you plan on digging into the ground, or you have well-water, you could also look at using some sort of geothermal heat pump system.  I know a turn-key system can be expensive, and they don't produce CO2 for plants, but they're probably reliable, low maintenance, can run unattended, and supposedly it is a method used in the Netherlands with a well and aquifer.  

Rather than to try to heat a translucent glazed-structure with such low insulation value, it can be easier to use more insulation (that is opaque) and artificial lighting.

This calculator might help.  http://www.ecosystems-design.com/climate-battery-calculator.html

Mike Phillipps wrote:I admire your ambition and it's a clever idea but I still think this is rather challenging to do with a smaller greenhouse.  It has a very high surface to volume ratio.  It requires a surprising amount of insulation and/or heat and there is relatively little space for plants.  "Season extension" is one thing, growing year-round is another.

The dimentions arn't dead set yet, and may end up a bit larger but as i mentioned i will be putting in a few different methods for insulating and keeping temperature, such as:
1) The fan assisted 3ft climate battery as mentioned before.
2) Surrounding the lower section in water barrels to increase thermal mass and provide a bench for the plants.
3) Sinking the greenhouse 3ft into the ground to limit wind chill and help insulate.
4) Using double wall polycarbonate to increase insulation vs glass.
5) In winter placing  bubble wrap around the walls to further increase insulation.
6) Placing a large insulated compost bin on the north wall and having a metal wall or fencing inbetween it and the greenhouse to provide passive heat.
7) The rest of the north wall will also be insulated.
8) The Rocket Mass Heater for winted heating.

The reason i want it to be year round is because i hope to eventually convert the greenhouse to be aquaponic. (After ive got the whole year round growth sorted).

Matt Coston wrote:P.S. I live in the UK too. Are you anywhere near London? I've never built a RMH before so there's a possibility I could lend a hand as long as it doesn't involve much travel.

Thanks for the help, unfortunatly i will be doing this on the isle of man.

Peter van den Berg's batch box design has been pretty well optimized, and following his proportions is the best way to ensure success. If you want a smaller bulk of unit, simply choose a smaller system size - the batch box is supposed to work reliably all the way down to 4".

I think you would be well served to build a 5" or 6" system, perhaps putting the core beneath one of the beds to save space. (That is, if you are planning raised beds.) This would allow one burn per day vs. several.