Rocket mass heater for NZ greenhouse

Hi everyone we are in the south of New Zealand and with help from woofers have built a 6m x 2.3m x2m high greenhouse. Very pleased to have found Permies and a huge wealth of information.
We're wanting to put a RMH in for heating in the winter and found the  brilliant book by Erica and Ernie Wisner.
Our concern is the amount of time required to stoke the fire before night fall. Would it need pretty much constant attention for two or three hours as we fear or could we come back and forth every so often until it gets up to temperature?
Any help/comments much appreciated
Carol
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Hi Carol;  Welcome to Permies!
Congratulations on getting your green house built!
A RMH will certainly keep your greenhouse warm in the winter.
Will you need to "baby sit" it ?  No , with a few qualifiers.
While it is still drying out , it will need watching.
First time lighting of the season , may need watching.

Once you have a dry warm mass . Depending on the size and style of RMH you build, 30-60 minutes between visits is common, a batch box style would burn longer.
They also relight extremely easy if the fire happens to burn out before you get back to it.

Thanks that sounds great.  We are up to putting the raised beds into the greenhouse so will arrange a space for the heater and possibly work on placing pipes under the soil of at least one of the beds. The greenhouse itself is made of recycled wooden frame windows. I'll try to send a picture. Might look at retrofitting double glazing at some stage too but that's another thread ...
Thanks for your help

If the greenhouse isn't too far from the house you could setup a RMH cooker/oven as well so you're not just using the heat to keep the greenhouse warm.

Hi Carol;  Nice looking greenhouse! With that roof, it looks like heavy snow is not an issue...Thanks for sharing the photo!
In our greenhouse, rather than burying our pipes in dirt (not the best for a mass) we built a brick encased stone & cob mass and then we set our starts on top of that. Keeps their little roots nice and warm.
This way we have a true heat holding mass that keeps our greenhouse  warm thru a Montana winter.
I'm not sure how cold it gets for your winters you might not need as dense a mass as we do.

Thanks for the cooker idea, the house is a fair distance away but will give that some thought.
Wow that's a decent snowfall. We are pretty temperate here and snow isn't that common. About 5cm is about the most we'd get at at time and nothing some years. I don't think the almost nonexistent pitch on our roof will be an ìssue. May add some extra timber and bracing if it is.
The greenhouse is really to extend the season a bit and  let us grow tomatoes peppers cucumber basil etc which are marginal outside
Thanks for the ideas and photos. It's great to see what other people are up to ☺
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Kia ora Carol, and welcome to Permies. I've been using a little RMH to take the chill off my glasshouse for a while now. We're into our third winter using it and it works quite well. It's a 4" J-tube and just a little undersized for the space, but since we're in the North Island that's not such a big deal. All I'm trying to do is keep the inside from dipping below 5 degrees, and if I can get some more insulation to stop the heat loss out the single glazing that will help immensely. A cold night in this location is rarely more than minus 2 or 3 and we've only had snow once in the 14 years we've lived here.

What I've found is that starting the fire earlier in the day, around mid-afternoon, is a big help. That charges up the cob mass and means that I can stop feeding it in the evening and let the fire die, and the radiant heat from the cob keeps things just warm enough. I find that I need to traipse out there every 30 minutes and feed it, on average. Since it's close to the house, this isn't a big deal, but I may convert it to a batch box at some point so I can load, light, and leave. I've keeping chiles, tomatoes, eggplants, lemon grass, a dwarf banana, turmeric, galangal, ginger, basil, and other tender plants going pretty successfully.

The cob bench on my system is atop concrete fenceposts and rubble to provide some thermal isolation from the ground, and also to keep it dry since we excavated 80 cm below grade for extra volume and to get the flywheel effect.

What size system are you contemplating? J-tubes are nice from a simplicity standpoint but do entail the feeding routine. A batch box is a little more advanced to build (not much) but has a big advantage on the running side of things.

Looking forward to seeing your progress...whereabouts are you?

Thanks for your reply. I would love to be able to grow ginger!
We are planning to build a 6 inch J tube heater but we are still very much in the planning stages.
We are inland from Oamaru in North Otago
Cheers
Carol

Hi Carol,
I'm in zone 10a in Oz. I'm currently building a brick 6" heater.
Excuse the mess. I'm in clean up stage at the moment.
Once it dries out and I can re-seal any cracking in the mortar,
it will be rendered in a lime plaster and I have a granite slab for the
bench, bell and feed ready to be applied.
I think this style would suit a green house well.
You could use the bench (or benches) above ground or below soil level.
Another method could be to use earthenware pipe under the soil.
If it hasn't been mentioned...metal pipe will rust out quickly and a cob bench
will likely detroiate quickly in a moist greenhouse environment so brick,concrete will suit the job better.
happy to help or even build.
Thanks
Dan

Dan Hatfield Ii wrote:Hi Carol,
I'm in zone 10a in Oz. I'm currently building a brick 6" heater.
Excuse the mess. I'm in clean up stage at the moment.
Once it dries out and I can re-seal any cracking in the mortar,
it will be rendered in a lime plaster and I have a granite slab for the
bench, bell and feed ready to be applied.
I think this style would suit a green house well.
You could use the bench (or benches) above ground or below soil level.
Another method could be to use earthenware pipe under the soil.
If it hasn't been mentioned...metal pipe will rust out quickly and a cob bench
will likely detroiate quickly in a moist greenhouse environment so brick,concrete will suit the job better.
happy to help or even build.
Thanks
Dan

Hi Dan, thanks for your reply and handy tips. Your heater looks very neat. I think we will use aspects of your design and info for ours. Still at the planning and collecting stage so will keep you posted.
Regards,
Carol