How many times would you need to fire to heat up the mass?

I thought this might have been discussed before but, I couldn't find it in my search results.

Okay, I love the idea of heating with a rocket mass heater. However, I am having trouble wrapping my head around using such a small firebox and burning the wood it holds up in 15-20 minutes, creating enough heat to heat the mass for any period of time. Wouldn't you need to fire the thing several times in order to get the mass up to temperature? If that is the case, couldn't the fire box be bigger? Just wondering how something like this could work say in a green house and not need to be fired several times a day to keep the green house warm? Thanks.

A greenhouse is not usually well-insulated, so a large rocket combustion system is recommended (8"). You would probably have to keep it fired for several hours to warm the mass sufficiently to keep the greenhouse warm over night.

Hi Denny; Where are you located ? If you are in the north and want to heat a greenhouse thru the winter you most definitely be burning it more often than not, depends alot on the temp you are trying to keep it at. As cindy stated, a greenhouse should have an 8" system to generate higher temps at your barrel and in your mass as well as a larger firebox. with my 8" system in the green house I have to check it once every hour to hour & half, certainly not every 15 minutes. The time of year , the low temps expected , the location and any insulation all have a bearing on how long or how many times you need to fire it a day. They will and do work better than any other wood stove anyplace, whether in a greenhouse a shop or your home.

To first answer your question. In a greenhouse it may take up to 2-4 hours 2-3X per day to satisfy your heating needs. Unless the GH is really small or you live way far south, an 8" system is required.
Here's what it boils down to. You'll need X amount of BTU's to heat your space. You can calculate this by the formula A/R x ^T=BTU's/hr. A= the surface area of the "structure", R= the insulative "R" factor of the walls, and ^(delta)T= the temp differential you want to achieve i.e. 20F outside 70F inside=^T of 50.
A pound of decent dry hardwood contains ~8k BTU's of heat energy.
So if you burn 5lbs of wood in an hour, you'll get approx 40K BTU's. Your RMH barrel will radiate around 40% of that and the rest is hopefully transferred into your thermal mass for later release.