Hi Diane;
We keep a 12x20 x16' tall
greenhouse above 40F all winter in northern Montana with an
RMH.
For the first seven years, we used an 8" J tube, and then we switched to a 6" batch box.
Depending on temps you will be burning it all day from 7 am thru 9-10 pm.
I lived in Putney, Vermont many years ago and indeed you will get cold temps.
If you build an 8" J tube you will be adding
wood every 45-60 minutes.
With a 6" batch box it will be every 2 hrs or so.
A brick bell is a better choice if you're wanting to remove it in the spring.
As long as you build your RMH with clay/sand mortar it can be disassembled and all the mortar saved.