Hi Philip; Welcome to the world of rocket scientists! Many things are possible, heating water can be done although it is challenging to do it safely. I have been heating our greenhouse with a
RMH for three years ... love it !!! went from over 12 cord of
wood to less than 5 !!! I simply removed all the dirt from one of our raised beds and used that space to create my mass 2.5 W x 3.5 tall X 14' long encased with clay brick and filled with stone and
cob. This bench has surface temps from 60-100 we simply set trays of starts on top and they stay toasty warm even when its well below freezing outside and air temp in the green house is 40-45 F . You mention that you plan on replacing your single wall plastic with double wall ... consider solexx instead, its not cheap but it has a 20 year warranty for UV degradation, and has an R value !!! most good greenhouse grade plastic lasts 3 years then need replacing! My solex has wintered 3 years thru wind ,snow ,hail and is as good now as when it went up! Do not buy the ridged sheets look for the linear role it is 4'1.5" wide and they will ship just over 100' free! Having your
rmh core inside your greenhouse will add huge amounts of heat to your building . Using a traditional "J" tube style like me or the newer batch box, either will provide all the heat you need . When it is below zero here we start the j tube in the morning and once its going well you will only need to check it every 40 minutes, we keep it going all day that way then let it go out between 9-10 at night . Room temp at the time will be 60-65 F next morning at 6-7 am , room air temp will be 40 F +and the temp on top of the mass will be 70-80 F near the core and 60-65 at the far end ! That is with outside temps of 0,-10 F!!! Good luck with your build!