Advice on thermal mass

Hello I would dearly love to set up a rocket mass heater. We have quite a large house in a cold climate and it is constantly, well, not warm! We have recently had a wooden floor laid in the living room which is where I would set it up. There is a large fireplace in there so could site the actual 'rockety bit' there. In order to have a cob bench I would need to lift some of the wooden floor as a health and safety {that old chestnut!) precaution. I realise I could lay insulation down instead but even I would be nervous about putting a large combustible set up on top of the wood. So I have been trying to explore other ways of getting the heat out into the room, and was thinking of the possibility of running the warm air through one or more of the old cast iron radiators, as the thermal mass?? I think I have noticed that the inlet and outlet on these are much larger than the standard radiators and also they are freestanding, and quite nice to look at. Would anyone know of any problems with that? Would it be unwise to run the warm air through them, rather than water? Does anyone else have sleepless nights trying to plan Rocket Mass Heaters?!

You need at least 6" diameter pipes for the rocket mass heater to work, and possibly 8". The ways through cast iron radiators are far too small to use for a RMH. The thermal mass of the RMH is to store lots of heat, so it will have significant insulating properties anyway. The radiators on the other hand pass heat quickly and would hold heat for a few tens of minutes not the hours of a thermal bench. There is a post on the forum about building a RMH on a suspended wooden floor, so it can be done.