Jonathan, I am pretty certain this is a fully viable option, though I've not seen it done yet. I would be very interested in any similar work others have done.
I would suggest a transfer coil in a large tank (200+ gallon - but larger is better, outside of other pragmatic factors of course).
You may get some good ideas by looking at the designs from other outdoor wood burners (Hardy, CentralBoiler, etc), which have built and run this sort of process (a cyclical run process where a large thermal mass is heated and released over time)
I'm quite interested in this as well, as we are working on several newer iterations of the large mass burn rocket design, with some control capabilities.
There are several significant factors to consider on this:
1. Do you run a single large volume tank, or a primary tank around the rocket and a secondary 'thermal reserve' tank?
If a single tank, you need to keep it between 175-190 F, (the higher temp is better for more rapid thermal transfer, but too hot causes boil-over and possible scalding)....
If a double tank, you're going to need a secondary pumping system (and significant control systems) to go between the two. However, this method will allow you to better manage the rocket stove output and to keep it at a more optimal temperature.
2. How do you plan to transfer heat to the water jacket? This is a significant process to overcome. We have worked on several dozen processes. The best seems to be a large stainless steel container which holds the water and the heat from the rocket dumps directly against it. Stainless because ferrous steel doesn't hold up well against the high temps of the rocket output.
I could spend a full day or two going thru the various aspects of this... Although you've got a great possibility to save a lot of money and be much more ecologically sane, you're facing several complex technical hurdles to make it run reliably and consistently, and you'd be best working thru these BEFORE you jump up and start building something....
Do you want to run the rocket all day at low output (to maintain a relatively consistent temperature in a large thermal mass), or to you want to run it as a batch process (ie., fire the rocket for several hours to more rapidly charge up a larger thermal mass, then let it cool down over time)? This will drive the size of your rocket stove as well as the sizes of your thermal storage tank and your transfer system.
IF you're planning to run the rocket non-stop (ie., small rocket stove, which heats only a small amount of water at a time), my initial guess on this would be to run a double tank system, with a good transfer pump between the two. You'd have say a 100-150gallon initial transfer around the rocket, with a 400+ gallon insulated thermal bank tank. You can then cycle the heat from the thermal bank to your baseboards, then back directly to the rocket tank (so the water will be colder coming to it, where it will heat more rapidly--you always get better thermal transfer with a higher temp delta), OR you'd have 2 pumps and cycle from the house only to the large tank, and from the small to the large (which would require a significantly higher complexity of control and thermastat/regulation setup)....
If you're planning to simply regulate temp by shutting down the rocket (burning/pulling all the wood out when it's run for xx number of hours), you'll be able to keep up pretty well.
This method is much more a batch method than a cycle system, so it will require a significant amount of involvement to understand how much burn/how long/how hot, etc. IF this is the route you take, then a single large tank on the rocket (400-500 gallon, well insulated, with the rocket directly heating the entire tank) will probably suit you well. This method might do even better, as you would simply pump water to the baseboards only when the house temp is low (which for you is 10 months of the year!), but would also mean that when it's not needed, the thermal reserve would all stay IN the large tank, rather than pumping back & forth (and thus cooling down some).
It would be best to start with an understanding of the parameters you need to understand.... There are a ton of factors playing into what you need though: What's the thermal load of your house? how many minutes/hours does your current system run on an average day? How much load can your current oil unit output (BTU's)? How big is your current tank (or is that for hot tap water only?) How efficient is your transfer system (pipes and radiators), how warm do you need the house, how hot does your water need to be in order to have the radiators run efficiently, how much water does your current loop use to circulate?
Remember that for every 100 gallons of water, you've got 834 BTU PER 1 degree (F) of thermal capacity. SO a 500 gallon tank at 190F has just over 63,000 BTU before the water gets down to 175F. i'm sure you've got the formula already - 1 BTU = 1pound of water at 1 degree of thermal change (F)...
I'd start by figuring out the normal operating temp of the water in your current system (temp when it enters the radiators, and temp when it leaves). That'll give you a couple good points of info, then when you understand what you're up against, you'll know how much water you're going to need and how large a rocket its going to take. Then you can calculate how many burn cycles your system does on an average day, to come up with a thermal load output for your heat system.
I've installed and run several large thermal reserve based systems, and my experience says that maintaining the water in the 175-190F range is optimal for thermal transfer to the house (that's with a water-to-air handler system - - I think that on average radiator systems run 145-150F, but that's merely hearsay- - - it would be good to know what yours run at).
Most outdoor wood burners run 120-200 gallon thermal tanks. However, the reasoning is different- - they're capable of cycling on/off reasonably well, and they need the thermal mass/reserve to be small enough that the unit will still self-light, when the temp has dropped far enough..
If you're thinking batch only, then you are following the design theories of the Garn wood stove system (which is essentially a rocket stove sitting on its side, using forced air induction, rather than the natural draft of a rocket).
garn.com/From a DIY perspective, i'd probably start with the batch-only process (ie., a larger rocket stove and a single large thermal tank), as this method requires only a fraction of the engineering issues to be resolved prior to design as would a cyclical based system. It would require more personal time (ie.,you'd have to feed the beast several times a day), but you'd likely spend a year in development time to get a reliable cyclical process developed. you'd also need a very good understanding as to the ash issues for your particular build, so a regular 'hands on' situation may well be best for you.
There's no reason you can't also incorporate your household hotwater for tap needs in the tank as well - most outdoor wood burners do this and it works great. For that, coiling pipe thru a large thermal mass ( greater than 150gallon) leads to far more consistent temperatures in the pipe, which is very important for both safety and comfort.
Dunno if i've answered any questions,or just raised a lot of new ones......
