Some of the units you mentioned if I’m thinking correctly have very little storage themselves so storage is added then batch burns are performed. One thats known for that is a Garn, they start at a 1000 gallons I believe and a fire is lit once a day then allowed to go out once the water reaches temp. Most OWB’s have enough water storage in them that adding storage usually isn’t required although I do have an additional 450 gallons of storage I’ll explain shortly.
My 450 gallons of storage also doubles as a waste oil boiler I built a few years ago, in the shoulder seasons I let the wood boiler go out and just fire up the waste oil boiler to heat my water once a day, usually at night so if we have any call for heat plenty of BTU’s are available. With my old boiler it couldn’t keep up so I’d run the waste oil boiler during the day as well to heat the shop. While you’ll have much longer burn times bringing all the water up to temp you’ll also have MUCH longer idle times, this became very apparent my first year with a G400. So figure while having to heat up 700 gallons of water I also had to wait for 700 gallons to cool off enough to call for the G400 to run, this lead to excessive idle times and a lot of creosote unless very dry wood was used. My solution was to install a solar differential controller, this monitors storage temperature and the temperature coming from the G400, I had a fifty plate flat plate heat exchanger between the two so the water would never mix, the solar differential controller would start the pump on the storage that ran to the FPHX only when the G400 was hotter than the storage temp, another nice feature is you can set at what temp it starts to run as well, I set it so it doesn’t start the pump until the G400 was up to 150℉.
This system works much better since I moved down to a G200, the 400 would come up to temp so quick the water in storage rarely got above 170℉, with the lower high end output of a G200 it runs much longer allowing the water in storage to average around 175℉ instead. I also use the 450 gallons of water as a buffer tank, normally unless I’m abosulutely positive I’ll be working in the shop the next day I usually keep the night temps at 45, going out in the morning and cranking 200,000 BTU’s worth of heaters up to 65 would really pull hard on about any boiler, however having 450 gallons of water at 175℉ works very well. Also would take a LOT more flow from the boiler to keep up with 200,000 BTU’s. I have a single 100,000 BTU air handler then a 50,000 BTU under the bolt bin directed at the lathe, bridgeport and drill presses, another 50,000 BTU is under the refrigerator I store welding supplies in pointing at the welding bench. I have the 50 plate HX installed thru the back of the fridge as well, the heat that radiates off that keeps all my rods nice and toasty and dry, a few holes drilled between the fridge and freezer on top keeps several forty-five lb spools of wire dry and rust free as well.
As are most OWB’s, HeatmasterSS is an open system so no need for expansion tanks, on my storage I just took a 3/4” pipe cap, chucked it in the lathe and cut the threads out, this I just drop on the 3/4” pipe that acts as a breather.
Being that your shop is radiant it will be kept at one temp, so the large heat load every morning won’t occur so no need for storage or a buffer tank.
Instead of glycol as long as you can install your heat exchanger after the furnace, the warm air from your furnace will keep your water above freezing if the fire is out for an extended period. I’ve thought about this a bit and maybe others will chime in, but I’ve thought that if a person was going to be gone for awhile, it would almost be best to let it go out before you leave, then place a metal bucket or cap over the stack to hold all the heat in the stove as possible instead of trying to cram it clear full before leaving.