Hobbyheater is right, more info on the system would be helpful.
I'll try to answer your questions below and hopefully someone can correct me if I'm wrong. First, a generic overview of how it should work, some of which you probably already know.
The boiler is probably plumbed to a storage tank along with the heat emitters (radiant floor, radiators, fin tube, etc.). This hot water storage allows the boiler to run at 100% output until the wood is gone. Some heat will will go to the heat emitters in the house and the rest into storage. Without storage, the boiler would have to reduce its output or overheat the house.
The storage probably runs between 120F - 190F (50C - 85C). The boiler will go into idle mode when the max storage temperature is reached if there is still wood in the firebox. It's good to avoid this since it will result in lower efficiency, more chimney smoke and probably creosote buildup.
The boiler output and stored hot water is normally mixed down to a lower temperature before going to the heat emitters (say 80F-100F for radiant floors, 140F-180F for radiators). This may be automatically adjusted based on the outdoor temperature (outdoor reset) or it may be fixed. Usually a thermostat calling for heat will turn on the circulating pump and/or open a zone valve to that room. Over time, the temperature of the storage will decrease as the heat is used to warm the house. Another fire must be started to recharge the storage with hot water.
The system should include a boiler protection valve, probably in the loading unit along with the boiler circulator. This valve keeps the boiler hot water output from going to the rest of the system until it is up to 140F (60C). This is necessary to keep water from condensing in the chimney / boiler which will destroy it by corrosion. If there is no boiler protection valve, the minimum storage temperature should be 140F.
how often should I reload?
When the storage temp is at the lowest desired temperature. If you only have radiant floors, maybe 120F / 50C. You may have to experiment with the amount of wood you load to prevent the storage temperature from going too high. If the storage is not up to the max when all the wood has burned you may want to load more to keep the fire going longer. It will depend on the amount of storage and how cold it is outside. It's probably best to load less wood to start and add more rather than hit the high temperature limit or idle until you get a feel for it.
What does the "Storage temp. low-> RECHARGE" error mean?
It sounds like the hot water storage tank has fallen to the minimum temperature set point. Time to start another fire.
What temperatures should the storage temp up and low setting be set to?
What are they now? If the system is working OK, I would not change them.
How do I stop the system tripping when it gets to 90 degrees?
Do you mean the boiler turns off or turns down its output? If so, you may have loaded too much wood or there is no storage. Don't increase this if it's 90C. Actually boiling the water should cause the pressure relief valve to open and can cause the water in the system to become corrosive. If there is no PRV, it could blow up! You may also have a thermal discharge valve which is designed to prevent the boiler from overheating (commonly used in Europe, not so much in the US). It will open and allow domestic cold water to run through a cooling coil in the boiler. This valve and the PRV could create a big mess depending on how their drains are plumbed.
You might contact the previous owner and find out what heating company they used for maintenance or repairs. Paying them to come out to familiarize you with the system and answer any questions might be a worthwhile investment if you have any concerns.
