Wow - there's a lot of great information in this thread!
I'll chip in with my two cents' worth... My boat came with a force 10 propane (exposed flame) bulkhead heater. It's essentially useless as far as I am concerned, because it simply loads the cabin with moisture, and has obvious carbon monixide risks, while you're at it. When cruising, I might use it on a very cold morning and then vent the cabin while we sail. As an on-again, off-again winter liveaboard in a wet climate, I found that an oil-filled electric heater coupled with a basic fan kept the boat toasty warm, and a dehumidifier running into the bilge kept the cabin bone dry, all at an affordable cost, with very little in the way of up-front cost.
That said, for cruising, I'll replace the force 10 with a dickensen propane bulkhead heater, but only because there's already propane to the heater. If I were starting from scratch, I'd use a diesel heater, partially because they are more efficient, and partly to cycle through the fuel in my tank more often. Even though I spend a lot of days sailing, during the winter my diesel fuel can sit for a long time, and that doesn't do anyone any favors.
So I'll substitute the open flame propane bulkhead heater for a closed flame dickenson for the time being, and then install a hydronic system running on diesel when the money faiery pays me her next visit. I like that you can capture heat from the heat exchanger during the day to keep the boat warm if you are running under power.
Friends who have such a system changed out their Espar diesel furnace for a german variant, which they say cut their annual maintenance costs down from $1-$1.5K per year, to about $1K every 5 years. They heat a 44 foot sailboat, and can achieve positively tropical temperatures when it's below freezing out.
I'd love a wood heater on board if it suited my needs, but the reality is that I simply don't have the space for wood, or the time to muck around with it when I have sailing students aboard.
Any thoughts on the best manufacturer and configuration for a hydronic system would be much appreciated.
SeaLife Sailing (Sailing School Operator)
s/v 'Ma Provence'