From the Sure Marine web site :
WHY THE RIGHT SIZE IS IMPORTANT
Many factors are involved when choosing the right heater for a particular boat. Merely purchasing a heater based on its compact size for the purpose of taking the chill off can be a costly mistake in the long run. A heater that is too small for a boat will work hard to meet the demand called by the thermostat. Even though the heater may only keep the boat at a minimum temperature for comfort, it will run excessively and cause maintenance problems due to wear. Smaller heaters can be attractive in boating because they take up a little less space. Many boaters are willing to accept a small temperature difference in exchange for that extra room, without looking at the extra costs in fuel and maintenance due to over working the heater. The wear that is caused from this excessive running would not be covered under warranty.
Bigger is not always better. Some customers have expressed interest in installing the largest heater possible, thinking it will cut down on operation and wear. If a heater is oversized it will be prone to short cycle. Short cycling is when the burner section does not run long enough for its proper heat-up. Like engines, burners must be run up to operating temperature and there is a minimum run time for this to occur. If the burners optimum running temperature is not reached, the unit will run inefficiently and it will be prone to carbon buildup which can lead to extra maintenance.
SIZING A BOAT
Considering these issues, choosing the right heater is obviously very important. Sure Marine Service offers a basic quick reference chart to guide an installer in the right direction. If there is concern about whether or not a heater is right for a boat, there are formulas that can be used to figure out the requirements. The internal volume of the spaces being heated should be multiplied by 12 for a sailboat ((length x width x height) x 12), and by 15 for a powerboat ((length x width x height) x 15). These numbers will give a round number of the BTU requirements, but other factors must be taken into consideration. When considering a heater, the construction of the boat must be considered as well as its size. Windows are sources of heat loss that need to be considered. If a main saloon has multiple windows the amount of heat for that space should be raised. Whether or not a boat is insulated is another factor. If your boat is not well insulated you should raise the heating requirements. Be sure to consider these variables if a boat size rests between heater sizing specifications. We have included tables to assist you in your sizing decision.
Properly sizing a heater for a live-aboard application is crucial. Air heaters should not be used. Since there is no way to store household temperature warm air, an air heating system must fire its burner every time heat is needed. This leads to excessive run times and maintenance issues.
In general, live-aboards and larger boats require a water heating system using a DBW 2010 or above. The DBW series are the only heaters in the Webasto line that we advise for such installations. These systems are designed to maintain a larger supply of hot water which provides a thermal mass to store heat. This cuts down on burner run time while providing good heating system performance.
A properly sized and installed heating system will run better and cost less in the long run. By selecting the right heater for your application the system will work well and the owner will be happy.