Dusty- Good advice! The Wallas heaters have, generally speaking, a fine record.
B-C: If I read into what you're asking correctly, you're asking if the smaller unit might be a better choice than the larger one because it would be running on a higher setting more of the time.
Indeed one of the most common problems with the Wallas and diesel units, in general, is sooting up over time, with the consequent inoperability. Most of the discussion concerning this problem indicates the best preventive is to run the unit one High for at least 5 minutes or more before shutdown in order to clear it out.
The logical implication is that the smaller unit would therefore stay cleaner by working in the higher operating ranges. My guess is that's you're probably right. Unfortunately, this issue has been clouded over somewhat and possibly confused with the great debate over the proper fuel selection, which seems to be settled for the most part.
The problems with questions like this where we're trying to compare two or more units, is that most of the time, each person only has experience with one heater, or or one windlass, one radar, one autopilot, etc. We all have our own experiences which are very worthwhile to share, of course, and this is where Les can be of great service with the broader perspective of more varied experience.
While we're discussing this, I'll mention that one of the problems in selecting a proper heater is to size it properly, which is directly related to your question. Ideally, a perfect heater would be able to operate continuously anywhere from zero to it's maximum BTU rating, without developing performance problems in the process. Rather than have a unit that constantly cycles on and off, putting extra stress on all the electric components, the battery, sooting itself up, and generally being a distraction, the infinitely adjustable output would allow the heater to just match the heat loss and maintain a constant temperature.
To my knowledge, no such unit exists. The nature of the diesel wick, diesel spray, propane burner, and other combustion systems all have a miniumum operating threshold. Diesel heaters soot up when run at lower temperatures, and propane heaters have to maintain a minimum to run the thermocouple and assure complete combustion, and operate within an efficent heat transfer range. The best advice is to get one that's not oversized and one that will operate more naturally in its most efficient mode.
Experience with electric heaters would indicate that the typical small space heater which has high and low settings of approximately 1600 and 750 watts, which are roughly equivalent to 6000 and 3000 btu's, seems to be able to accomodate the CD-22 Cruiser for commonly encountered conditions with it's termostatic control. To put a heater in a CD-22-C that operates on, say, a high/low range of either 12,000 or 6,000 btu's would seem to be asking the heater to cycle itself on and of constantly in moderate conditions.
These numbers would have to be sized up and down for the different D-Dorys. The CD-25 has about 50% more interior volume than the CD-22 Cruiser, the CD-16 Cruiser about 40% less, and the CD-22-Angler, about 20% less. Surface area of the cabin is probably more directly related to heat loss than volume , but these figures are probably close enough to get by with.
So you're very wise to look carefully at these two units and make every comparison possible to match your needs more exactly and to get the one the will operate the most trouble free. Heater selection is a $500-$2000 choice (including all installation parts), and worth doing carefully.
Sorry go drone on, but this subject is a complicated one, and very interesting to me. HTH. Joe.
