Okay, so a LOT of questions, concepts and misconceptions in this post. I will try to clear up what I can:
Wallas builds two different lines of marine products, kerosene products and diesel products. All diesel products have a "D" in their name (as in "30D"). The kerosene line is comprised of products that are older in design, since Wallas started making kerosene (aka "paraffin" in Europe) products more than 35 years ago.
Wallas diesel products came along in the 1990s and any of the diesel products can burn diesel #2, diesel #1 or any of the kerosene fuels approved for the Wallas kerosene products. Wallas kerosene products can't burn diesel.
All of the Wallas stoves (with or without heater lids) and the model 40D furnace draw their combustion air from the cabin, burn it and send it all overboard. While the volume is low (<15cfm), it will typically change out cabin air a couple times a night, drawing fresh air in through the cracks and frames of the boat, replenishing the oxygen and removing respiration moisture from the cabin. If you have an unusually airtight boat, crack a window in the cabin on the side opposite the stove.
There seems to be some confusion about furnace air and exhaust we need to clear up. For all forced air heaters, we need to consider combustion air, makeup air (fresh and return) and exhaust gas.
Unlike other brands, Wallas furnaces and heaters have two separate fans. One is the combustion air fan and the other is the warm air blower. The speeds or rates of these fans are controlled independently of one another, allowing broad power range and continuous use at any power setting.
Combustion air is drawn into the furnace and fed into the combustion chamber, where it is burned and sent overboard as exhaust gas. It can be taken from either the cabin or from outside, depending on preference and on the required volume. Wallas products all have relatively low combustion air demand, so can take their combustion air from either outside or inside the boat, depending on the model.
Makeup air is the air that is drawn in to the furnace to cool the "cold" side of the heat exchanger. Through this process, that air gets warm and is blown into the cabin to heat the room. This air can be taken from inside the cabin ("return air") or from outside the cabin ("fresh air"). Fresh air is rich in oxygen and generally drier than return air. return air is already warm, so will improve the net heating effect. We consider the best installations to include a mix of return and fresh air for the makeup air.
Of the five furnaces in the Wallas line today, three are kerosene models and two are diesel models. All but one (40D) of the furnaces uses what we call "duplex" or "coaxial" exhaust. This system incorporates an exhaust pipe that is fitted inside another pipe. Combustion air is drawn in through the space between these pipes. This preheats the combustion air as it approaches the combustion chamber, adding efficiency. It also means that when wind blows against the through hull fitting, it will be pushing against the exhaust outlet as well as the combustion inlet, meaning it will not effect the burn very much.
The 40D is the highest output furnace Wallas makes. It uses a simple single exhaust pipe, as the stoves do. It has makeup air inlet nipples, which mean it can be installed in a diesel engine room and take makeup air from elsewhere, so the smells and potentially noxious air from the engine(s) will not be introduced into the living quarters. Since it takes its combustion air from the engine room, it will help pull a mild vacuum in the engine space, helping keep smells from entering the living area.
As a final note, nearly all modern forced air diesel furnaces, heaters and stoves burn diesel or kerosene at a very high temperature (about 600C), resulting in extremely efficient burn. This also means there is very little CO produced in the burn process, making these products very safe to use, but all exhaust gas does need to be vented overboard and the exhaust systems should be inspected regularly for cracks or loose fittings.
Here's hoping this helps. Feel free to contact us at Scan Marine if we can be of assistance.
Doug