The furnace system
This was a major requirement based on winter experience with the version 1 vehicle. To compliment both the heating and cooling systems, the body was well insulated. Essentially the body has the same treatment as a frozen food delivery vehicle, polyurethane insulation injected into every body cavity. The floor was raised above the chassis rails to allow maximum floor insulation.
Control
RV style furnaces are available with the same sophisticated electronic control capabilities as residential furnaces. Our system has a requirement that ultimately limited the sophistication of the controls and, in the Atwood product family, guided the selection to the 8400 series. Besides heating for human comfort, the system must provide anti-freeze protection for the water-based systems in the vehicle.
This must be automated, allowing the vehicle to be unattended, and disconnected from external power sources, for extended periods during cold weather. The extended period here would be a 1 - 3 day period. The reason this limited the sophistication of the control system was my inability to locate a thermostat that would function in the room temperature range and the range around 35 degrees Fahrenheit. Using two separate thermostats in parallel ruled out the Atwood Excalibur series, which might have otherwise been desirable because of its two stage capabilities.
Details of the thermostat selection are available from the link at the bottom of this page.
Capacity
Given the relatively small space, 422 cubic feet in grooming area and 111 cubic feet for equipment; and the insulated shell, recommended heating capacity was small. By some calculations the requirement might be as low as 2,000 BTU. I had a hard time comparing that to what I saw in general use in RVs and other commercial grooming vehicles; typically in excess of 24,000 BTU.
Consumption
When examining available systems, I decided to use an Atwood product. Within that product family, and presumably with all furnaces, the amount of gas consumed is directly proportional to the heat required; with an efficiency factor based on the furnace unit.
Another limited power source required by the system is electricity, 12 VDC, used for the blower. This did vary by model. The Atwood 16,000 and 20,000 BTU units use the same blower and consume 4.6 Amps while running. The higher capacity models consume 7.6 and 9.8 Amps. So I decided to go with the highest capacity in the low electric power consumption bracket.
Placement
The location of the furnace and associated ductwork was one of the more difficult design issues. The furnace requires a lot of airflow, both output and return air, to breath properly and not overheat. The output air requirements were clearly defined, and required four 3" ducts. Coordinating this requirement with the limited space in the equipment compartment was a challenge.
The furnace itself must be cut through the body with an external access door. A location on the curb side at the bottom-forward corner of the equipment space fit the bill. The unit would be rotated 90 degrees from its typical horizontal installation. This would allow taking output from the rear of the unit and directing return air to the top, along the equipment space bulkhead. Click on the thumbnail below for an enlarged view. The red areas show the furnace and the duct that runs to the bathroom. The blue area is the return air duct from the grooming space.
