I just took a look at NFPA-54:
188.8.131.52 This code shall not apply to the following items (reference standards for some of which appear in in Annex L):
(18) Fuel gas systems on recreational vehicles manufactured in accordance with NFPA 1192, Standard on Recreational Vehicles.
Guess I shall now go read NFPA 1192.
I took a look at:
The words "direct vent" occur four times in the standard, all on the same page, (24):
5.4.6 Requirement for Direct Vent System Appliances.
184.108.40.206 All fuel-burning appliances, except ranges and ovens, shall be designed and installed to provide for the complete separation of the combustion system from the interior atmosphere of the recreational vehicle.
220.127.116.11 Combustion air inlets and flue gas outlets shall be listed as components of the appliance.
18.104.22.168 The required separation shall be obtained by the installation of direct vent system (sealed combustion system) appliances.
22.214.171.124 A fuel-burning refrigerator
shall be permitted to be installed to meet the requirements of 5.4.6, using panels supplied by the recreational vehicle manufacturer, provided that the refrigerator
manufacturer furnishes the necessary vents and grilles as specified by the listing requirements and, in addition, the refrigerator
is equipped with the necessary means to ensure the integrity of the separation of the combustion system when the refrigerator is removed for field service and reinstalled.
126.96.36.199 A fuel-burning appliance shall not need to be of the direct vent type, provided that it conforms to all of the following:
(1) It is a vented appliance.
(2) It incorporates provisions for introduction of combustion air from outside the vehicle.
(3) It incorporates a safety control system that prevents burner operation under any operating conditions that allow products of combustion to discharge into the interior of the recreational vehicle.
(4) It incorporates provisions either integral to the appliance design or by use of a safety control system(s) to protect against ignition of flammable materials that could come into contact with any heat source or part of the appliance.
(5) It is listed for recreational vehicle installation and is installed with the terms of the listing.
5.4.7 Exterior Appliances.
First Revision No. 14:NFPA 1192-2011
[FR 31: FileMaker]
188.8.131.52 Exterior Ffuel-burning appliances installed or intended to be used only outside the and attached to recreational vehicles shall be listed for recreational vehicle use but shall not be required to be of the direct vent, sealed combustion type.
184.108.40.206 The installation shall preclude the possibility of appliance operation or propane
flow when the appliance is in its storage (travel) position.
220.127.116.11 The appliance manufacturer shall specify clearance to adjacent surfaces as applicable in both the operational and storage positions.
The apparent typos at 18.104.22.168 are the result of changes to that section. The link shows it better.
My observation is that it does not exclude a direct vent furnace
. At least not on the basis of CO danger.
I am aware that they are no longer available, and are probably not legal to install, but I fail to see why. Neither document examined has provided any answers.
It has been my assumption that this was more a political decision then anything technical, and I see no reason to update that assumption.