(1) For the purposes of this Subsection a non-solid fuel-fired appliance shall be classified as,
(a) direct vented whereby the combustion air is supplied directly from the outdoors to the combustion chamber via a sealed passageway, and the products of combustion are exhausted directly outdoors through an independent sealed vent,
(b) mechanically vented induced draft whereby combustion air is supplied from within the building envelope and the products of combustion are positively conveyed to the outdoors by means of a dedicated sealed vent, or
(c) natural draft whereby combustion air is supplied from within the building envelope and the products of combustion are conveyed to the outdoors through a chimney or Type B vent.
(2) For the purposes of this Subsection, a dwelling unit shall be categorized as,
(a) Type I when,
(i) all fuel-fired combustion appliances serving the dwelling unit are direct vented or, except for fireplaces, are mechanically vented induced draft, and
(ii) the dwelling unit is not served by a solid fuel-fired combustion appliance,
(b) Type II when a solid fuel-fired combustion appliance serves a Type I dwelling unit,
(c) Type III when a mechanically vented induced draft non-solid fuel-fired fireplace or a natural draft appliance is present, or
(d) Type IV when electric space heating is present.