This is interesting. I see now that there are two uses and definitions of "Zero Clearance" used in the field. From
http://www.aer-online.com/aerpage.phtml?page=ar-vent1
Quote: "What specifically does Zero Clearance mean? The Zero Clearance rating refers to the amount of airspace between the chimney liner and the inside of the masonry chimney. It also refers to the airspace between the exterior of a masonry chimney and combustibles. For this reason the listing is sometimes called "Zero - Zero" in the field.
If the chimney liner you are installing carries a full UL 1777 listing to Zero Clearance you can install, within the liner's installation guidelines, with no airspace between it and the inside of the masonry chimney. This also is the case when the chimney has no air space
A liner which carries the full UL 1777 listing can be installed to Zero Clearance within the chimney without insulation so long as the masonry chimney it's installed into meets NFPA 211. Again, if the masonry chimney does not meet the NFPA 211 standard, insulation is required."
Which really has nothing to do with the definition of Zero Clearance when referring to a prefab fireplace. From
http://woodlandstoves.com/Faq_Detail.asp?id=917
Quote: "A zero clearance fireplace, whether wood or gas, refers to a manufactured fireplace that you can frame right up to. Think of it as a stove in a box with the purpose of the box being to contain and/or convect the heat into the room while maintaining a cool enough exterior skin temperature that wood framing can placed nearly against it.
What differentiates the various zero clearance wood fireplaces?
There are 3 basic categories: air conditioners, pretty good heaters, and very good heaters. The air conditioners are characterized by pulling more air out of the house than they give back in heat so their net effect on the house as a system is to cool it. Glass doors are usually an option rather than a standard feature and they are not gasketed or sealed, nor should they be closed when the unit is burning. The optional outside air will not provide more than a quarter of the combustion air needs.
The pretty good heaters are a big step up in terms of heating ability. They will generally have sealed, gasketed doors and outside air sufficient to provide all combustion air needs. They are not E.P.A. certified. As exempt units, you cannot control the heat output other than by the fuel load.
The primary difference between the pretty good and really good heaters is that the latter are E.P.A. certified. What that means to you is that you can control the combustion air intake, thereby controlling the heat output and, consequently, the burn time."
Madness!
