The very last page of the product literature on the official heat'n glo site states "18.1% efficiency". It is on the lower right-hand corner in a box. Heat n'Glo is NOT a "better" brand. To confirm this further, I checked to see who carries these in my state. NOT to my surprise, no reputible stove/fireplace shops carry these. Only wholesale "builders" supply places carry these. That is a big give-away as to the low quality of this brand. There are many other brands that have something similar that you would be happy with that wouldn't cost much more, if anything. I suggest you visit your local stove/fireplace shops and see what they offer. Also ask them about this particular fireplace. This brand is not a good choice. If you have read the other threads on this brand you will see that loud banging noises seems to be a problem in addition to paint problems. Of course, you can purchase whatever you wish. We are just warning you about this particular brand.

I agree that you need 2 flues, and the chimney can be built to accomodate two, especially since you're starting from scratch. Unless you are wedded to the idea of burning wood on the outside, I suggest that you consider going with gas direct vent on the inside, because that's the safest and most energy efficient system you can have, and having a plain vented gas fireplace on the outside. For the outside, most of the heat will go up the chimney, but you probably won't care too much because your priority for the outside is the cozy feel and appearance of a fire when when the outside temp drops to between 45 - 60 degrees and you want to take the chill out of the air. If the porch is screened and therefore exposed to the elements, you probably don't want more heat than that on the outside. But I think your priority for the indoors would (or should) be generating heat safely, efficiently and as cleanly as possible. A direct vent system will do that for you.

We've had problems this year, during warm humid days only. The cheminy had been cleaned, There is a "open" insert into the fireplace the damper has been removed. If the insert were true air-tight that wouldn't (and hand not in the past) mattered, as it blocks air-flow. I tried sprays, and baking soda in the fire box, still it smelled bad on humid days (not on hot humid days as the airconditioning seemed to prevent back flow of air from the flue. Finally I put a small electric heater in the firebox, set to come on about 1/4th of the time. This made the lower chamber warmer than outside and any air-flow was up, not down the cheminy. We're thinking about a new true air-tight with a stainless steel flue liner. I will at minimum remover the insert this spring and put the damper back in.

The Isokern Fireplace system is a great option comes in 3 sizes and can be vented or vent less. Great for outdoors or indoors. Lightweight and easy to assemble and is gas operated.