If you go to the website hearth.com you might find the kind of fireplace aficionado who has expertise on your particular problem. I suspect that you will not know the answer to your questions without removing the bricks that seal your fireplace. If your home is in a subdivision of similar homes and one of your neighbors has a fireplace that was not sealed up, you might be able to get some idea of what's possible for your fireplace.

I suspect that getting the fireplace to work, after undergoing the messy job of unsealing it (probably very messy), will be conmplicated because the vent is used by your furnace. I think there are some gas fireplace inserts that are retrofitted into an existing firebox and that vent horizontally to the outside, although most vent up through the chimney. I'm not sure from your description of how your house is situated next to your neigbor's whether the 18 inch space is sufficient to accomodate such an exhaust. It may be possible, however, to vent to the outside horizontally through the wall behind the fireplace, being careful to avoid the furnace vent, and then to install a vertical duct attached to the side of the house up to above the roof line.

The optimal gas insert would be one that uses direct vent technology. This is a sealed unit that has no effect on indoor air quality because it has 2 vents to the outside, one to exhaust the fumes from the fire and the other to draw fresh air in from outside to feed the combustion. These units are costly, but they put out heat like a blast furnace, and they are highly efficient.

Another consideration depends on the age and condition of your gas furnace. If replacing it some time soon is a possibility, the new ones are so efficient (over 98%) that they can safely vent horizontally, because the amount of heat and fumes to move out is so small. Installing such a state of the art furnace would save you fuel in heating your house, and would eliminate the need to vent through the fireplace chimney, freeing up the chimney (maybe) for retrofitted gas insert.

So, I would first check with neighbors who might have a similar fireplace. Then, I would try to get as many free estimates as possible from gas insert firms. They are usually happy to come to your house & size things up. From my own experience, which is limited, the choice of a really competent, honest installer is at least as important as the product you are installing. So ask around. Good luck.

There is another web site that I administer. that can help answer your questions Code wise there are a few potential problems. One can not share the same flue with another appliance. Your burner may be the only use for your existing flue

Code also addresses direct vent locations and definitely there is language concerning close proximity to neighboring homes and existing walk ways. To advise you more we need more details We have a gas appliance forum you can post on at www.hearthtalk.com

Here is a link that might be useful: Hearthtalk

Thank you for the feedback.

haus_proud, I think you're right -- time to ask an installer to come by and take a look. I did speak with one neighbour, but she has a chimney with two flues, so it wasn't a problem for her. On the other side, not sure what they did -- worth checking out, as you note. On the age and efficiency of the furnace, it's not old, but it is only mid-efficiency (previous owners' choice). Might be less expensive to replace that than get "creative" with the fireplace and venting. As you can likely tell, this is all new to me ...

And thanks, elkinmeg. I'll check out the site you mention.