First, DO NOT get a ventless system. Many building codes actually outlaw them. I think they all should because the build up of fumes can be dangerous. Therefore, the choice is between a VENTED gas log system and something called DIRECT VENT system. A vented system works pretty much like your present conventional wood burning fireplace. It will provide some heat, but most of the heat will go up the chimney. It is not an energy efficient system. A direct vent system is comprised of a sealed firebox equipped with 2 vents that go up the chimney -- one to exhaust the fumes and the other to bring in fresh air to feed the combustion. It is very energy efficient, and can give off some serious heat. Direct vent systems are costly and take close to a whole day to install, assuming you already have a gas line in place. You really should consider this option if you want more than just the pleasant appearance of a flame. One more consideration: Since direct vent system can get very hot, it's a good idea to get one with a remote thermostat. Otherwise you'll wind up turning the thing on and off over and over again. But if you live in a climate zone where a kit of heat is not the primary consideration, for instance in Florida, a vented gas log system would be a reasonable option. If a really DRAMATIC dancing flame is what you're looking for, I think the Eiklor brand is the gold standard. But it will use a LOT OF GAS, and most of the heat will go up the chimney. So you're almost literally burning money with that one. I'm partial to direct vent. We have one here in North Carolina, and we love it. Make sure you find someone with a good local reputation to do the installation. Otherwise you'll wind up with buyer's remorse.

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 had an gas insert put into our main fireplace in the living room and we all LOVE it. It has a remote control with a timer and you can set the temperature as well. For example, set the temp for 68 and it will automatically come on and the house will be toasty warm when we get up. It has a blower to circulate the heat through the house but it so quiet, I really don't hear it. The blower can be turned off as well if we really don't want the heat but want the fire on just for "show." We loved it so much that we added another gas freestanding stove to our sunroom off the kitchen. That one has a thermostate on the wall to set the temp. Both have real "fire"-to me it doesn't look one bit different than our old "fire" but oh boy, what a world of difference. No work-nothing-push a button! we use them all the time and never even use the whole house heat anymore. Please go look at the stores before you decide because this was one of the best improvements we ever made! The things I hated about the "real" wood fireplace, getting the wood and having it in a buggy stack outside, hauling in pieces and the mess it made inside, "tending" the fire, sparks flying, cleaning the mess, worry of having it still burning if we had to leave the house and trying to put it out, and most of all, our pediatrician told us it was really bad for the babies. She said even the best of fireplaces contributed to indoor air pollution. Things I love about the new insert and freestanding stove-pushing the button! Being able to set the temp to come on automatically, being able to raise and lower the flame to change the look, the realistic looking embersm NO cleaning other than once a year the company comes in and cleans the glass and checks it out for a minimal cost. Just for your info, they really can get HOT, you don't want to underestimate the power just because it is an insert. Ours seems to be much more efficient than the real wood fire and I would not put it too close to my kitchen table!

Another wood fireplace lover here. That being said, we're in the process of having a wood-burning insert installed in our fireplace (Pacific Energy Vista). At the rising costs of fuel it just makes sense. We're planning on using it as an alternate heat source, to keep the oil barons at bay! The warmth, feel, and visual effect of a wood fireplace can't be beat. Now it will be behind glass, but we'll still get to watch it, while watching our oil bills drop. It's also good exercise ;o) You don't get exercise from flipping a switch or pushing a button on a remote LOL. And with the carbon footprint being minimal using a wood burning appliance I know we're doing our share for the earth.