I would not recommend that you treat the living/dining room area any differently from the rest of the house with regard to heating.
My reasons are as follows:
- You want to maintain a relatively consistent temperature/humidity throughout the entire house (save the garage) so that you can maintain the integrity of your finishes and furnishings (moldings, wall coverings, floor coverings, furniture, etc.). Any wood products in the room such as baseboard, door trims, hardwood floors and furniture naturally expand and contract with heat and humidity. Therefore, if the room(s) are excluded from the heating system this can cause wide fluctuations in the temperature and humidity, which will be hard on not only the wood products but also paints, wallpapers and upholstery, especially if the room is brought up to temperature quickly when entertaining. Lastly, by closing off the rooms from the heating system you may also make the room susceptible to mold and mildew growth and their offending odors.
- Consider the life of the house. Your future use or a future occupant's use of the living/dining room may be different from what you anticipate today, so don't limit your options.
- If you block access to the living/dining room with a door, you may be giving up opportunities for maximizing daylighting and/or passive solar heating.
A gas fireplace
Adding a gas fireplace to the living/dining room can add value to a home and increase your overall enjoyment of the home.
- By spending the extra money upfront on a high-efficiency model, you will most likely break even when you factor in the savings on your utility costs over time.
- And these units help preserve your indoor air quality better than standard units.
As for a recommendation of various manufacturers, that is challenging only because I do not know the style of architecture of your home, the layout of your home, or your budget. A few things to consider:
- Compare the efficiency rating in BTUs for the various models you are considering -- direct vent models are going to be the most efficient models.
- Consider the overall style and availability of finishes and grill options.
- How realistic do the "logs" look when the unit is on?
- What are the electrical requirements and does it have a battery back-up that allows you to light the unit during a power outage?
For more information:
Also, read Lee Hall's Q&A "What is the greenest, best fireplace replacement?"