Vented fireplaces must be installed in a fully functioning wood burning fireplace, they look the most realistic and lose the most heat as it rises up the chimney. Vent Free fireplaces deliver more heat to your home than vented but some states do not approve, check your local codes. Direct Vent fireplace systems come in various sizes to fit most fireplaces and have an adjustable heat output. 

Most fireplaces are available in either natural gas or propane. If you have an existing system, check to see what it is prewired for. If not, check to see what lines your other appliances have. Homes that are wired for natural gas in their stove or dryer will typically use the same in the fireplace.

Measure the width, height, and depth of your fireplace opening to be sure the one you buy will fit your space. If you are looking at inserts or gas logs, be sure to leave enough space for them to fit. 

In standalone grills be sure to determine whether you want it to come in propane or natural gas. The cooking surface and BTUs are important to how much you will be using your grill and how many people you will cook for.

Be careful with built ins to check and see if you will have a power source. Some models require it while some do not. Measure the height, width, and depth of your grill opening to be sure you are purchasing the right size and be careful to choose natural gas or propane depending on your needs.