
This was a question I was asked frequently as a kid.

Kids have little concept of hot and cold—let alone what it takes to heat a home. Leaving doors open as they run outside is common. It always has been common, and probably always will be common.
But doors and windows are not the only thing that wastes energy when they are left open.
As an inspector, one of the most common things I find left open on a home is the fireplace damper. When the wind blows past the top of the chimney it creates a negative pressure on the chimney resulting in drawing huge amounts of conditioned air out of the home—especially if windows and doors are left open.
This is incredibly wasteful in both summer and winter when we are expending energy to either heat or cool the home.
I would bet that more than 50% of older style fireplaces that I inspect have their damper open at the time of inspection. Of course modern fireplaces have glass doors that help with this problem. Most likely the best solution is a gas insert that will actually contribute to the heating of the home instead of making your house more like a barn.
Do you know if your fireplace damper is open or closed right now?
By Charles Buell, Real Estate Inspections in Seattle
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I too find a significant number of dampers open during inspections and in some cases they are held open by the required clip necessary for vented gas logs. If there are glass doors that can help conserve energy in those cases and I always recommend doors on the fireplace if there are vented gas logs, although I do not consider that to be a repair item.
You are correct that many are unaware of the significant energy penalty that results.
I always consider the damper when setting up a radon test. If it is open (with no clip) I close it and drop the first 12 hours on the test when I download the data. If it has a clip I do not take the clip off and close the damper but do make my client aware of the conditions.
Bill that is but just one of the many complications in obtaining “meaningful” measurements when it comes to radon.