Considering that a sealed fireplace can reduce costs from heat loss during the winter and wasted air conditioning during the warmer months, it is not likely to reduce a home's property value. While there may be some aesthetic concerns with a permanently-sealed fireplace, there are ways to do it that don't negatively impact its decorative value.
Increase Property Value
With efficient central heating systems, old-fashioned masonry fireplaces are mostly aesthetic. That doesn't lessen the fact that they are a major source of heat loss. Sealing a fireplace, then, reduces money a homeowner has to spend on utilities. That can't help but be beneficial in terms of the home's value. Even with the firebox sealed with brick, the combination of a mantelpiece, fireplace doors and ornamental accessories maintains the charming, aesthetic value of the hearth area.