While you may be tempted to keep the same box springs every time you replace your mattress, it may not be the best decision. An old box spring may lose its ability to support a mattress even if it still looks good, and some mattress warranties require a matching box spring; otherwise, the warranty may be void.
New Mattress, New Box Spring
Replacing the box spring whenever you replace the mattress is generally a good idea. If the old mattress feels worn out and uncomfortable, the box spring most likely isn't offering its original level of support. Without proper support, a new mattress wears out prematurely. Many stores offer package deals in which purchasing the box spring and mattress as a set is more economical than buying both pieces separately. As a set, the box spring you get pairs perfectly with the new mattress, so there is no question of fit or concerns over proper support.
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Pay Attention to the Warranty
The warranty on a new mattress you're considering has specific terms and conditions that must be met for the warranty to remain valid. In some cases, the mattress warranty is only valid if the matching box spring is purchased with the mattress. This assures the manufacturer that its mattress has the support it requires with no chance for questionable wear due to insufficient support beneath it.
Your Box Spring and Mattress Should Be Aligned
Modern mattresses are quite a bit different from the basic old-fashioned models that have been around for some time. For instance, pillow-top and plush mattresses do not require flipping, nor do mattresses with a top layer of memory foam or other specialty cushioning materials. However, some of these mattresses require a specific type of box spring or foundation. Using a new style of mattress with your old box spring may not be good for the mattress, even if the box spring appears to be in good shape. For example, a memory foam option designed to remain still isn't meant to rest atop a spring-filled, bouncy box spring, so that old box spring may rob you of all the benefits of your chosen mattress.
When to Throw Your Box Spring Away
If the box spring went through a flood or smells musty and shows signs of mold and mildew, it should definitely be replaced. In some cases, even the most thorough cleaning cannot remove mold, mildew, or moisture-based problems deep within the layers of the box spring cushioning. If the outer fabric on the box spring is torn and stained and the inner springs don't bounce back like they used to, it is time for a new box spring. Worn-out springs may cause even a new mattress to sag a bit, making the mattress less comfortable. This type of wear may even mean backaches or reduced sleep quality.