Using an old box spring with a new mattress can affect how long your new mattress will last. One purpose of a box spring is to support the mattress; if that support is faulty or old, your mattress will not last as long as the manufacturer intended. Most mattresses are made to last 8 to 10 years. According to the website What's the Best Mattress.com, if an old box spring is used the mattress will last only one third as long.
Most mattresses come with a warranty that covers any defects in the design, materials and construction for a specified amount of time. Mattress companies typically require you to purchase a companion box spring with the mattress for it to be valid. Carefully read the warranty information to fully understand what is required before making any mattress purchases. Keep the receipt, warranty card and tags from the mattress in a safe place, such as between the mattress and box spring, so you can refer to them if your mattress has a defect.
Box spring foundations are not only used to make the bed higher; they are integral to the comfort of a mattress. Mattresses are designed with levels of comfort, and using an old box spring will not give the comfort that a new, matching box spring will offer. Over time the heavy mattress will begin to sink into the worn areas of the box spring and begin to sag and feel lumpy and uncomfortable; this will be exaggerated when an old box spring is re-used.
A mattress and box spring wear out together; every time the mattress is used, so is the box spring. Not only does the foundation provide support to the mattress, it also provides support for the people sleeping on the bed, acting as a shock absorber for the entire set. As the coils inside the box spring age, they become fatigued and will stop providing the support required for the mattress. In turn, the mattress will begin to prematurely sag and wear out without the proper support.