Like most things, box springs do not last forever, even when well built. You will know that it is time to replace them by the way they feel when you lay on your bed. While your mattress may wear out first, sagging box springs happen given enough time and constant use.
When It’s Time to Replace Box Springs
Box springs consist of a grid of coils contained within a softwood frame. Atop the coils there is a layer of material that serves as a barrier between the coils and the mattress. A box spring provides extra softness when used with a mattress. Its coils give slightly with weight and are designed to provide for a better night’s sleep. After time, though, the coils fail to spring all the way up, sagging in spots where the most weight is distributed. This is sometimes prevented by a piece of hard foam or some kind of rigid frame installed beneath the coils. It keeps them from sagging too much.
Test Your Box Spring
If you are unsure whether or not the sagging is from an old mattress or the box spring, remove the mattress and test out the box spring by itself. If, when laying on it, it is noticeably saggy in places, the likelihood is that it needs replacing. Coils lose their spring after time and constant use, so there is little that can be done to prevent other than alternating the weight placed upon it.
It may not just be your mattress that is causing an uneven bed. Your box spring might need replacing if the coils have started to sag in places, losing their spring and affecting their ability to support you comfortably while sleeping.