Hello. We opened up a 13 year old mattress yesterday to determine what could be reused in rebuilding it. We removed the comfort layers to check the flatness of the innerspring. I believe we have a continuous coil open ended type innerspring. Laying a straightedge across the areas where the hips lay, the worst measurement was 1/2". Laying a 7 pound weight in those same areas gave us a worst measurement of 11/16". For comparison, I laid the same 7 pound weight in the middle of the bed (which sees little use) and got a measurement of 3/8". There is definitely wear on this innerspring. I have been experiencing lower back pain in the mornings. What are your thoughts about reusing this innerspring? Feel free to ask questions if any of this is unclear.
Hi PLohr.
Welcome to our Mattress Forum 
Generally speaking, a weight is going to distribute itself much differently on the springs than your body would. Laying a weight on the springs is more similar to if you were standing on one foot on your bed.
You did mention that the mattress is 13 years old - depending on your weight, the overall durability of the materials, the base used (whether it was correct for the mattress or not), activities on the mattress, etc are all going to factor into wear.
If you’re experiencing back pain it’s more likely due to failing comfort layer material but if you believe it could be due to wear in the springs, I would not recommend re-using them.
Of course, you wouldn’t know for sure until you lay a new comfort layer atop these same springs. It’s the safest and surest option to purchase a new set of springs but if you want to try and DIY the ones you have first, that’s totally valid!
NikkiTMU