Hi Diddlydo,
I think that the steps and linked information in post #1 here will help.
As you will see … you are probably falling into a similar trap to most other consumers and looking at brands rather than the materials that your mattress is made of. A mattress is only as good or as durable as the materials that are inside it and this is perhaps the biggest “secret” to successful mattress shopping. This is especially important for the upper layers of the mattress (called the comfort layers) because these are the ones that will soften and degrade faster than the lower layers or components because they take most of the stress. The problem with “hills and valleys” is usually not the innerspring (which is a deeper support component) but the upper layers made of polyfoam.
This is particularly important with your heavier weight because if these comfort layers are too thick and/or lower quality … when they soften they will leave you sleeping in a gully. This will happen faster for you than other people who are lighter. The problem is that the gully is because of softened foam when you are lying on the mattress and when you are not on the mattress then it still has enough resilience to come back close enough to level that the warranty is worthless (warranties have exclusions that mostly protect the manufacturer from warranty claims) . This loss of comfort and support that comes from softened foam is not warranty covered because all manufacturers have exclusions for depth of unweighted impressions (although some are more than others).
This would be the same issue with any mattress and any brand that used the same materials. The innerspring is not generally the issue and if you were to take your mattress apart you would likely find that it was still fine but that the foam on top of the innerspring had softened.
The solution to this is to buy a mattress that uses higher quality materials in these layers. This means either higher density polyfoam, latex (the most durable type of foam) or high density memory foam. Firmer comfort layers, thinner layers, and a two sided construction (if you flip it) will also extend the life of your mattress. I suspect that the issue with your current mattress is that even though the materials in the comfort layers used by Original Mattress are higher quality than most larger brands and they are two sided … they ars still not a high enough quality for your weight and this is probably made worse because the layers may be too thick (as in a pillowtop). Posts #326 and 329 in this thread are very similar to your situation and would also be well worth reading.
So the first “secret” to durability is to make sure the materials in your mattress (especially the comfort layers) are more durable.
The second “secret” is to buy a mattress that only uses comfort layers that are just thick and soft enough to provide good pressure relief in all your sleeping positions and no more. Comfort layers that are a little firmer/thinner are better than a little softer/thicker because all foam materials will soften over time … it’s just that the better quality types of foam will soften much more slowly.
The third “secret” to durability is to either buy a two sided mattress (which will last longer than the same type of mattress that is only one sided if you flip it) or a mattress where the top layers which are the weak link can be replaced if they soften or wear out faster than the rest of the mattress (this way you don’t have to replace the whole mattress).
The final “secret” to durability is to connect with a better local manufacturer or sleep shop that understands all of this and won’t just sell you whatever they can for the sake of a sale. When you connect with “experts” they will already know what you would otherwise need to learn.
Some of the better options and possibilities in the St Paul area are listed in post #2 here.
Most importantly … no matter who you buy from … make sure that you know the details about every layer of your mattress and preferably work with someone who can tell you the differences between different qualities of material (or check here if you are in doubt and they don’t seem to know).
Your box spring will probably still be fine as long as your new mattress is designed to be used with a flexible box spring rather than a rigid non flexing foundation (most all foam mattresses …but not all … are designed for a rigid foundation).
Hope this helps
Phoenix