Why are my relays dying?
If possible increase your time base to double or triple the 15 seconds. 

A better relay will last longer- you could also go to an SSR, but it would require a fair size heatsink to dispose  of around 9-10W. 

The lifetime of a mechanical relay will be specified in the data sheet, but typically it’s around 100K at full load, so at 15 seconds  that’s a bit over 2 weeks 24/7. If it’s 50K, obviously half that.