Pool heater problems

The pool heater for my home recently broke down, and I'm a little unsure as to how to go about repairing it. I think there's a problem with the pressure valve, but it is beyond my own knowledge to fix. I was wondering if it would be cheaper to fix the  or simply buy a new one? I would much prefer not buying new, because it seems like they're quite pricey. Is it risky to buy a used/refurbished model? Thanks in advance!

For reference, I have a Pentair natural gas pool heater (175,000 BTU?), here's a picture I found: 

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