where to vent pressure relief valve if no access to outside
Given the location of the heater, I'd still take the T&P into the crawl. Keep it in the joist area and pipe it out the rim joist. Once outside terminate it with a 90 pointed down. Keep it 6" above finished grade.
Codes very across the country so this may not be allowed in your area but we do it often.
If you're worried about length, you could always upsize to 1.5" pvc pipe.
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