My residential oil-to-gas boiler conversion would have had a 6-7 year payback, and that's with oil going up past $4. I'm still staying with oil for now.

In your case it makes prefectly good sense due to the space you're managing. What's heating the tap water?

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma