No cap necessary. Rain is not an issue, the exhaust pipe has a drain. Rodents can happen, but a cap doesn’t prevent that, and a screen is typically not recommended (at least in cold climates). Tennis ball, I suppose it could happen, lol. Once in a lifetime shot!
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If you play tennis like I do, I wouldn’t count it out…lol
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There’s a better chance that a squirrel eats the rubber roof boot than climbing into the flue.
Never heard of a tennis ball making it into a flue. Back when I was in the HVAC trade, you could always count on 3 or 4 service calls for locked out furnaces during Easter. For some reason, people love to hide eggs in the intake.
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Good point. Well taken.