I have a dryer exhaust that crosses over a flexduct HVAC line, could the dryer vent cause a fire?
I really doubt it since your dryer duct should never get hot enough.
With the said, a couple things to consider though 1) how close to the dryer does the HVAC line cross and 2) are they touching? If the HVAC tub is plastic covered, I would make sure that they are not touching as it might melt it.
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