If my house needs another natural gas vent, should I get a plumber or hvac person to install it?

And how much is it likely to cost? And/or how many hours is it likely to take them to do it? The type of vent that would be used for a tankless water heater, or any appliance with a fan-powered vent. It's a typical A-frame house. One story, slab foundation, moderately steep roof, unused attic (just insulation and access)