Making It Up. High-volume exhaust hoods can create negative pressure in a tightly built house, which can cause backdrafting of gas-heating appliance flues. Broan’s make-up air duct fittings open the damper to an outdoor vent as needed to balance the fan’s exhaust volume. The LinkLogic SMD damper is controlled by voltage pulses sent through existing house wiring by the exhaust appliance, while the MD model must be directly wired to compatible hoods and fans. Both come with 6-inch ducts (to vent up to 500 cfm) and 8-inch ducts (up to 1,000 cfm) and cost from $149 to $253. Broan-NuTone, 800/558-1711. broan.com