My home was built in 2005, I have a shower/tub combo that has only been used once since it was installed. I recently had issues with hearing sucking/draining noise coming from the drain in this bath when toilet was flushed, I discovered the water in the trap had evaporated.. Easy fix! But when I turned on the tub faucet I only had cold water, and when the handle was moved to hot the water nearly shut off completely. My home builder advised me to tap on the valve/handle with a hammer to free a possible stuck valve. Is this a plausible fix, or is a new valve needed? And how hard is a valve replacement? Thanks Tony

The cartridge may be installed backwards.