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If the problem is only in the tub then it's not the water heater that's the problem. Can you post a picture of the faucet. Click on ADD FILES next to the SEND button to post pics here
Debris is possible. He can flush out the debris but turning off the water to the faucet then remove the stem then turn the water back on.
If this is a single handle faucet then the cartridge would have to be removed to flush it out
A loose washer (assuming a 2 or 3 handle faucet) or a part that isn't original equipment can also cause this problem.