I recently replaced my water heater, but I still have no pressure for hot water going into the bathtub. What is the best way for me to clear out the suspected calcium deposit in that line?

Rather than calcium buildup, I suspect dirt in the bathtub valve. Pull it apart and flush everything out. I suspect when the valve is apart you will have plenty of flow. If it is a single handle cartridge type, you could need a new cartridge.

Do what EPMiller suggests first. If you don't have a cartridge valve, then pull out the valve stems and flush through them.

If that doesn't work, then if you have galvanized pipes, you might actually have plugged pipes. It's not uncommon for galvanized pipes to corrode on the inside. That corrosion mixed with calcium deposits can really make a mess with your pipes. When that happens, they can totally occlude the pipe and you get very little water. The only good fix for that is to replace the pipes -- preferably with something other than galvanized, and that can prove to be a big job.

If you have good pressure and volume everywhere else then its just debris, probably from the installation. Debris or calcium will find its way to the first hot valve opened and , in this case, your tub. It really doesn't matter much what it is, just where it is. Can you describe your tub valve, maybe a picture or nrand.

The tub fixture is a single knob. Left for hot (that is little to no pressure) and right for the cold. That is the only fixture in the house that has a problem with the hot water flow.

Turn the hot water off at the water heater, now disassemble the faucet, have someone help- you by holding a heavy towel in front of the tub valve, to divert the water into the tub, while you turn on the water heater valve back on for just a few seconds ( 10 seconds should work, you don't want it too hot for the person holding the towel,) this should wash out any trash.

If you only close the hot water at the heater you will be in for a very big surprise when you take apart the faucet. I'd recommend closing your main and then open a few faucets to bleed off pressure. Now remove the mixing valve which is likely some sort of cartridge. The flushing is a good idea after you clean away all debris from the valve.