Chiang218

That is my main worry, but the contractor quoted $5k to remove bathtub, replace floor and wall tile. Do not want to spend so much money.

Is this a instant water heater? Set the temperature lower and knock the flow switch. Most likely is that water flow switch is partially stuck so when it sense that the water is flowing, the heater will cut in, once it sense no water flow it will cut off. Otherwise check your shower spray, see if the water pressure is good.

I need to remove my damaged Bathtub but do not want to incur additional cost. Can I remove the Bathtub without replacing the floor and wall tiles? Anyone done it before and how much does it cost for such job?

If you get other brands, the two outlet pipe will not be in the same position. so you need to alter the pipework. Getting the same brand of heater will not have such problem. Simply unscrew the nuts, remove the heater and fix back.