I have a big tub and a small water heater. I live in an apartment so there's not much I can do about that right now. I've looked into immersion heaters and from what I can tell they aren't meant for bathtubs (don't produce enough energy), and/or take a long time to heat the water.
What are alternatives that I can combine with the under-powered water heater to get my tub water hot?
What about good-ol-fashioned heating or even boiling water on the stove? You'll need a thermometer to ensure your water doesn't go over 120 degrees.
When I was working on my water heater recently, I used spare aquarium heaters to heat up 5 gallon buckets. You can buy up to 300w on those....some aquarists guide will say what wattage heater for say, a 60 gallon or a 100 gallon tank. You can calculate the number of gallons you intend to heat in the tub before you buy any heater.
Many modern bathtub faucets have temperature limiters.
Perhaps yours is set too low. If so, it can be adjusted higher.
My bathroom is like a lot of homes and is on an outside wall and is sitting on top of the floor, or subfloor, I haven't looked that closely. My tub is cast iron and gets ice cold even when it's not freezing outside. I was thinking of removing enough flooring to uncover the bottom of the tub and place electric coils underneath but not actually touching the tub. They would be inside a metal enclosure and placed to the rear of the tub since the front, closest to the faucet would heat up quickly from the hot water directed onto the tub surface. This is just an idea and I haven't put it to test yet, but I will build a mock-up on a small scale to see if it is worth making one on a large scale. As soon as I have finished my model, I will provide details.
Use a PROPANE WEED BURNER to heat it up! A good propane torch costs about 25$ and a small green tank at about 10$ and a good one can generate heat to EASILY melt most hard metals!
