can a tankless water heater fill an oversized bathtub? Are you one of the lucky few who have an unusually large bathtub or potentially even a dedicated whirlpool tub inside of your home? It’s lovely to enjoy, so long as you can get enough hot water in it to enjoy it. Many people find that their water heater does not have enough water in it to fill up their tub. They get it filled up about halfway and the tank runs out. It could take a half-hour, or more, for the tank to reheat new water. In the meantime, the water you put in cools off. What’s the solution?

One option people consider is switching to a tankless water heater. Here’s what you need to know about that option, and your other options, to finally enjoy that large tub.

Can a Tankless Water Heater Fill an Oversized Tub?

Yes, usually. A tankless water heater can provide hot water indefinitely, which means it will fill up the tub. However, you still need to consider how fast the heater can provide the temperature of water you want. Faucets work by mixing in hot water with the cold. When you demand very hot water, it may mix very little cold water in. That means there is less overall flow to your tub, which means the tub gets full slower. Depending on how long this takes, your water may be colder than you like by the time you step in.

Flow Rate Comparison

There are different sizes of tankless water heaters with different flow rates. A higher flow rate will mean the unit can deliver more water and potentially fill your tub faster (although this will also depend on if you’re using other faucets or making other demands on the hot water at the same time).

While the average home bathtub holds less than 100 gallons, a soaker tub may hold 250 gallons, and a hot tub may hold 300 gallons. But you won’t fill them to the brim because you need to fit in them too. How fast will different tankless water heaters fill this up?

GPM Time to Fill 250-gallon tub, to the 200-gallon mark
2 100 minutes
5 40 minutes
8 25 minutes
12 16 minutes

As you can see, undersized tankless heaters of 2-5 GPM are not ideal for soaker tubs. A more average tankless heater of 8 GPM may be satisfactory to you. The largest tankless water heaters, which can achieve 12 GPM, fill up the tub the fastest.

If you know how much your specific tub holds, you can do the math and figure out how quickly different tankless water heaters will fill it up.

While many soaker tubs have larger faucets to allow them to fill faster, you may find you’re limited by faucet size and not getting the full GPM of your tankless water heater. In this case, you may need to change out the faucet.

Tank Size Comparison

You also have the option to upgrade to a larger tank-based water heater which may be able to fill most of your tub on the first run of water and recover fast enough to potentially fill the rest.

It is wise to talk over your options with the experts to be sure you will get the performance you need from your water heater.