The best water heater for a soaker tub is a 60 to 75 gallon gas tank, or a 50 gallon tank stored at 60 °C behind a thermostatic mixing valve. A standard 50 gallon (189 L) tank delivers only about 70% of its volume, roughly 132 L, before the water cools, and a 250–400 L tub needs more than that in winter. A tankless never runs out, but it fills the tub more slowly, so the trade-off there is fill time.
The Arithmetic: A 250–400 L Tub Against a 50 Gallon Tank
Start with how much water the bath takes, then how much of it must be hot. You fill a soaker to about two thirds of its rated capacity before you get in, so a 300 L tub takes about 200 L and a 400 L tub about 265 L. Bathwater at 40 °C, mixed from 49 °C hot and 5 °C winter cold, is about 80% hot water.
- 300 L tub: about 200 L in the bath, about 160 L of it from the hot side.
- 400 L tub: about 265 L in the bath, about 210 L from the hot side.
- 50 gallon tank at 49 °C: about 132 L usable, plus roughly 30 L the burner adds during a 12-minute fill.
- Result: the 300 L tub drains the tank completely and the 400 L tub cannot be filled hot.
That is why a soaker installed during a bathroom renovation so often disappoints. The tub was chosen from a showroom floor and the water heater was never part of the conversation. Nobody gets a hot shower for the next hour, either.
Option 1: A 60 or 75 Gallon Tank
A larger tank is the direct fix. A 60 gallon tank holds 227 L with about 159 L usable, and a 75 gallon tank holds 284 L with about 199 L usable, each normally backed by a larger burner than a standard 50.
Stored at 49 °C, the 60 fills a 300 L tub with some in reserve and the 75 covers a 400 L tub. The costs are floor space and fit: a 75 is noticeably wider and taller, its burner input is higher, and the vent and gas supply have to be checked against it. It also keeps more water hot around the clock for a bath that may happen twice a week.
Option 2: Store Hotter and Mix Down to 49 °C
Raising the storage temperature makes the same tank go further, because less hot water is needed for each litre of bathwater. From a tank at 60 °C, a 40 °C bath in winter is about 64% hot water instead of 80%. A 50 gallon tank's 132 L then makes about 205 L of bathwater, enough for a 300 L tub.
It is only acceptable with a thermostatic mixing valve. The Ontario Building Code limits hot water delivered to fixtures in a home to 49 °C, with dishwashers and clothes washers excepted, because water at 60 °C scalds skin in seconds. The valve sits at the tank outlet, blends in cold water, and sends 49 °C or less to every tap while the tank stays at 60 °C.
- The valve is a wearing part. Scale from hard water in Halton and Hamilton makes it stick, and a stuck valve gives lukewarm water or temperature swings; see mixing valve replacement.
- The tub is filled with 49 °C water like every other fixture, so the gain comes from the tank, not from a hotter tap.
- Standby loss rises a little, and scale forms faster in a hotter tank.
- Storing at 60 °C also keeps the tank above the range where Legionella bacteria grow, which is a point in its favour.
- The dial setting belongs with the installer, who verifies the delivered temperature with a thermometer at a tap.
Our answer on what temperature a water heater should be set to covers the safety side in more detail.
Free quotes, written pricing.
Option 3: Tankless, and What It Does to Fill Time
A tankless fills any tub without running out, but only as fast as it can heat the water. In an Ontario winter a 199,000 BTU condensing unit heats about 19–20 L/min to its setpoint, which works out to roughly 24 L/min of 40 °C bathwater. A 300 L tub takes about 8 to 9 minutes to fill and a 400 L tub about 11.
A tank feeds a tub filler as fast as the pipe allows, and a high-flow filler can run at about twice that rate until the tank is spent. With a tankless, opening the filler wide does not help: the heater either limits the flow or the water arrives cooler. A smaller 150,000 BTU unit stretches the 400 L fill to about a quarter of an hour, and a shower running at the same time slows it further.
The upside is everything after the bath. The shower afterwards is hot, the second bath is hot, and there is no hour of recovery. For a household that uses the tub several times a week, that matters more than a few extra minutes of filling.
Details That Change the Result: Filler, Pipe and Tub Material
- Tub filler flow: a high-flow filler empties a tank quickly but also fills fast, so less heat is lost to the room during the fill.
- Pipe size and length: a long half-inch run to an upstairs ensuite slows the fill and cools the first few litres. A renovation is the time to run three-quarter-inch.
- Tub material: cast iron and stone absorb heat from the first water in; acrylic absorbs far less. A heavy tub needs a hotter fill or a top-up.
- Season: in late summer the cold supply is far warmer, the hot fraction drops, and every option on this page performs better. Size for February.
- Gas line: a tankless or a 75 gallon tank draws more gas than a standard tank, and upsizing the run can add $400–$1,200.
What Each Soaker Tub Option Costs
- Gas tank replacement, including a step up in size: $1,200–$2,500 installed, depending on size and venting
- Tankless conversion: $3,500–$6,500 installed, including gas-line and venting work
- Thermostatic mixing valve added to an existing tank: priced on a written quote, because it depends on the piping at the tank
If your current 50 gallon tank is under eight years old and sound, the mixing valve route is the least expensive way to get a full bath. If the tank is near the end of an 8–12 year life anyway, put the money into the right size of replacement.
Verdict: Match the Heater to the Tub Size and How Often You Use It
A 60 or 75 gallon tank is the right heater for most soaker tubs, the mixing-valve route is the cheapest fix for a 300 L tub, and tankless suits a tub that is used most days.
- Tub up to about 250 L, used occasionally: keep or buy a 50 gallon tank, store at 60 °C, add a thermostatic mixing valve.
- Tub of 300 L: a 60 gallon tank, or a 50 at 60 °C with a mixing valve if nobody needs a shower straight after.
- Tub of 350–400 L: a 75 gallon tank, or a 199,000 BTU tankless if you accept a fill of about 11 minutes.
- Tub used most days in a busy household: tankless, because recovery time stops being a problem.
Tell us the tub's rated capacity in litres, which is on the manufacturer's sheet, and we will size the heater to it instead of to the number of bedrooms. Tank options are on our water heater service page and on-demand options under tankless water heaters; to get a written quote, contact ZK Mechanical with the tub model and a photo of the current heater.
Frequently asked questions
How much water does a soaker tub really use?
Less than its rated capacity. The listed volume is to the overflow with nobody in the tub. Your body displaces a good share of the rest, and most people fill to well below the overflow, so a 300 L tub takes around 200 L and a 400 L tub around 265 L. In winter about 80% of that comes from the hot side.
Can I turn my water heater up to 60 °C without a mixing valve?
No. Water at 60 °C can cause a serious scald in a few seconds, and children and older adults are most at risk. The Ontario Building Code caps hot water at residential fixtures at 49 °C. Storing hotter is a sound way to stretch a tank, but only with a thermostatic mixing valve at the outlet, set and tested by the installer.
Will there be hot water for a shower after filling the tub?
With a tank, usually not right away. A full soaker fill takes most of the usable volume of a 50 or 60 gallon tank, and a gas burner needs 30 to 60 minutes to bring it back. A 75 gallon tank leaves some reserve. A tankless gives a hot shower immediately, which is its main advantage for bath-heavy households.
Would two tankless units fill a tub faster?
Yes, two units piped together roughly double the flow, but the gas supply must carry both, and many houses would need new gas piping and possibly a larger meter. For a single soaker tub that is rarely worth it. A large tank, or one 199,000 BTU tankless and a few more minutes of patience, does the job for far less.