Heating Your Soaking Tub – Basic Science

Portable Tankless Water Heater for Cedar Hot Tub

One major difference when buying a soaking tub compared with a conventional hot tub is the heater is not built-in. For many people this gives them the benefit of flexibility to design a system that works best for their unique circumstances. You may not necessarily need to hire an electrician for a special 220V outlet, or you do not even need electricity to enjoy a hot soak! Our tubs are designed to be off-grid friendly. 

How Much Energy Does it Take To Heat My Cedar Hot Tub?

For our standard size tub, interior dimensions are about 45” long x 25” wide x 26” high. Since the fill depth will be lower than the height to allow for water to go up once you get in, let’s say fill height is about 22”. That means there are about 107 gallons of water to heat in a standard Elemental Works soaking tub

Now we are closer to calculating how much electricity, propane, or wood is needed to heat our soaking tub to that nice 104 degree Fahrenheit temperature.

The most common way of figuring this next part out is with BTUs. A BTU is an abbreviation for British Thermal Unit, which is the amount of energy required to raise 1 pound of water 1 degree Fahrenheit at sea level. Since a gallon of water is 8.34 pounds, we have about 892.38 pounds of water in our tub, requiring about 900 BTUs to heat your tub 1 degree

If the water coming out of your hose into the tub is 54 degrees, we need to raise the temperature about 50 degrees, so in total that would be about 45,000 BTUs to get your tub up to temp. That sounds like a lot, but let’s translate that into what’s required of different heating types. Note that all number are approximate and will vary 10-20% depending on specific set-up and other variables. 

  • 1lb of wood has about 8,500 BTUs when burned. Wood can be used for a thermosiphon heat set up but will require more planning and fire maintenance, which to some of us is a good thing! 
  • A small portable tankless water heater using propane at 1.5GPM puts out about 37,500 BTUs. These portable water heaters are an excellent choice if you prefer to heat your tub on demand with about an hour and a half advanced planning. 
  • 110V heating elements include 1400 Watts have about 5,000 BTUs, and 2000 Watts at about 6,800 BTUs. While these are lower BTUs than other options and generally require about 8 hours to get the water to temp, paired with a thermostat they offer the ability to maintain constant temperature while running intermittently. This is how standard electric hot tubs work, but with higher wattage since they need to heat larger volumes of water. 
  • 220V heaters can have 3,000 watts or more, with 3,000 watts being about 10,000 BTUs, adding about 3,300 BTUs for each additional 1,000 watts. 220V heaters are more in line with traditional fiberglass hot tubs. If you do not already have a special 220V outlet for a spa, this usually requires an electrician to set up and generally is the most expensive. This will be the most powerful and convenient option, but not required to enjoy a hot Elemental soak.

Heating From Cold Water

Assuming you are heating your tub from cold water, which will be done periodically as you change the water, the formula is roughly:

Gallons of Water * Degrees Fahrenheit to heat * 1 BTU = Total BTUs required

So that  

107 Gallons @55 degrees F warmed to 104 degrees F would need roughly 45,000 BTUs.

Note that this assumes perfectly efficient heat transfer and no heat loss, which needs to be taken into account. For this we will add an additional ~10% assuming a top cover is used to keep the heat in, for a total of about 50,000 BTUs required. 

Time needed for different heat types to warm 107 gallons 50 degrees. All values are estimates:


Heat Source BTUs per Hour Total Time to Heat
2lbs Wood Fire* 17K BTUs 3 hours
1.5 Gpm Tankless Water Heater 37,500 BTUs 1 ½ hours
1.5K Watt 110V Electric Heater 5,000 BTUs 10 hours
3K Watt 220V Electric Heater 10,000 BTUs 5 hours

*Wood fire heating assumes some kind of heat exchanger set up such as a copper or stainless tell coil, thermosiphon or separate pump. We will cover this topic in a future post!

We will post next about how much different heating systems cost including popular system and where to buy. 


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Contact us to find out about options and how to order. We’ll work with you to provide the right design for your needs.