Having a tankless water heater sounds excellent, but if you’re used to the big tank in the utility closet, you might wonder how to size a tankless water heater. After all, both work differently to achieve the same result.
Traditional water heater tanks are rated by the amount of water in gallons they hold. Tankless water heaters, on the other hand, are rated by the maximum temperature rise they can deliver at a given flow rate.
If this sounds confusing, you’ve come to the right place. In this article, we’re going to discuss tankless water heaters and calculate tankless water heater size. Let’s get to it!
How to Size a Tankless Water Heater
To size a tankless water heater, you’ll need to determine the temperature rise required and calculate flow rate. It’s essential to get this right, as an undersized tank will struggle to deliver.
Step 1: Estimate Your Total Flow Rate
To calculate tankless water heater size, you first need to estimate how much water you want to heat at once, or your desired flow rate. For this, you’ll need to decide the maximum number of devices you want to run simultaneously and their flow rates in gallons per minute.
If you installed the fixtures yourself, you could look at the packaging to determine its flow rate. However, if you’re unsure (like most of us), here’s a general guide to help:
- Showerheads (standard and older models): 2.5 – 3.5 GPM
- Low-flow showerheads: 1.2 – 2 GPM
- Bathtub: 3 – 4 GPM
- Faucets: 0.75 – 2.5 GPM
- Kitchen faucet: 1 – 2 GPM
- Dishwashers: 1 – 2 GPM
- Clothes Washers: 1 – 2 GPM
Once you’ve determined the flow rate of your devices, then decide how many of them you would like to run with hot water at the same time. Add up the total GPM, and that’s your flow rate. Of course, you can always consider installing low-flow fixtures to help reduce this total.
Step 2: Determine the Required Temperature Rise
Next, you’ll need to determine how much your heater will need to raise the water temperature before delivery. Take the needed output temperature and subtract the temperature of the incoming water.
Of course, you’re probably wondering how you’re supposed to know the incoming temperature of your water. If you live in a warm climate, it’s likely the incoming temperature is higher, while in colder climates, your heater will have to work harder. Here’s a map of average groundwater temperatures in the US.
For most applications, you’ll want to heat your water to a temperature of 105-115 degrees. Assuming water is coming in at 50 degrees, that means 105 minus 50, and you’ll need a water heater that produces a rise in temperature of 55 degrees.
Step 3: Calculate Tankless Water Heater Size
Finally, to calculate tankless water heater size, take both the required temperature rise and your total flow rate to determine your needs. For example, an average shower uses 2.6 gallons of water at a temperature of around 105 degrees.
If you want to be able to produce enough hot water to run two showers at the same time, you need the ability to heat 5.2 gallons of water. If the incoming water temperature is 50 degrees, you’ll need a heater that can produce a temperature rise of 55 degrees at a rate of 5.2 gallons per minute.
Should You Get a Tankless Water Heater?
Tankless water heaters are designed to rapidly heat water as needed, rather than heating and storing water in a tank. They use either an electrical element or gas burner to heat water as it is required.
The truth is that no matter how insulated your traditional hot water tank is, it’s going to lose heat and need constant reheating. Standby heat loss, as it is called, is responsible for 10-20% of water heating costs annually.
When you think about it, it makes a lot of sense to heat water only when you need it. Plus, without the limitations of a tank, hot water can be produced for as long as required without ever running out. All in all, tankless water heaters are more efficient than traditional ones and save you money on energy bills.
How Tankless Water Heaters Are More Efficient Than Traditional Ones
Another way tankless water heaters are more efficient is how they use fuel (either electric or gas). Because they do not need to be continually heating a tank of water, the heat source only needs to burn fuel when the tankless heater is being used.
For instance, if a faucet is only drawing 1 gallon per minute (GPM), the water heater only needs to use as much fuel as is required to heat this amount of water. However, if a shower faucet runs at 5gpm, the heater can burn more fuel as necessary but never more than it needs.
2 Types of Tankless Water Heaters
There are two types of tankless water heaters. Depending on your water habits and needs, you can get a centrally-located whole house heater or a point-of-use (POU) heater.
Centrally-located whole house heaters are designed to replace an existing tank heater. For small to average-sized homes, this kind of heater can be sufficient. Nevertheless, multiple units may need to be installed for larger homes.
POU heaters can be installed either onto a single fixture or for two or three fixtures in the same room. POU heaters are often used to supplement a central water heater, such as in locations far away from the central unit, to reduce the time it takes to get hot water.
Tankless Water Heater Sizing
Interestingly, although we talk about “sizing” a tankless water heater, determining the unit that’s best for your needs has more to do with its power than its actual physical size. Always look for one that’s rated a little above your calculated needs to ensure the best results.
In general, most gas-powered tankless water heaters are capable of a 70-degree rise in temperature at a rate of 5 gallons per minute. An electric equivalent unit will produce the same results for 2 gallons per minute.