What Size Tankless Water Heater Do I Need? (How To Select The Right Size, GPM And More)

by Curtis Grossi | Last Updated: March 19, 2024

Tankless water heaters heat water to the specified temperatures and dispense it at the desired pressures. These heaters are fast, efficient, and most durable options in the market.

They’re the household devices to add to your home if you want to keep your water and electricity bills low. Tankless heaters are designed to handle different sorts of hot water demands. Bigger families will require more powerful units or several less-powerful units.

What Size Of Tankless Water Heater Do I Need For My Family?

The perfect size of the tankless water heater is not entirely determined by how big your family is. It’s solely determined by the number of people who will be using the water heater concurrently. Electric on demand water heater produces hot water instantly.

They heat the water as soon as it reaches the heat exchanger and releases it immediately it reaches the specified temperature. So, the number of faucets the unit can power at once determines how many people it can serve. If you’re a family of 4, 5, or 6 members, the best unit will be the one with a high flow rate. Flow rate is measured in gallons per minute (GPM). The higher the GPM rating, the higher the flow rate and the more water a heater can offer per minute.

To determine the perfect tankless water size for your size family, you’ve to calculate your home’s peak flow rate and look for a unit that can handle such size of a heating load. Determining the perfect size of a tankless water heater based on the maximum water demand during peak hours ensures you will have enough water supply at all times.

You should factor in the number of bathrooms, shower, washing machine and faucets available in your house when calculating, as well. If your family is made up of five members and only four use the shower early in the morning, you should calculate the peak demand of four users. When calculating, the flow rate of the tankless water heater, as well as its optimal output (water temperature and the optimal water temperature rise), should be the primary considerations.

How Big Of A Tankless Hot Water Heater Should I Buy?

When deciding on the ideal hot water tank size , you should check the flow rate at which the unit can raise the groundwater temperature of the incoming water to the desired water temperature.

That’s mostly determined by the BTU rating of the tankless water heater. Units with higher BTU lead to a much quicker temperature rise in groundwater temperature than those with lower BTU. In deciding how big your tankless heater should be, you’ve to ask yourself these questions:

How big is your family?

While determining your family size, factor in the number of people who take showers at a given time, and how often they shower. You should get an accurate figure of the people who use hot water at the peak hours for you to choose the perfect tankless water heater.

The ideal model here should have a flow rate that meets the overall demand in gallons per minute and in the required temperature rise.

As an illustration, a tankless water heater with a flow rate of around 5 GPM to 6 GPM should service a 1 bedroom house well while achieving the required temperature rise. On the other hand, a 2-bedroom with 2 shower bathrooms will require a tank with 8 GPM to 10 GPM.

How many bathtubs available and their GPM?

Find out how many functional bathtubs there are, factoring in ones that are whirlpool, and ones that are soaker. You should also determine the capacity of each of the tubs in gallons (GPM) plus the ideal flow rate for each.

How many appliances will use hot water from the heater?

What’s the required temperature rise for each appliance? Determine the appliances in your home that will use hot water from the heater and how frequently they are used. Importantly, your target heater’s GPM should be able to sustain all the appliances that are frequently used together without compromising its temperature rise capabilities.

A standard size shower head has a flow rate of 2.5 GPM. Most people prefer taking a shower in hot water with a temperature rise of 102 to 106 degrees Fahrenheit. A kitchen sink has 4 GPM of use, dishwasher 4 GPM, and hose bibb 5 GPM.

How To Choose A Tankless Water Heater?

In deciding the size of the tankless water heater to get for your family, you should consider various things. These are the most important considerations when you are deciding which on-demand water heater to buy.

Electric vs. Gas

The majority of American households use gas tankless heaters. Gas-powered units offer better GPM and guarantee higher levels of efficiency in achieving the required temperature rise. As for electric heaters, they are good but not suitable for bigger families.

They are cheaper compared to gas-powered units, but not as efficient and reliable as gas water heaters. For a big family, the overall cost of achieving the required temperature rise can add up pretty quickly.

How Many BTU’s Do You Need?

Another consideration when you’re shopping for tankless water heaters is the BTU rating. One BTU is equal to the total energy it takes the unit to raise the groundwater temperature of 1 lb. of water by one degree Fahrenheit.

That means, the higher the BTU rating, the more the water the unit can heat to the required temperature. Big families of 5 or 6 members will require a water heater with a BTU rating of at least 200,000 BTU to ensure their peak hot water demands are met.

Point Of Use vs. Whole House

Whole house water heaters have higher BTUs and can seamlessly supply the whole house with enough hot water at the required temperature. However, these units are quite costly and may not be good choices for those on low budgets.

If you want more hot water supply but cannot afford the whole house heaters, you can opt for multiple small tankless heaters. These will feed your house with enough hot water on demand without raising your energy or water bills. However, each unit’s GPM and its efficiency in achieving the required temperature.

Installation Requirements

Installing a whole-house tankless gas water heater can prove more expensive than installing a point of use unit. That is due to the extended distance from the heater to the fixtures and venting system. Also, you may need to drill through brick walls, which could raise the costs. The cost will be lower in the case of electric water heaters.

These units don’t require venting and exhausts that add to the cost of gas tankless water heaters. However, you have to pay a qualified electrician to get the wiring done professionally to avoid issues with wrong connections.

What Size Tankless Electric Heater Do I Need?

One of the biggest problems of tankless heaters is the high amount of energy they need to run to raise the water temperature. Electric heaters require a substantial energy supply, and in most instances, your household power output will not be enough. Most units will use several 240-volt circuits, meaning you may have to modernize your electrical infrastructure to accommodate the needs of your tankless water heater.

The most conventional tankless water will require 100 to 120 amps and at least 3 circuit breakers, which will total to around 240 volts, meaning you should get not lower than 200 amp service.

If installing more electric water heaters, you will need more amp service. The output of electric water heaters is not the best, and so to ensure quality supply to every faucet in your house, you may have to get several units.

What Size Tankless Gas Water Heater Capacity Do I Need?

On-demand gas heaters are mightier compared to electric heaters. They output more and use less energy to take groundwater temperature to the user’s desired water temperature. The fact that they are more powerful means that you don’t need multiple units to supply your entire house with hot water on demand.

When deciding which tankless water heater to buy, you have to check the weather conditions in the area you live in. Colder areas will require more powerful units as raising the temperature of the input water to the desired temperature will require more power and energy.

When choosing, it’s as well essential to match the heater design to the type of gas you’ve in your home. There are two main types of tankless gas heaters, which are propane and natural gas. The two boast good energy factor and GPM, but they can never be interchanged.Here are the sizing for different family sizes:

• If you want 1 to two fixes supplied with hot water for a family of 1-3, you should look for a tankless water heater with an output of not less than 4 GPM. 5 GPM is much more recommendable.
• If you want to supply two to three faucets or two faucets and a shower head at once, you should get a unit capable of outputting at least 5GPM. This will be the ideal choice of water heater for families of 2 to 4 members.
• If you want to supply hot water to 3 to 4 fixtures, for instance, 3 faucets and a shower head faucets, get a unit capable of outputting at least 7 GPM. These tankless heaters will be the ideal choice for families with 4 to 6 members.

Bottom Line

There’s no doubt tankless water heaters are much more efficient than their tanked alternatives. The fact that they don’t constantly maintain the temperature of a tankful of water means a lot to your electricity bills.

Gas-powered tankless water heaters can bring water to the required temperature much quicker than electric units. But keeping mind that the former has a higher upfront installation cost. To narrow down to your ideal heater, determine how much GPM that your shower head, faucets, dishwasher, and other appliances need.

Curtis is the founder and owner of softeningwater.com. He is the lead guy concerning all the water-softening accessories in this site's guides and reviews. If he is not writing a reviews, guides or any other useful tips, you will find him testing them to find out their suitability. He is passionate about softening water. You will find him extremely helpful and always willing to educate the general populace all there is concerning water and its effects on their health and appliances.