A proper tankless water heater, also called an on-demand water heater, can be one of the best investments you make for your home, due to its energy efficiency, superior lifespan, and ability to provide hot water on demand provided you purchase the correct size for your home’s needs.

Otherwise, you risk falling among the crowd of others who complain of simply not having enough hot water to supply their household, which experts warn is often due to an ill-fitting on-demand water heater.

Therefore, if you are considering upgrading to a tankless hot water unit, be sure to select the right size in order to reap the full benefits, which this short guide will instruct you on how to do.

**Understanding How Tankless Hot Water Heaters Work**

On-demand water heaters contain a GPM (Gallons Per Minute) rating, which is based on the highest temperature increase possible at a given flow rate. Therefore, to correctly size a tankless water heater, you first need to determine the rate of flow, in GPM, as well as the hottest temperature output of each appliance in your home.

**Calculating the Flow Rate**

The flow rate is simply the amount of water you require from a fixture at a given time.

To determine the amount of water your household requires, first, decide how many appliances you want to run, then calculate the flow rate for each appliance, which is usually also listed on the appliance in GPM. Next, add all the flow rates together to arrive at the total flow rate you need for your home.

For instance, if a faucet runs 2.5 GPM, while a shower head is running, which utilizes 2.5 GPM, and a running tub uses 3 GPM, then you need an on-demand water heater with an 8 GPM rate to efficiently run your household.

To determine an appliance’s GPM, you could also just simply hold a gallon sized container under the faucet to see how many seconds or minutes it takes to become full.

To reduce your flow rates and make your tankless hot water heater more efficient, you can purchase faucets that are low-flow.

**Calculating the Temperature Risk**

The temperature rise is simply how hot the water streaming from your appliances gets.

To calculate the temperature rise, use a thermometer to measure the starting temperature of the flowing water. It is this temperature that will determine the temperature rise your tankless unit will need to sustain and at the highest flow rate.

For instance, let’s say the desired temperature for your bathtub is 106°F, and the initial temperature of the water is 65°F, then you will require a tankless water heater that can increase the temperature of the water by at least 40°F degrees at the highest flow rate in order to sustain the bathtub’s desired temperature of 106°F.

The average starting water temperature is about 50°F, so when in doubt about your water’s starting temperature, simply use this average temperature as a base to avoid purchasing a water heater that is too small.

**How Ground Temperatures Play a Rol**

In some parts of the country incoming water temperatures, which are determined by the ground temperature, can dip below 40 degrees, while in other parts of the country incoming water temperatures can reach as high as the 70s.

The warmer the temperature of the incoming water, which is determined by the ground temperature, the less it will take for the tankless water heater to heat it to the desired temperature.

**Why Size Matters**

To give you a general idea of how the correct sizing or your unit affects its performance, let’s examine the following example.

Say your starting water temperature is 60° F, and one person is using the washer, which is 2 GPM with a desired temperature of 110°F, while another person is using the sink faucet, which is 2 GPM with a desired temperature of 104°F, and perhaps another person is washing the dishes, which is 1 GPM with a desired temperature of 115°F, then the sum GPM needed during that time is 5 GPM.

Therefore, you will require a water heater that is capable of increasing the water temperature 55°F at 5 gallons per minute in order to maintain all of these appliances at the same time.

Since gas on-demand water heaters provide higher temperature increases per gallon per minute than electric heaters, they are more than capable of doing this. In fact, a gas operated on-demand heater can produce a temperature increase of 70°F a 5 gallon per minute flow rate, whereas an electric water heater produces a 70°F temperature increase per 2 gallon per minute flow rate.

However, gas powered tankless heaters ratings are preset for a certain temperature rise over a set amount of gallons per minute , such as 100,000 BTU gas powered unit that is capable of increasing total water temperature 40°F at a maximum 4 GPM, which is great if you are running multiple appliances at once.

But if you are only using one appliance, this could be a bit too much because each GPM taken away from the sum allows at least a 10°F increase in water temperature, so all the flowing water would be heated way above the desired temperature, which can be too much.

**Determine the Space Needed to Mount the Unit**

One of the perks of tankless water heaters is that they are smaller than traditional tank-style water heaters, which means they require less space. They are also designed to be wall mounted, so they go virtually unnoticed.

However, gas-powered on-demand water heaters require enough space to allow the unit to perform its process and to vent. Therefore, be sure to set aside a wall space of about 20-inch wide and 30-inch high, if you opt for a gas-powered unit. On the other hand, electric heaters typically only require very little space for mounting; therefore, be sure to plan accordingly.

Hopefully, this gives you a general understanding of how to purchase the right size tankless water heater for your home. For more information, be sure to consult a knowledgeable sales professional who will be more than happy to assist you with selecting the right unit.