How to select the right size tankless water heater

Modern technology has provided man with better, convenient and easy to operate models of household fixtures.  So you’ve decided to shift to a tankless water heater. It’s probably one of the best decisions you’ve made this season.  But again, you don’t know where to start and are unsure of what to purchase. Do not fret. This read will guide you in a simple yet informed layman’s language on how to figure out the size of a tankless water heater that fits your needs and all the factors you need to consider.

Tankless water heaters come in two types, gas and electric. In comparison, gas fired heaters are capable of heating huge amounts of water faster than electrically powered units. On the other hand, electric tankless heaters are easier and less costly to install.

Selecting the right size of tankless water heater is the primary factor which will determine the efficiency you will get. Therefore, to be able to purchase the right size, you need to learn a few things regarding heating units. The factors mentioned below will guide you through to determining the right size for your house or business.

The Number of Devices

Different devices have different flow rates. It’s important to first and foremost consider the maximum number of devices you want to run in your establishment and further examine your expectations.

Do you expect to have several devices simultaneously running or not? For example, a standard household is likely to have devices such as the kitchen sink, laundry sink and the shower being used concurrently. At that particular time, the total flow rate per minute is higher compared to instances where only one device is running.  It might not be practically realistic to put a policy of running only one device at a time in your household.

Therefore, the best thing to do is put all the devices into your calculations before doing your purchase. You can invest in low-flow water fixtures also to help you obtain higher water temperatures fast. This is because the more water that is being used at any given moment, the lower the water temperature that can be obtained and vice versa.

The Required Rise in Temperature

To estimate the required temperature rise, you can subtract the temperature of incoming water from the required water temperature rise (output temperature). Unless you have used a thermometer to check the actual water temperature, you can assume that the incoming water temperature is 50°F.

It is safer to assume a low temperature than overrating it to avoid ending up with an undersized tankless water heater. For most uses, water is typically heated to around 105–115°. In this example, you’d need a water heater that produces a temperature rise of 55°. That is by subtracting 50° from105°.

People living in very cold regions with temperatures in the 30s will, of course, need a significant rise in temperature compared to warm areas. You will need to adjust accordingly depending on how much of a temperature rise you need in the winter.

An efficient tankless unit should most importantly be able to maintain your desired temperature at a maximum flow rate.

Availability of Mounting Space

Where in your place will the tankless unit be fixed or installed?  Don’t wait until the delivery man is at your door to start running around worrying about which point is convenient or wondering where the vent pipes will run.

The good side of Tankless water heaters is that they are much smaller and can easily be mounted on the wall. You can find electric units measuring 10 inches in height and 7 inches in width. These require very little space.

On the other hand, gas-fired units need adequate space for mounting and more space to run the pipes to vent the fumes from combustion.  The space available to you will determine the type of tankless water heater to purchase.

Determine your Water Flow Rates

Water flow rate affects the efficiency of your Tankless water heater to a great degree. Most appliances have the manufacturer’s model number tag which includes the gallons per minute (GPM) rating of the unit. A standard shower may use 1.5 gallons of hot water per minute, or GPM as it is called.

If for instance, your sink runs at 1.5 GPM through the faucet while doing dishes and your bathtub uses 4 GPM. This simply means you are consuming 5.5 GPM at that particular moment. Therefore, the flow rate should be able to produce 5.5 gallons of water in a minute to satisfy your multiple needs.

Here is an expert’s but easy example of determining your peak demand. By adding all your device needs together, you are able to determine the total peak demand.

Fixture Average Flow GPM Average Temperature
Tub 4.0 GPM 102°F
Shower 2.5 – 3.0 GPM 104°F
Washing Machine 2.0 GPM 120°F
Dishwasher 1.5 GPM 110°F
Kitchen Sink 1.5 GPM 110°F

If you are hard pressed, you can try to save on the size of water heater you need by coming up with household policies to not run multiple water sources at a go.  Also, if your incoming water temperature is above 67F, you can comfortably manage with slightly less considering that you won’t need as much of a temperature rise.

GPM Heating Basics

Even as you consider your water flow rates, be informed that the type of tankless unit you will purchase differ in their GPM requirements.  To cause a temperature rise of 70°, a gas-fired tankless heater requires 5 gallons per minute while most electric tankless units would require a maximum of 1.5 to 2 gallons of water per minute. The less water that is being used at any given moment, the higher the water temperature that can be obtained.

Power Source

Put into consideration whether you have access to natural gas, electrical service or both. What you need know is that even though tankless water heaters reduce your overall electricity consumption, they have a high peak electrical load.

This means that it works on demand and thus must transmit a substantial amount of heating power straightaway to the water. For that reason, consideration must be given to the number of AMPS of electrical service your home has in place.

Is your source enough to support the operation of the heater and other needs at once? For those with access to natural gas, it is more economical and efficient. This is especially if your goal is to provide hot water for an entire home. One way to get to know the number of electricity amps your home has is by checking on the main breaker in your electrical panel.

Dwelling Size

The size of your house and the number of people who live there dictates on the hot water demand.  Below is a simple illustration to act as a guide on size. Please note that this is for approximation purposes only.

Dwelling    Bathrooms GPM
Apartment / Condo 1 3.0 GPM
Small House (<1000 sq. ft.) 1 4.0 GPM
Average House (<1600 sq. ft) 2 7.5 GPM
Large House (1600+ sq. ft) 3+ 10+ GPM

Put in mind that a big house requires several water heaters.

Size of the Tankless Water Heater

This is based on the approximate total number of gallons your establishment will use in one day. If you buy a small container which is anything less than the required gallon amount, the heater will not be able to meet your needs. For example, an average shower makes use of 2.6 gallons. If you wish to use the shower twice a day, you would need a heater that can comfortably manage about 5.2 gallons efficiently in a day.  Therefore, take note of the size of the container. After all, it can impact the temperature of the water. The bigger the container is, the more power is needed.

Other Sizing factors

Water Hardness

Type of water dictates the maintenance schedule. This indirectly affects your decision of the category of the electric tankless water heater to go for. It’s still a significant aspect to think through. We have infrared electric tankless water units in the market. These do not build up calcium and limescale. Lest you forget, all heating units do need maintenance at some frequency. The frequency is determined by how hard the water is.


You will learn that it’s not advisable to have a heating unit that has a faster flow rate or cooler inlet. This brings about some negative effects on the heater that end up making the temperature of water to go low down much faster.

Also, inlet water for tankless units should never be pre-heated.

The rating of a tankless water heater is centered on the power of its flow rate and the maximum temperature it can provide. It’s thus essential to get a good knowledge regarding all the factors you need to consider in as far as size is concerned, prior to purchasing your tankless unit. With the above in mind, you are now fully armed to make an informed decision.