What Tankless Water Heater Do You Need?

If you decide that a standard tank-style water heater is no longer suitable for your family, tankless water heaters are a viable choice. Not only do they provide unlimited hot water, but they also reduce your utility expenses and decrease standby energy losses. Furthermore, they are compact and tiny, which saves storage space and may endure for decades with regular care and cleaning.

One issue with choosing a tankless water heater is determining the size of your family. When the water heater is too tiny, the entire investment is rendered ineffective, while a tank that is too large may cost you too much in the long run.

What should you know about tankless water heater sizing?

Put the maximum output water flow rate for tankless water heaters’ given temperature increase rate. When deciding on the size of your tankless unit, there are two factors to consider:

  • How many litres of hot water per minute do you require at peak use? There are no tanks with tankless water heaters since they only provide hot water when needed.
  • The temperature differential between groundwater and output water determines the required temperature rise.

If the required hot water demand exceeds the maximum output of your tankless unit, the flow may be automatically throttled.

There will be less hot water at all outputs and a decline in temperature and pressure.

When you select a unit that exceeds your requirements, it is not a drama. You pay extra in advance.

Rheem 9.5 GPM Propane Indoor Tankless Water Heater

Is GPM that significant when it comes to tankless water heater sizing?

Begin by determining the peak hot water demand in your house. Many charts are available that detail the normal flow rate of various types of water outlets in the United States; all you need to know is which appliances you want to operate simultaneously and the number, and then add up the flow rates.

For example, you could utilize one shower and one kitchen faucet simultaneously. The greatest water flow is as follows:


It’s not the only thing to think about, so keep reading for additional pointers:

  • Will the unit power the entire house or just a portion of it?
  • You must not only count the number of bathrooms in your home but also the number of people that live there. Two individuals may use two showers simultaneously, although your home may have four bathrooms.
  • When considering peak demand (for example, in the morning), you will always appreciate having hot water. Planning is necessary because it provides a reality check, allowing you to significantly lower peak demand.
  • Install some low-flow aerators/fixtures to reduce the flow rates.
  • Flow rates seen on the internet include both hot and cold water. When using a 2.5GPM showerhead, not all of the water will be heated. Nine times out of ten, you’ll also have some cold water, lowering your real hot water consumption.
  • Flow rates are often greater in older fixtures.
  • Taking the flow rates of washing machines and dishwashers into account takes a lot of work. Flow rates can range from 1.5GPM to 2GPM and even 3GPM. You may improve accuracy by running each appliance individually and checking the water meter while monitoring the time. You’ll have a basic notion of the flow you’ll require.

Aside: To get a more accurate measurement of peak water flow, fill a 1-gallon bucket with water and watch how long the shower head/bathroom tap/kitchen faucet takes to fill it. Here’s the formula for calculating the flow rate for each appliance:

The flow rate necessary to fill a bucket is 60 seconds.

Does it important that the temperature rises?

You must have a precise understanding of the desired temperature rise. It is easy to discover. Subtraction of the entering water temperature from the intended output water temperature yields the temperature rise:

Output water temperature – Feed water temperature = necessary temperature rise

It would be best if you now determined the temperature of the feedwater, which may be accomplished in one of two ways:

  • Make use of a thermometer.
  • Look at the map of average groundwater temperatures in the United States. Remember that these are typical temperatures, and actual temperatures may fluctuate from season to season.

Groundwater temperatures may reach 77 degrees Fahrenheit in southern Florida if you reside in a warm climate. On the other hand, groundwater temperatures in Alaska and freezing locations can drop below 37F degrees.

Because the temperature rise in Florida is less than 40F degrees (110-77= 33), a tankless water heater will not have to work hard to heat the water. On the other hand, a tankless water heater would have to work harder because the temperature spike is greater (110-37= 73F degrees). When a tankless water heater works harder, its performance and durability also suffer.

To summarize…

It is easy to determine the size of your tankless water heater if you know the hot water demand during peak hours and the required temperature rise. You only need some time and patience and a look at the models available to see which one best meets your needs.

All reputable manufacturers include sizing charts with their goods, but they also specify the maximum flow rates at a given temperature rise and vice versa. Some manufacturers may also provide flow rates for input/output water temperatures.

Even if oversizing is better than under-sizing your tankless water heaters, you should select the right size for your family.

Remember to be prudent and to accept information with a grain of salt. The possibility of manufacturers exaggerating the performance of the devices is not nil. Always do your bits and bobs and read client reviews. They should have known better…