A Tankless Water Heater Sizing Guide

A tankless water heater is an excellent option for many homes, but the sizing really matters, and if you install a unit that’s too small, you will not have enough hot water to meet your needs. But, if you install an oversized tankless water heater, you will have too much hot water, the system will be less efficient, and you will spend more on your energy bills. The type of water heater must be chosen carefully to balance your need for hot water and the expected energy efficiency benefits. Let’s take a closer look at tankless water heater sizing for your home.

The Flow Rate

The tankless water heater will have a flow rate expressed in gallons per minute (gpm), and this is how much hot water that unit can produce. The tankless water heater that you install in your home must be capable of providing an adequate flow rate to meet the needs of your home. It’s important to bear in mind that the hot water using activities of your family may be restricted to one or perhaps two instances depending on the size of the water heater that you install. Let’s take a look at some typical examples of flow rates from various water efficient sources to get some benchmark figures:

  • The Kitchen Faucet: 1.5 gpm
  • The Tub Faucet: 4.0 gpm
  • The Bathroom Sink Faucet: 1.0 gpm
  • A Showerhead: 2.5 gpm
  • A Dishwasher: 1.5 gpm
  • A Washing Machine: 2.0 gpm

Using these water use figures allows us to make fairly accurate predictions on the hot water demands during peak times. As an example: let’s say that you have two bathrooms and two people want to shower at the same time each morning. This would require a tankless water heater flow rate of approximately 5 gpm. We say approximately because each home plumbing system is slightly different, and people install specialized showerheads or use power showers.

Using these flow rate figures is very enlightening; you can quickly notice that certain activities can be a major drain on the demand for hot water. Running a tub to take a relaxing bath after a hard day at work doesn’t mesh well with running a dishwasher at the same time. Generally speaking, it’s a better idea to use the dishwasher and washer at off peak times in your home if that’s possible. This significantly decreases the demand for hot water at peak times when there is a pressing need. A prime example would be the need for showers in the morning to get ready for work and school.

The Rising Temperatures

The other key part of the equation when it comes to tankless water heaters is the temperature needs. The water temperature has to be hot enough to handle all of the needs of your home. This aspect of water heaters is known as the temperature rise, and it’s a key part of any type of water heater unit. A seasonal shift can alter the temperature of incoming water significantly, and this needs to be taken into account. When it comes to sizing your new tankless water heater, it’s only important to know the average groundwater temperature in your area.

Let’s say for the sake of argument that the average groundwater temperature in your area is approximately 52º. Most people want a temperature of around 102º at their sink, tub or shower, and perhaps 120º for the washer and dishwashing machines. When you choose your tankless water heater, start with the 52º average groundwater temperature, and you can work out the difference. So, you want a water heater that can deliver a temperature rise of 50º at the sink, tub and shower, and 68º for your water using appliances.

Shopping for Your Tankless Water Heater

When you start to look for a tankless water heater for your home, you will notice that they are presented with the two factors we’ve previously mentioned. Every new tankless water heater will have its size listed using the flow rate and the temperature rise. But, it’s important to understand that the very same unit can perform differently depending on the local conditions where it’s installed.

Let’s say that you’re looking at a specific tankless water heater that has a flow rate of 8 gpm, and your required temperature rise is 30º. That very same water heater could drop to a flow rate of 5.3 gpm if it was installed in an area with a temperature rise of 65º. These two factors are very closely linked, and balancing them together is crucial if you want to size your prospective tankless water heater correctly.

This process may seem tricky, and in truth, it is difficult if you’re not sure about the groundwater temperature rise and how that will affect the flow rate. The last thing that you want to do is engage in guess work or try to wing it when you make your final choice. The whole idea behind choosing a tankless water heater is to make your life easier and improve the energy efficiency. For this reason, many homeowners consult a professional plumber for advice and to get their new water heater installed.

Choosing a Professional Installation

A tankless water heater represents a significant investment in the supply of hot water to your home and the comfort of your family. So, it makes good sense to get some professional help from a local qualified plumber with plenty of experience. A good plumber will have the local knowledge that’s vital if you want your tankless water heater to be energy efficient. They can help you to find the best tankless water heater to meet your needs with the right balance of performance and efficiency. A professional installation also takes the guesswork out of the entire process for a hassle free experience.

If you’re interested in a tankless water heater installation for your home, contact a local certified plumber for expert help and advice to guide you through the available options that are best suited to your home.

By Giovanni Longo President Flood Brothers Plumbing
Giovanni Longo is a 3rd generation master plumber who has been practicing his craft and trade in the greater Los Angeles area for well over a decade and a half. A plumbing and hydraulics-engineering innovator, Giovanni’s particular world-class expertise focuses on dealing with challenging sewer system designs as well as resolving complex commercial and residential draining issues. As a certified Flood Mitigation expert, he is also well versed in a wide variety of water damage and remediation solution.