Replacing your current tank water heater is a sound decision. A tankless unit is economically sound, environmentally friendly, and provides almost unlimited hot water. However, to take advantage of all the benefits a tankless water heater offers, it’s vital that your tankless water heater is properly sized for your needs.
Although the adage “bigger is better” is applicable in some instances, it isn’t when it comes to sizing a tankless water heater. In fact, if your tankless is too big for your needs, you probably won’t be happy with it. You may run out of hot water, your utility bills may increase, and your unit may not last as long.
Unlike conventional tank water heaters, tankless units aren’t sized by gallons of storage but by flow rate and temperature rise. This guide will show you how to properly size your tankless so that it best fits your needs, and remember: At Benjamin Franklin Plumbing of Orlando, our punctual plumbers help homeowners pick the perfect-sized system for endless, efficient hot water.
Quick Takeaways
- Tankless water heaters are sized by flow rate, or GPM, and temperature rise rather than tank capacity.
- Flow rate is the maximum amount of hot water you could need simultaneously and includes appliances, showers, and sinks.
- Temperature rise is the difference between the temperature of incoming cold water and the desired temperature of the hot water coming out of the tap.
- The correct size ensures maximum efficiency, comfort, and availability of hot water.
- Oversizing wastes money and causes additional wear and tear on the unit.
- Undersizing causes a lack of hot water and additional wear and tear on the unit.
- A small home may only need a flow rate of 5-7 GPM. A larger home or one with several occupants may need 8-10 GPM or more.
How Does a Tankless Work?
Instead of storing water in a large vat and maintaining it at a preset temperature, even when it isn’t being used, a tankless system only provides hot water when the hot water tap is turned on. This can be a substantial energy saver. It also means that the tank must be correctly sized so that it can instantly heat your water.
When you turn on your hot water tap, your tankless water heater immediately ignites and begins to heat the water flowing through it. When the hot water tap is turned off, the tankless stops heating water and doesn’t begin again until the hot water tap is opened. This on-demand system enables you to have an almost-limitless supply of hot water, but it also means you must have an accurately-sized tankless water heater.
What Determines the Size Unit I Need?
The maximum amount of hot water that you can need simultaneously is part of the determining factor for size. This is called the flow rate, or GPM.
Calculating GPM
The total GPM demand is estimated as follows:
- Bathroom sink faucet ~ 1 GPM
- Dishwasher ~ 1.5 GPM
- Shower ~ between 2-2.5 GPM
- Washing machine ~ 2 GPM
By adding all these together, you get the GPM you’d need if you were using them all at the same time. If you have more than one shower and bathroom sink faucet, you’d include the total number of showers and bathroom sinks you have. You should choose a unit that slightly exceeds the total possible demand, but not one that’s smaller or significantly exceeds the total possible demand.
Calculating Temperature Rise
Temperature rise is the difference between the temperature of incoming cold water and the desired hot water temperature, which is usually around 120°F. Colder climates may require a higher temperature rise than warmer climates. It’s important to note that GPM capacity decreases as the temperature rise increases, so always take this into consideration when sizing your unit.
Household Size and Usage
If you have a larger household size, you’ll have a larger demand, and this needs to be part of your calculations.
For example:
- 1-2 people is usually 5-7 GPM
- 3-4 people is usually 8-10 GPM
- 5 or more people or homes with multiple baths is usually 10-12 GPM or more
In addition, lifestyle habits such as showering, doing laundry, and running the dishwasher matter as much as household size.
Fuel Type: Gas vs Electric
The type of fuel your tankless uses is a factor in the size you’ll need.
Gas units have a higher heating capacity, making them ideal for larger families. Gas units are sized by BTU rating.
Electric units are easier to install, so they’re good for smaller households. Electric units are sized by kilowatt power.
However, both types, either gas or electric, must be correctly sized if they’re to provide the best service.
Steps to Calculate the Size You Need
Step 1 – Identify Fixtures and Appliances
List all your hot water points that can be used simultaneously, such as dishwashers, faucets, showers, and washers.
Step 2 – List the GPM for Each
Use the manufacturer specifications or use the following:
- Bathroom sink faucet 1 GPM
- Dishwasher 1.5 GPM
- Shower between 2-2.5 GPM
- Washing machine 2 GPM
Step 3 – Total the GPM for all units
This is the maximum amount of hot water you can expect to use simultaneously.
Determine the Temperature Rise
Subtract the inlet cold water temperature from the targeted hot water temperature desired. This result is your temperature rise.
Match the Result to a Manufacturer’s Sizing Chart
Select a unit that delivers your desired GPM at your targeted temperature rise. If you’re between sizes, opt for the one that’s slightly larger than you need, but never select one that’s even slightly smaller than your calculated needs.
Common Sizing Errors: Avoid These
- Using your current tank capacity rather than your GPM.
- Undersizing to save money: You won’t.
- Oversizing just to be safe: You won’t be.
- Ignoring temperature rise: Colder water limits flow capacity, or GPM.
- Skipping professional sizing: All homes have unique plumbing and GPM rates, so call our punctual plumbers for a professional sizing.
How do Professionals Size a Tankless?
You should always have a professional install your tankless water heater. Trying to DIY your tankless installation might invalidate your homeowner’s insurance, and most municipalities require permits and inspections. Our punctual plumbers can install your tankless system according to local regulations and building codes, so you can have peace of mind knowing the installation is done correctly and efficiently.
Licensed plumbers:
- Use manufacturer data, specs, inlet water temperature charts, and fixture audits.
- Verify gas line size, electrical capacity, and venting requirements before recommending a model.
- Account for humidity and groundwater temperature before recommending a size.
- Benjamin Franklin Plumbing’s punctual plumbers use digital flow meters to accurately size systems.
FAQ: Common Questions About Tankless Water Heater Sizing
What size tankless water heater do I need for my family of four?
Most families of four need a unit that’s rated for 8-10 GPM, depending on usage and local water temperature.
Can a tankless water heater be too large?
Yes. A tankless water heater that’s too large for your needs can increase installation costs and energy consumption without providing a benefit. It may also cause your unit to wear out more quickly.
What size tankless water heater do I need for a two-bedroom home?
Typically, a 6-8 GPM unit can handle two showers or faucets running simultaneously. However, there are other factors to consider, so you need to perform a size calculation.
Does incoming water temperature affect size?
Yes. The colder the water, the greater the temperature rise, which will reduce your flow capacity.
Should I select gas or electric for my Orlando home?
The type of unit that works best for you depends on several factors, such as the size of your home and your lifestyle. You need an energy calculation to determine whether gas or electric is best for your circumstances.
Get Expert Tankless Water Heater Sizing in Orlando
If you need a tankless water heater, don’t guess on the sizing. At Benjamin Franklin Plumbing of Orlando, our punctual plumbers can accurately size, install, and maintain both gas and electric tankless water heaters. You’ll have maximum efficiency and endless hot water when you have our punctual plumbers size, install, and maintain your tankless.
Call us today to schedule your in-home consultation today. We value your time and comfort and look forward to working with you.