Pipe Size Based On Fixture Units

We don’t know of any specific minimum in the International Building Code (IRC) or Florida Building Code (FBC), but the sizing of water distribution pipes is based on a combination of “fixture units,” which is a number assigned to different types of plumbing fixtures according to their likely water usage, and the length of piping in the system. To complicate things further, the calculation also requires an allowance for the friction in the pipes based on type of pipe and number of elbows.

  But, on average, a 1/2” distribution pipe is only good for about 3 fixture units, while a 3/4” pipe is rated at approximately 10 fixture units. The hot water fixture rating of a bathroom is 1.5 fixture units, and a kitchen with sink and dishwasher adds 1.9 fixture units That’s 3.4 already, and we haven’t even added a laundry at 1.8 fixture units. So a 3/4” inlet pipe would be the minimum for most homes. 

    Just to be sure, we checked with our plumber, James Freeman, of J.W. Freeman Plumbing in Gainesville. “You need to know how many fixtures are being fed by the water heater, and then use the tables in Appendix E of the Florida Plumbing Code,” according to James. “Most homes will need 3/4”. Some of the older houses in town, though, piped the entire hot water system in 1/2”, to get the hot water faster and cut down on waste.”

• • • • • • • • • • • • • • • • • • • • • • • • • • • • 

Here’s links to a collection of more blog posts about WATER HEATERS:

• Are water heaters required to be raised off the floor? 

• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?

• Where are gas water heaters not allowed to be installed?  

• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code?

• Can you use a light switch for a water heater disconnect? 

• How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

• Can you wire a 240-volt water heater with 120 volts? 

• Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?  

• What is the minimum clearance to doors and windows for an outdoor tankless gas water heater? 

• What is required clearance for access and working space in front of an electric water heater? 

• Why is the water heater older than the house? 

• Does a water heater need a shut-off valve? 

• Why should a tankless water heater have an isolator/service valve kit installed?

• When was a gas water heater first required to be elevated 18 inches above a garage floor?  

• Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home? 

• What is the purpose of a thermostatic mixing valve above a water heater? 

• Does a tankless gas water heater still work with no electricity during a power outage?  

• Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom? 

• Where do I find the water heater in a mobile home?

• Does a tankless water heater in an attic require a drain pan? 

• Does an electric water heater require a disconnect? 

• Is a catch pan and drain piping required for a replacement water heater?

• What is the difference between a single element and dual element electric water heater? 

• What's that powdery crust on the pipe connections at the water heater? 

• What are the most common installation mistakes with water heater replacement?

• Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises? 

• What can I do to make my water heater last longer?

• How can I determine the age of a water heater if the serial number is missing or decoding it is impossible? 

• How does a hydronic heating system work?

• What is the difference between a regular water heater and a direct vent water heater? 

• What is the difference between a regular water heater and a power vent water heater?

• What is backdrafting at a gas water heater? 

• How do I determine if a water heater is gas or electric?

• What does it mean when a gas appliance (water heater, furnace, or range) has been "red tagged"?  

• What's the valve with the flip-up handle on the water heater for?

• Why is an older water heater an insurance problem?

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.