How to Size a Water Heater for Residential and Multi-Unit Buildings

How to Size a Water Heater for Residential and Multi-Unit Buildings

Installing an undersized water heater is one of the most common complaints contractors hear from homeowners after move-in. An oversized unit wastes energy and money. Getting the sizing right from the start prevents callbacks and keeps clients happy. Here's a straightforward guide to sizing both tank and tankless water heaters for residential projects.

Sizing Tank Water Heaters: First-Hour Rating

The most important spec on a tank water heater for residential sizing isn't the tank size — it's the First-Hour Rating (FHR). The FHR indicates how many gallons of hot water the unit can deliver in the first hour of use, starting with a full tank. This is listed on the EnergyGuide label.

Step 1: Determine Peak Hour Demand

Estimate the hot water use during the busiest 1-hour period in the home. Here are average usage estimates per activity:

 

• Shower: 10-15 gallons
  
• Tub bath: 20 gallons
  
• Shaving/handwashing: 2-4 gallons
  
• Dishwasher: 6-10 gallons
  
• Clothes washer (warm): 7-12 gallons
  

Step 2: Match FHR to Peak Demand

Add up the expected usage during the peak hour. Your target water heater should have an FHR at or above that number. General guidelines by household size:

 

• 1-2 people: 30-40 gallon tank (FHR ~40-60 gallons)
  
• 2-4 people: 40-50 gallon tank (FHR ~60-80 gallons)
  
• 4-6 people: 50-80 gallon tank (FHR ~80-100 gallons)
  
• 6+ people: 80-gallon or two-unit configuration
  

Sizing Tankless Water Heaters: Flow Rate and Temperature Rise

Tankless water heater sizing is based on two variables:

 

• Required flow rate (GPM): Total gallons per minute of simultaneous hot water use
  
• Required temperature rise: Difference between incoming groundwater temperature and desired output temperature (typically 120°F)
  

Step 1: Calculate Required Flow Rate

Add up the flow rates of fixtures that may run simultaneously:

 

• Shower: 1.5-2.5 GPM
  
• Kitchen faucet: 1.5-2.0 GPM
  
• Bathroom faucet: 0.5-1.5 GPM
  
• Dishwasher: 1.0-1.5 GPM
  
• Clothes washer: 1.5-2.0 GPM
  

Step 2: Calculate Temperature Rise

 

Groundwater temperatures vary by region:

 

• Southern states: 55-65°F incoming → 55-65°F temperature rise needed
  
• Northern states: 35-45°F incoming → 75-85°F temperature rise needed
  

A unit rated at 8 GPM at 60°F rise may only deliver 5 GPM at 80°F rise. Always consult the manufacturer's performance curve for your climate.

Multi-Unit Building Sizing

For apartment buildings, multi-family housing, or commercial applications, a diversity factor is applied — not every unit draws hot water simultaneously. Common approaches include:

 

• Central tank bank: Multiple large tanks or commercial units serving the whole building through a recirculation loop
  
• Individual unit heaters: Each unit has its own water heater (common in garden-style apartments). Simplifies maintenance and avoids shared system failures.
  
• Point-of-use tankless units: Small electric or gas units at individual fixtures — effective for supplementing a central system or for additions
  

Additional Sizing Considerations

 

• Homes with large soaker tubs (60+ gallon capacity) should upsize to an 80-gallon tank or specify a recirculating system
  
• Radiant floor heating systems that use domestic hot water need a separate dedicated heater or larger commercial unit
  
• Always check local energy codes — some jurisdictions require heat pump water heaters above a certain square footage threshold
  

Pro Tip

When building multiple identical units (townhomes, condos, apartments), standardize on a single water heater model. Bulk ordering of a single SKU from a wholesale supplier reduces cost and simplifies maintenance for property managers down the road.