Water Heater Sizing Guide for New Mexico Homes

Tank sizing — first hour rating, not gallons

First Hour Rating (FHR) is what matters, not raw tank size. FHR is the gallons of hot water a unit can deliver in the first hour of use. Manufacturer's spec sheet lists it. Match FHR to your peak hour demand.

• 1-2 person household: 30 to 40-gallon tank, FHR 50 to 60
• 3-4 person household: 40 to 50-gallon tank, FHR 60 to 70
• 5+ person household: 50 to 75-gallon tank, FHR 75 to 90

Tankless sizing — flow rate at temperature rise

Tankless units are sized by gallons-per-minute at a specific temperature rise. ABQ's incoming water hits 50 to 55 F in winter, 65 to 70 F in summer. To deliver 120 F at the tap, you need a 65 to 70-degree rise in winter.

• Single bathroom, 1-2 people: 5 to 6 GPM at 70-degree rise
• Two bathrooms, 3-4 people: 7 to 9 GPM at 70-degree rise (Rinnai RU160 or Navien NPE-180A territory)
• Three bathrooms, 5+ people: 9 to 11 GPM (Rinnai RU199, Navien NPE-240A2)

Altitude derating — the ABQ catch

Gas appliances at 5,300 feet are derated by manufacturer spec — most by 4% per 1,000 feet above 2,000. A 199,000 BTU/hr tankless rated at sea level delivers about 165,000 BTU/hr in Albuquerque. Size up to compensate. We do this math on every quote.

Recovery rate for tank gas units

Recovery rate (gallons per hour at a 90-degree rise) tells you how fast the tank refills with hot water once you've drained it. ABQ's lower BTU output drops recovery 5 to 10%. A 50-gallon tank rated 41 GPH at sea level delivers about 37 GPH here.