What Size Tankless Water Heater Does a Tampa Home Need?

A storage tank is sized in gallons. A tankless unit has no stored water, so gallons tell you nothing. What matters is flow and temperature, and getting those two numbers right is the difference between a tankless upgrade you love and one that runs lukewarm every morning.

Step 1: add up what runs at the same time

Sizing starts with your household's busiest moment, not its total daily use. The U.S. Department of Energy's Building America Solution Center, in its guide to gas-fired tankless water heaters, says that "sizing is based on the practical maximum flow rate needed at any one time," and suggests making a list of the hot water draws that happen together.

Its worked example allows about 1.5 gallons per minute of hot water for a shower, because the shower mixes hot water with cold, and arrives at a practical maximum of 4.5 gallons per minute for the household in the example. The guide also points out that you do not need to assume every fixture runs at once.

For your home, think about a weekday morning: two showers running while the dishwasher or washing machine is on is a common peak. Two showers alone is often around 3 gallons per minute of hot water.

Step 2: work out the temperature rise

The second number is how far the heater has to lift the water. Hot water is usually delivered at about 120°F. The temperature rise is that figure minus the temperature of the cold water coming into the house.

The same DOE guide sizes its example for Seattle, where it puts groundwater at about 52°F, so the heater has to add 68°F. Tampa Bay's incoming water is much warmer than that for most of the year, which means a smaller rise. As an illustration, if your cold water arrives at 70°F, reaching 120°F takes a 50°F rise.

That is good news for Tampa homeowners. The same unit delivers more gallons per minute here than it would in a northern state, because it has less heating to do. Size for your coolest months, when the incoming water is at its coldest.

Step 3: match the flow to a unit's rating

Manufacturers publish how many gallons per minute each model delivers at a given temperature rise. Pick a unit that covers your peak flow at your winter rise, with some margin. The DOE guide puts it simply for its example: look for a model "that can provide at least 4.5 gpm with a temperature rise of 68°F."

Two practical points:

• Gas units usually cover more flow. The DOE guide notes that tankless burners typically run from 120,000 to 200,000 Btu/h, far more than a tank's burner. That output is why a single gas unit can often run a whole house.
• Electric units are limited by your panel. Whole-house electric tankless heaters draw heavy current, so the electrical service decides how large a unit you can install.

When a home needs a larger unit, or two

If your peak demand is above what one unit can deliver, the options are a larger model or two units linked together. Large homes, homes with a soaking tub, and households where several people shower at once in the morning are the usual cases.

Undersizing is the most common tankless complaint: the water turns lukewarm when a second fixture opens. Oversizing costs more up front and may need a larger gas line than the home has. Our guide to whether a tankless water heater is worth it in Tampa covers the wider decision, and tank to tankless conversion in Tampa explains the gas, venting and electrical work.

Get your tankless water heater sized in Tampa

Mr Water Heater provides tankless water heater installation across Tampa and West Central Florida, from Carrollwood and Odessa to Wesley Chapel and New Port Richey. Mark Lowrie holds Florida Master Plumber Licence CFC1433539, and the business is licensed and insured. Sizing is part of the assessment, including checking the gas line or electrical panel the new unit will need.

Call Mr Water Heater at +1 727 203 8201 or contact Mr Water Heater to book a tankless assessment.