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For contractors and builders relatively new to tankless water heating, determining the right size tankless water heater or configuration for your client’s home or project requires getting used to some new calculations.

Tankless water heater size is determined by two main factors: how much water the home uses, in gallons per minute (GPM), at peak demand; and how much that water needs to be heated compared to its incoming temperature.

A technician from HVAC and plumbing contractor Roscoe Brown installs a tankless water heater. A single unit can often meet the hot water needs of small or medium-sized homes.

For example, a typical showerhead delivers 2.5 GPM. The average incoming groundwater temperature, according to DOE, is around 50 degrees — though it can be lower in northern states during the cold winter months and higher in southern states during summer months. Generally, water heaters are set to deliver water at 120 degrees.

That means for your clients to enjoy a hot, uninterrupted shower, you’ll need to install a tankless unit that can heat at least 2.5 GPM by at least 70 degrees to reach the 120 degree goal. Nearly all propane tankless water heaters on the market can handle this load, with most capable of at least 5 GPM at a 70 degree temperature rise. Lower flows can be heated to a larger temperatures rise, and higher flows less.

But that’s just for a dedicated unit at a single point of use. Things get a little more complex when determining what capacity unit you’ll need for the whole house, or whether it’s better to install multiple units run in parallel or at points of use to achieve the best results. Also, keep in mind that while tank storage heaters are fired at between 50,000 to 75,000 Btus, tankless units can demand up to 200,000 Btus for peak loads, so make sure your gas lines are sized accordingly.

Here are four ways to tackle tankless water heater sizing.

1. Add up each faucet and point of use, such as washing machines and dishwashers.

According to the DOE, you can determine your peak demand by counting the faucets, showers, and appliances a home will use at any one time and then using their ratings to determine total GPM. If you can’t find the rating, or the fixture doesn’t have one, time how long it takes to fill up a gallon jug. If the kitchen faucet takes 30 seconds to fill the jug, then the faucet flows at 2 GPM. Add up all your ratings. That’s the minimum GPM you need at peak demand.

2. Time how long it takes to run out of hot water.

For projects where you’re replacing a storage tank water heater, this might be the simplest way to determine your peak demand, suggests David Crawford, CEO of home maintenance and service firm Service America. Turn on each tap and appliance that uses hot water, and track how long it takes for the water to turn cold. Then, look at the size of the traditional water heater in the home today, and divide by the number of minutes you ran the water. For example: 40 gallons ÷ 12 minutes = 3.2 GPM. “This is your peak demand and minimum GPM your clients need for a tankless heater,” Crawford says.

3. Decide how many units you need.

According to Bobby Patterson, plumbing service manager at Murfreesboro, Tennessee–based HVAC and plumbing contractor Roscoe Brown, once you’ve determined peak GPM, consider the number of bathrooms in the home to decide if it’s better to install multiple units run in parallel. “In smaller houses with one to two bathrooms, we would use one tankless based on what’s needed when everything is running at once,” Patterson says. “In homes with three or more baths, we’ll look at linking multiple units together, depending on capacity.”

“In certain cases, where a fixture has a long supply run from the primary heater, it may be more desirable to install an additional tankless unit on that side of the house.”

Having multiple tankless units makes a home’s water heating system more reliable, allowing the two units to augment each other according to demand. “The gas valve on a tankless heater works a lot like the throttle on a car,” Patterson explains. “When you turn on more fixtures, it throttles up, and when you turn them off, it throttles back down. If you add a second heater in parallel, that second heater will only kick on when the first one reaches its capacity. When it does, the second heater becomes the primary unit, and the first unit backs it up. Setting it up like this wears the heaters evenly and is why they heat so efficiently, because neither has to run at full capacity for very long.”

4. Decide whether to run multiple units in parallel or at point of use.

Running multiple units in parallel works best for homes already set up to have centralized water heating, such as a retrofit where there was a single storage tank heater. However, if you’re building new construction, or in cases where an existing home had multiple water heaters previously, installing point-of-use heaters to run independent of each other can ultimately be a more energy- and water-efficient solution.

For larger homes, having multiple tankless units makes the water heating system more reliable, allowing the two units to augment each other according to demand.

“In certain cases, where a fixture has a long supply run from the primary heater, it may be more desirable to install an additional tankless unit on that side of the house,” Patterson says. “Doing so will decrease how long it takes hot water to get to the fixture.” Adding a recirculation system can achieve the same results, but reduces energy efficiency, since the heater will turn on more often to keep the water inside the system hot. “If the house previously had multiple heaters, we would likely install multiple tankless units with the existing configuration.”

By considering your client’s peak demand uses, as well as the temperature rise needed in your area, you can get a good gauge on the capacity needed for a tankless water heater. Add the size of the home and number of baths into the equation to determine if you need one or more units, and then decide whether you should run them in parallel or install them at points of use. Once you have, you’re well on your way to helping your clients save water and energy — as well as money. That’s a clear advantage all the way around.

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Joe Bousquin