Tankless Water Heaters: In Search of Endless Hot Water

Tankless Water Heaters – Homeowners love hot water. Want proof? Call them right after their nice hot shower has run cold and you’ll be sure to get an earful. Hence the plethora of sales pitches making the promise of endless hot water.

Unfortunately, the wrong- sizing of units and improper installation practices have left some early adopters of tankless technology wanting. There is an easy fix though. With a proper load demand calculation, and ensuring that the install meets the basic needs of the unit, the warm showers can last for as long as the homeowner desires.

And don’t worry, the math is easy, and the basic needs of tankless water heaters are very simple. They consist of three things: water, air and fuel.

If you get all three of these things right, the chances of failure for the unit have all but disappeared and the unit should work maintenance- and trouble-free at high-efficiency while meeting that number 1 customer request of “never running out of hot water” for years to come.

Meeting demands with a calculator

The number one request that I get when sizing a domestic hot water load f or a customer is that the end user must never run out of hot water.Thankfully, this all comes down to simple math, and the definition of a BTUH is where to start on our path to meet this request.

A British thermal unit per hour, or BTUH, is the approximate amount of energy needed to heat one pound of water one degree Fahrenheit in one hour.

To calculate the number of BTUHs necessary to maintain hot water indefinitely, let’s toss some numbers on the wall.The mass of water needed, a flow rate, and a delta T will all help us land at our magic number.

We know that one gallon of water weighs 8.33 pounds; and basing our delta T on an incoming water temperature of 40°F on the coldest day of the year, and 120°F for typical use and storage, we have a differential of 80°F.

Our flow rate will be reported in gallons per minute, or 8.33 pounds per minute, but we are looking at an hour’s worth of energy, so let’s multiply the 8.33 pounds of water by 60 minutes.That gives us roughly 500 pounds of water, and a delta T of 80.

Multiplying these figures together, we find that we need 40,000 BTUH to maintain a one-gallon-per-minute flow of 120°F water.

That’s the math, and the numbers don’t lie, but we are not exactly done yet.To give a customer a system that will never run out of hot water, we need to calculate the total load.

If the end customer has one standard shower at 1.5 gpm, and one kitchen faucet at 2.5 gpm, then the total load is four gallons per minute. Given the flow requirements of the home, 160,000 BTUH will be required to meet this total load demand. And at that input rating, let the warm times roll.

Follow the instructions, to a tee

Following installation instructions may sound simple, but I’m always amazed at the creative ways that some appliances get installed, and that creativity can cause problems with how a water heater performs.

If the unit comes with two pipe connections for direct-vent application, use two pipes and direct-vent the unit. If it is necessary to terminate the exhaust with a tee, use a tee; if it says to use a 90, use a 90.

And keep in mind that there is still water in tankless units when in standby mode. Since they do not fire unless called for, there is a chance they can freeze, but if the installation instructions are followed and the unit is properly vented, there should not be any freezing problems.

Deliver what the unit needs

Tankless water heaters are high BTUH units. Feeding them with half-inch half-port gas ball valves attached to 90 feet of half-inch corrugated flex isn’t going to cut it. These are units where it will be necessary to use the gas charts, since shortcuts and rules of thumb can cause operating problems for the appliance.

Size for the home’s total load, measure the distance from the main to the unit, and run the proper size of pipes based on all of these factors. And there is nothing wrong with corrugated gas lines, just size them correctly.

Once the unit is up and running, conduct a combustion analysis, as well as an intake analysis to make sure there is no cross contamination between the intake and exhaust.

A cold water sandwich

There are a few idiosyncrasies with tankless water heaters that are worth thinking about and discussing with customers, especially those who have not had a tankless water heater before, and the cold water sandwich, where hot water is followed by cold and then hot again, is one of them.

The cold water sandwich is something that doesn’t come up much as it did in the early days of the technology, since tankless installers and users have learned how to avoid it, but it will take some adjustment for some end users.

Unless additional equipment has been added to eliminate this phenomenon, when using a tankless heater, the user should leave the hot water running. If they turn it off and on continuously, perhaps when shaving, the hot water flow will be interrupted with cold water as the water heater cycles on, off, on, off, creating this “cold water sandwich.”

There are items that can eliminate this effect, like in-line potable hot water expansion tanks, or the installer can just educate their customer.

By Matthew Reid