Is Your Condensing Water Heater Really Delivering Its Rated Efficiency?

Your commercial water heater may advertise outstanding efficiency. But is your building actually getting it? The answer depends on more than the equipment itself.

Imagine purchasing a high-efficiency condensing water heater for a hotel.

The manufacturer provides an impressive efficiency rating. The equipment is installed, the guests have hot water, and everything seems to be working perfectly.

But there's a question nobody may have asked:

Is the water heater actually operating at its advertised efficiency?

Not necessarily.

And the reason may have more to do with the way the system is designed than with the water heater itself.

How Does a Condensing Water Heater Work?

When natural gas burns, it produces hot exhaust gases containing water vapor.

Conventional gas water heaters transfer heat from those gases to the water, but much of the water vapor leaves through the exhaust.

Condensing water heaters are designed to recover additional energy.

Their heat exchangers can cool the exhaust gases enough for some of that water vapor to turn into liquid.

That process releases additional heat that can be transferred to the water.

It's called condensation, and it's one of the reasons these heaters can achieve such high efficiency ratings.

But there's a catch.

Condensation depends heavily on the temperature of the water entering the heat exchanger.

What Happens When the Water Entering the Heater Is Already Hot?

Imagine the heater receives incoming water at 60°F (16°C).

That relatively cold water helps cool the combustion gases, creating favorable conditions for condensation.

Now imagine the same heater receives water at 120°F (49°C).

Because the incoming water is already warm, the heat exchanger has less ability to cool the exhaust gases and recover their latent heat.

As inlet-water temperature rises, condensing efficiency generally decreases.

Eventually, the equipment may recover little or no heat through condensation.

The heater is still a condensing model, but it may not actually be condensing during those operating conditions.

There's no universal temperature at which every condensing water heater suddenly stops condensing. Actual performance depends on the equipment design, combustion conditions, firing rate, and heat exchanger temperatures.

That's why a single advertised efficiency number doesn't tell the whole story.

The Hotel Example: When System Design Changes Everything

Consider a hotel with a central hot water system.

Hot water circulates through the building so guests can receive it without long delays.

During periods of low consumption, relatively little fresh cold water enters the system.

Instead, the heating equipment may spend much of its time reheating water returning from the distribution piping.

That returning water is already warm.

If it flows directly through a condensing water heater, the equipment may have limited opportunities to recover the additional heat associated with condensation.

Now imagine that the same installation includes a storage tank.

In some arrangements, cold makeup water enters the tank while the heater receives predominantly warm water from that tank.

The heater may repeatedly reheat water that's already close to the desired temperature.

The installation can deliver hot water reliably while operating at a lower efficiency than the manufacturer’s rated value.

That doesn't mean every system with storage is poorly designed.

It means the temperatures reaching the heating equipment matter.

What Do Actual Tests Show?

This isn't just theory.

ASHRAE's Service Water Heating Handbook documents laboratory studies examining the effect of return-water temperature on condensing water heaters.

In one study, a condensing storage water heater's measured efficiency declined from approximately 90% to 84% as return temperature increased from 90°F (32°C) to 120°F (49°C).

Another tested tankless heater experienced an even greater reduction, from approximately 90% to 76%, under the specific recirculation conditions examined.

These results don't mean every storage heater operates at 84% or every tankless heater at 76%.

Different heat exchanger designs behave differently.

But the studies demonstrate an important principle:

A water heater designed for high efficiency can lose a substantial portion of that advantage when it receives water that's too warm for effective condensation.

Then What Does the Advertised Efficiency Mean?

A manufacturer's certified efficiency rating can be completely legitimate.

Commercial water heaters are tested under standardized conditions so their performance can be measured and compared.

For example, U.S. federal thermal-efficiency tests generally use supply water near 70°F (21°C), with a specified temperature rise and continuous burner operation at rated input. Some equipment requires a special recirculating test arrangement.

These conditions may differ considerably from those in an operating hotel, gym, or commercial building.

The U.S. Department of Energy's commercial water heater testing procedures define how these measurements are performed.

There's also an important distinction between efficiency ratings:

• Thermal efficiency: How effectively the equipment transfers fuel energy to the water under specified test conditions.

• Operating efficiency: How efficiently the equipment performs over an actual period of operation.

• Overall system efficiency: How much of the supplied energy ultimately reaches the points where hot water is used.

Different markets and equipment categories may also use different rating methods and fuel-energy reference values.

These numbers are not interchangeable.

A water heater's thermal-efficiency rating is useful for comparing equipment, but it doesn't automatically predict the gas consumption of the complete installation.

Does This Mean Condensing Heaters Shouldn't Be Used With Storage Tanks?

Not at all.

Storage tanks can be valuable when a building requires large amounts of hot water during short periods.

The challenge is making sure the heating equipment operates under conditions that support its intended performance.

Certain system configurations can provide more opportunities for condensing heaters to receive cold makeup water.

Others may repeatedly send warm water through the heaters.

These arrangements can produce very different results, even when they use similar equipment.

There's also another distinction worth remembering.

Heat lost from a storage tank is not the same as reduced condensation efficiency.

And neither is the same as heat lost through the building's hot water recirculation piping.

All three can affect operating costs, but they're separate energy considerations.

Should You Lower the Return-Water Temperature to Improve Efficiency?

Not simply for the sake of condensation.

Commercial hot water systems must also control bacterial growth and protect occupants from scalding.

The CDC's water management guidance addresses safe storage and circulation temperatures as part of Legionella prevention.

Lowering water temperatures without considering those requirements can create serious problems.

The better approach is to evaluate how the complete system is arranged and operated while maintaining appropriate water safety conditions.

Energy efficiency should never come at the expense of water safety.

Five Questions Every Building Owner Should Ask

Before purchasing a commercial condensing water heater, ask your engineer or equipment supplier:

1. What efficiency rating are we comparing?

Make sure the quoted number uses the same test method and measurement basis as competing equipment.

2. What water temperature will enter the heater during normal operation?

Consider both periods of high consumption and periods when the equipment is primarily maintaining temperature.

3. How will storage and recirculation affect that temperature?

Understand how the system operates when little or no fresh cold water is entering.

4. Does the manufacturer provide efficiency curves for different inlet-water temperatures?

A performance curve can reveal considerably more than a single number printed on a brochure.

5. What is the expected performance of the complete installation?

Equipment efficiency matters, but so do operating temperatures, distribution losses, controls, and actual consumption patterns.

Where necessary, field measurements can help determine real operating performance.

High Efficiency Starts With Good System Design

At Xelavic, we believe choosing quality equipment is only the beginning.

Water heaters, pumps, piping, valves, controls, and storage systems must work together.

That's especially important in modular and packaged hot water installations.

Proper integration can improve reliability, simplify installation, and help equipment operate under conditions that support its intended performance.

Learn more about preassembled water heater racks and packaged hot water systems.

The number on the brochure matters. But the way your system operates matters just as much.

Contact Xelavic to discuss your commercial hot water project.