How Water Heating Can Affect Your HERS Rating

When builders think about the impact of a water heater on a HERS rating, the first thing that usually comes to mind is the efficiency of the equipment.

Is it a heat pump water heater?
What is its UEF?
Is it electric or fossil fuel?

Those specifications matter, but they aren't the whole story.

Where the water heater is located and how far hot water has to travel can also affect the HERS rating.

A recent Massachusetts project provided a good example of just how significant those details can become. A change made between the preliminary model and final construction increased the project's HERS score by three points — enough to cause the project to miss its required rating.

A Real-World HERS Example

The project involved a new ADU created on the second floor of a multifamily building.

In the original plans and energy model, the water heater was located inside the conditioned space of the dwelling unit.

The planned hot-water distribution run was approximately 25 feet.

By the time of the final inspection, the water heater had been relocated to a different location outside the conditioned space. At the same time, the hot-water distribution run had increased from approximately 25 feet to 57 feet.

That's an additional 32 feet of hot-water piping.

The changes were significant enough that, once the actual installation was reflected in the HERS model, the rating increased by approximately three HERS points.

In this particular project, three points mattered.

The revised rating no longer met the required HERS target, which meant additional improvements had to be identified before the project could pass.

The lesson isn't that a water heater can never be moved.

The lesson is that water-heating changes need to be considered as part of the overall HERS design — especially when the project is already close to its required rating.

Your Water Heater Is More Than Just the Tank

A HERS model doesn't look only at the efficiency rating of the water-heating equipment.

The overall system includes the relationship between the water heater and the fixtures it serves.

Consider two otherwise identical homes.

Home A

• Efficient water heater

• Water heater located inside conditioned space

• Short, direct hot-water runs

• Well-insulated hot-water piping

Home B

• Same efficient water heater

• Water heater located farther away

• Longer hot-water distribution runs

• More piping between the water heater and fixtures

The equipment may be identical, but the water-heating systems aren't.

That difference can show up in the energy model.

RESNET's HERS guidance recognizes hot-water distribution characteristics such as piping length, recirculation, and pipe insulation as relevant information for evaluating the system.

Why Does Hot-Water Pipe Length Matter?

Every time you extend the distance between the water heater and a fixture, you're adding more piping that has to be supplied with hot water.

The longer the distribution system, the greater the opportunity for heat loss from the hot water sitting in the pipes.

There is also a practical homeowner issue: the farther hot water has to travel, the longer someone may have to wait for hot water to reach a fixture.

DOE building-science guidance recommends compact plumbing layouts that keep water-using rooms relatively close to the water heater and minimize the amount of water stored in the distribution piping.

From a HERS perspective, the important point is that the distribution system is part of the energy picture.

That means moving a water heater farther away isn't necessarily an insignificant construction change.

Why This Can Matter Even More in an ADU

Small dwelling units can be particularly sensitive to changes in modeled energy use.

An ADU might have only several hundred square feet of conditioned floor area, but it still needs:

• Hot water

• Mechanical ventilation

• Heating and cooling

• Lighting

• Refrigeration

• Appliances and other modeled loads

Some of these energy uses don't decrease proportionally just because the home is smaller.

As a result, relatively small changes to one component of the energy model can have a noticeable effect on the overall HERS rating.

This is one reason I recommend paying attention to water-heating design early in the project rather than treating it as an equipment decision that can be finalized at the end.

The Location of the Water Heater Matters

A water heater located inside conditioned space isn't automatically the right answer for every project.

There are legitimate reasons a builder might place water-heating equipment in another location.

But if the HERS model assumes one location and the equipment is installed somewhere else, the model may need to be updated.

The same applies when a change in location substantially increases the distance to the fixtures being served.

In the project described above, both things changed:

Water heater location:
Inside conditioned space → relocated

Hot-water distribution:
Approximately 25 feet → approximately 57 feet

Together, those changes had a measurable effect on the final HERS rating.

Don't Forget Hot-Water Pipe Insulation

Pipe insulation is another detail that shouldn't be overlooked.

Massachusetts projects subject to applicable energy-code requirements may have specific requirements for insulating service hot-water piping, and insulation can also be relevant to the HERS model.

Even when the water heater itself is highly efficient, there is still a distribution system between the tank and the fixtures.

The goal is to make that entire system work efficiently — not just to select an efficient water heater.

What About Recirculation?

Recirculation can be useful when hot-water fixtures are located far from the water heater.

Instead of waiting for hot water to travel from the water heater every time a fixture is used, a recirculation system can keep hot water available closer to the point of use.

But recirculation isn't automatically a free efficiency improvement.

The type of recirculation system, pump operation, controls, piping, and other characteristics can affect how the system performs and how it is represented in the HERS model.

If a recirculation system is being considered, it should be incorporated into the energy model rather than added as an afterthought.

Think About Plumbing Layout During Design

One of the easiest ways to improve hot-water distribution is to think about plumbing layout before construction begins.

A compact plumbing design can place bathrooms, kitchens, and laundry areas relatively close to the water heater.

This can reduce the amount of piping required and shorten the distance hot water needs to travel.

For a new ADU or multifamily dwelling, that can mean thinking about the plumbing layout at the same time you're thinking about:

• Mechanical-room location

• HVAC equipment

• Electrical equipment

• Ventilation

• Service access

• HERS performance

The best location for a water heater isn't necessarily the most convenient location for one trade.

It's the location that works well for the entire building system.

Don't Make Significant Changes at the End of Construction

The biggest lesson from this project wasn't that the original water-heater location was the only possible solution.

It was that the change happened without accounting for its effect on the HERS rating.

By the time the final inspection occurred, the equipment and plumbing were already installed.

That left fewer options for correcting the problem.

If a builder needs to relocate a water heater, change the type of water heater, substantially alter the hot-water distribution, or make another mechanical change, it's much easier to evaluate the effect before the work is completed.

A quick update to the HERS model during construction can reveal whether the change is likely to affect compliance.

A Three-Point HERS Change Can Be Significant

A three-point change may not sound dramatic.

But consider a project with a required HERS target of 45.

If the final rating changes from 45 to 48, the project doesn't simply become "a little less efficient."

It may no longer meet the applicable requirement.

That can mean going back to the model and finding other opportunities to recover those points.

Depending on the project, that might involve changes to:

• Air sealing

• Windows

• Insulation

• HVAC equipment

• Water heating

• Ventilation

• Lighting

• Solar

• Other modeled efficiency measures

The exact solution depends on what the project has available.

The important part is identifying the problem early enough to have options.

The Takeaway for Builders

A high-efficiency water heater is an important part of a high-performance home, but the equipment rating isn't the only thing that matters.

When you're working on a HERS-rated project, pay attention to the entire water-heating system:

• Where is the water heater located?

• Is it inside or outside the conditioned space?

• How far is it from the fixtures?

• How long are the hot-water distribution runs?

• Are the hot-water lines properly insulated?

• Is recirculation being used?

• Has the actual design been reflected in the HERS model?

The Massachusetts project described above is a good reminder that seemingly small construction changes can have a measurable impact on a final HERS rating.

In that case, moving the water heater and increasing the hot-water run from approximately 25 feet to 57 feet contributed to a roughly three-point increase in the HERS score.

When a project is close to its required target, those three points can make the difference between passing and needing additional work.

Water heating isn't just about choosing an efficient tank. It's about designing the entire system.

Frequently Asked Questions

Does water-heater location affect a HERS rating?

It can. The location of the water heater can affect the modeled water-heating system, particularly when it changes the relationship between the equipment and the hot-water distribution system.

Does longer hot-water piping affect a HERS rating?

Yes. Hot-water distribution is part of the HERS evaluation, and piping length is one of the characteristics considered in the system.

Should a water heater always be located inside conditioned space?

Not necessarily. The appropriate location depends on the building design, equipment, plumbing layout, and applicable requirements. The important point for a HERS-rated project is that the actual installation should be consistent with the energy model or the model should be updated when the design changes.

Can moving a water heater really change a HERS score by several points?

It can, depending on the project. In one recent Massachusetts project, a water-heater relocation combined with an increase in hot-water distribution length from approximately 25 feet to 57 feet contributed to an approximately three-point increase in the final HERS score.

When should the water-heater location be finalized for a HERS project?

Ideally, the location and hot-water distribution layout should be established during the design and preliminary modeling process. If construction changes are necessary, the HERS rater should be notified before the work is completed so the potential impact can be evaluated.