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HVAC VFD Retrofit: Fans, Pumps, Energy Savings, and Retrofit Pitfalls

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HVAC VFD Retrofit: Fans, Pumps, Energy Savings, and Retrofit Pitfalls

An HVAC VFD retrofit is one of the most common energy efficiency upgrades applied to existing fans and pumps in commercial buildings. When designed and commissioned correctly, a VFD retrofit can significantly reduce energy consumption at part-load conditions. However, many HVAC VFD retrofit projects underperform due to overlooked mechanical limits, poor controls integration, or incomplete commissioning.

However, in real projects, many VFD retrofits fail to deliver their expected savings—or introduce operational and comfort problems—because the system was never designed for variable-speed operation.

This article walks through the actual retrofit process, the engineering constraints, and the common pitfalls encountered when retrofitting fans, pumps, and motors with VFDs.

1. What a VFD Retrofit Really Is

A VFD retrofit does not simply “add a drive.”
It fundamentally changes how a mechanical system operates.

Before the VFD retrofit:

  • Motor runs at constant speed (typically 60 Hz)
  • Flow or airflow is controlled mechanically (dampers or valves)
  • Energy is wasted during low-demand conditions

After the VFD retrofit:

  • Motor speed varies based on demand
  • Mechanical throttling is reduced or eliminated
  • Energy consumption drops significantly at partial load

The key takeaway:
Energy savings come from reducing speed—not simply installing a drive.

2. When a VFD Retrofit Makes Sense

Not every fan or pump is a good candidate for a VFD retrofit.

Strong VFD candidates include:

  • Systems with variable demand
  • Long annual operating hours
  • Equipment that can tolerate turndown
  • Systems with a measurable control variable

Poor VFD candidates include:

  • Systems with strict minimum airflow requirements
  • Fans operating near surge or stall conditions
  • Equipment with no viable process variable
  • Systems with unresolved comfort or control issues

A VFD does not fix a poorly behaving system—it exposes it.

3. The HVAC Retrofit Process (Step-by-Step)

Step 1: Existing System Evaluation

Before any hardware is specified, the existing system must be understood.

Key questions:

  • What type of fan or pump is installed?
  • Is the motor original or a replacement?
  • Is the system belt-driven or direct-drive?
  • What safeties and interlocks exist today?

Field verification is critical. As-built drawings are often unreliable on older systems.

Here are two possible applications.

Pumps serving Two-way valve system

Remove the existing 3-way control valves and associated bypass piping, and replace them with 2-way modulating valves. This allows chilled water flow to vary with load instead of maintaining constant flow from the supply to the return.

Chilled water constant flow system with 3-way valves before retrofit.
Chilled water constant flow system with 3-way valves before retrofit.

Install a Variable Frequency Drive (VFD) to control the pump motor, first confirming that the motor is rated for VFD duty. If the motor is not VFD-rated, it must be replaced.

Next, install a differential pressure sensor to monitor system pressure as the control valves close. As differential pressure increases, the controller sends a signal to the VFD to reduce pump speed, lowering energy consumption.

Chilled water constant flow system with VFD and 2-way valves after retrofit.
Chilled water constant flow system with VFD and 2-way valves after retrofit.

Finally, verify that the chiller receives the minimum flow required for proper operation. Install a bypass line between the chilled water supply and return near the chiller, sized to maintain minimum evaporator flow when most valves are closed.

Garage Exhaust Systems.

Instead of operating the garage exhaust fan at full speed continuously, install a carbon monoxide (CO) monitoring and control system with multiple CO sensors throughout the garage. These sensors are connected to a Variable Frequency Drive (VFD), which modulates the exhaust fan speed to maintain carbon monoxide concentrations within safe limits.

Garage exhaust system with VFD controlling the fan based on CO levels.
Garage exhaust system with VFD controlling the fan based on CO levels.

When vehicle activity is low and carbon monoxide levels are minimal, the VFD reduces fan speed, significantly lowering energy consumption. As vehicle traffic increases and CO levels rise due to combustion engine operation, the controller automatically increases fan speed to provide the required ventilation and maintain code-compliant indoor air quality.

Step 2: Mechanical Constraints

Mechanical limitations are one of the most overlooked retrofit risks.

Key considerations:

  • Minimum airflow or flow requirements
  • Fan surge and pump cavitation limits
  • Bearing lubrication requirements at low speed
  • Resonance issues on belt-driven equipment

If the equipment cannot operate safely at reduced speeds, energy savings will be limited or nonexistent.

Step 3: Electrical Compatibility

Electrically, a VFD changes how the motor is powered.

Items that must be evaluated:

  • Motor insulation and VFD-duty rating
  • Distance between VFD and motor (reflected wave voltage)
  • Grounding and bonding
  • Harmonics and power quality impacts
  • Existing feeders and disconnect ratings

Many older motors survive VFD retrofits—but not all, and not without risk.

4. Controls: The Most Common Point of Failure

Most underperforming VFD retrofits fail due to controls, not hardware.

A VFD requires a process variable to modulate speed effectively.

Common control strategies:

Frequent control mistakes:

  • Poor sensor placement
  • No minimum speed limits
  • Improper PID tuning
  • Conflicting safeties and overrides
  • No integration with occupancy or schedules

If the control logic is unstable, operators will override it—and energy savings will disappear.

5. Why HVAC VFD retrofit Energy Savings Disappear Over Time

A common retrofit timeline looks like this:

  1. System is commissioned correctly
  2. Occupants experience comfort issues
  3. Operators increase minimum speeds
  4. System slowly returns toward constant speed

Without clear sequences, trending, and operator training, VFDs quietly revert to 60 Hz operation.

Energy retrofits only succeed if the control strategy survives turnover.

6. HVAC VFD retrofit: Estimating and Scope Pitfalls

From an estimating perspective, VFD retrofits often fail because the scope is incomplete.

Frequently missed items:

  • Pressure or flow sensors
  • Control wiring and tubing
  • BAS programming and graphics
  • Commissioning and trend logs
  • Operator training

Most HVAC VFD retrofit change orders originate from controls scope gaps, not mechanical work.

7. Commissioning and Turnover

A successful VFD retrofit includes:

  • Trend data at multiple load conditions
  • Documented sequences of operation
  • Verified minimum and maximum speeds
  • Operator training and setpoint lock-in

Commissioning is not optional—it is where savings are protected.

Final Note

VFD retrofits can deliver substantial energy savings when applied correctly.

They succeed when:

  • The mechanical system can tolerate turndown
  • Electrical limitations are addressed
  • Controls are designed—not improvised
  • Commissioning and training are taken seriously

They fail when VFDs are treated as plug-and-play energy devices.

How to Size Wire for an Air Conditioner

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How to Size Wire for an Air Conditioner

Sizing electrical conductors for an air conditioner is a common task for HVAC professionals, electricians, estimators, and inspectors—but it is also one of the most misunderstood.

This article walks through how to properly size wire for an air conditioner using the National Electrical Code (NEC), explains why air-conditioning circuits follow different rules than standard branch circuits, and clears up common points of confusion related to ampacity tables, temperature ratings, and nameplate values.

Disclaimer:
This article is for educational and informational purposes only. Electrical systems can be hazardous. Always comply with applicable electrical codes and regulations. Electrical work should be performed or reviewed by qualified or licensed professionals.

Why Air Conditioners Follow Different NEC Rules

Air conditioners are not treated like general lighting or receptacle circuits under the NEC.

The compressor inside an air conditioner uses a hermetic motor, meaning the motor is sealed inside the compressor and cooled by refrigerant rather than open air. Because of these unique operating characteristics, air-conditioning and refrigeration equipment is governed by Article 440 of the National Electrical Code, not the general motor rules found elsewhere in the code.

This is why air-conditioning circuits often look “wrong” to people accustomed to standard wiring rules—but are fully code-compliant when evaluated correctly.

Always Start With the Equipment Nameplate

The most important rule when sizing conductors for an air conditioner is simple:

Always start with the equipment nameplate.

Every listed air-conditioning unit is required to have a visible nameplate that provides the electrical information needed for proper installation. Two values are critical:

  • Minimum Circuit Ampacity (MCA)
  • Maximum Circuit Breaker or Fuse Size

These values are determined by the manufacturer and already account for motor characteristics, starting current, and internal loads. When MCA and maximum breaker values are provided, you do not recalculate them—you verify and apply them.

Where the 125 Percent Rule Fits In

The NEC requires that branch-circuit conductors supplying air-conditioning equipment be sized at 125 percent of the rated load.

In modern equipment, this calculation has already been performed by the manufacturer. The result of that calculation is shown directly on the nameplate as the Minimum Circuit Ampacity (MCA).

In other words:

MCA already includes the 125 percent factor.

That is why you will not see us manually multiplying values by 125 percent when MCA is provided.

Most Air Conditioner Manufacturer include the 125 Percent in their MCA numbers.
Most Air Conditioner Manufacturer include the 125 Percent in their MCA numbers.

Typical Residential Outdoor Condenser Wiring

For a typical residential outdoor HVAC condenser, the most common wiring method is:

  • THHN / THWN-2 copper conductors
  • Installed in PVC or metal conduit
  • Transitioning to liquidtight flexible conduit at the unit

These conductors are rated for wet locations, which is required outdoors. Although THHN/THWN-2 insulation is rated for higher temperatures, conductor ampacity is still governed by termination ratings, not insulation alone.

Why We Intentionally Use the 60°C Column

Even though many conductors have insulation rated for higher temperatures, ampacity must be based on the lowest temperature rating of any termination in the circuit, including:

  • Circuit breaker terminals
  • Disconnect lugs
  • Equipment terminals
  • Wiring method limitations

In many small HVAC branch circuits, these terminations are rated 60 degrees Celsius. Because of that, the safest and most universally applicable approach is to size conductors using the 60°C ampacity values from NEC Table 310.16.

This avoids assumptions, aligns with real-world inspections, and prevents misapplication of higher ampacity columns.

Simplified copper conductor ampacity chart for HVAC air conditioner circuit wiring
Simplified ampacity reference for educational purposes only.
Always verify conductor sizing using the current NEC and applicable termination ratings.

Important Clarification: THHN / THWN and the 60°C Column

A common point of confusion is that THHN and THWN conductors do not appear under the 60°C column headingin NEC Table 310.16.

This does not mean they cannot be sized using 60°C ampacity values.

Manufacturers produce THHN and THWN conductors with 90°C insulation, which is why they appear under higher temperature headings. However, the NEC requires installers to limit conductor ampacity to the lowest-rated termination in the circuit.

That means installers can install 90°C-rated conductors and limit their ampacity to 60°C values when terminations require it—and this practice is completely normal and fully code-compliant.

Electrical wire sizing should consider the temperature rating of the weakest link including Breakers and Lugs.
Electrical wire sizing should consider the temperature rating of the weakest link including Breakers and Lugs.

This practice is standard in the field and expected by inspectors.

When a Higher Temperature Column May Be Used

If—and only if—all terminations in the circuit are clearly identified as rated 75°C, including:

  • Breaker terminals
  • Disconnect lugs
  • Equipment terminals

and the wiring method permits it, the NEC allows conductor sizing using the 75°C column.

This requires positive verification, not assumption.

In this article, we intentionally use the 60°C column because it is the most conservative and defensible approach, and it avoids relying on termination ratings that may not be clearly marked or verifiable in the field.

Worked Example Using the 60°C Column

Let’s walk through a real-world example using an actual air-conditioner nameplate.

Nameplate Information

  • Minimum Circuit Ampacity (MCA): 17.8 amps
  • Maximum Circuit Breaker: 30 amps

Step 1: Select the Conductor

Using NEC Table 310.16 and the 60°C copper column, we find:

  • #14 copper = 15 amps
  • #12 copper = 20 amps

Because the required MCA is 17.8 amps, #14 copper is not sufficient. The next standard conductor size is #12 copper, rated at 20 amps, which exceeds the MCA.

Result:
✔ #12 copper conductor is required.

Step 2: Verify the Circuit Breaker

The nameplate allows a maximum circuit breaker of 30 amps.

Under the special rules of Article 440, the Code permits this breaker size even though electricians typically associate #12 conductors with smaller breakers in general-purpose circuits.

As long as:

  • The conductor meets or exceeds the MCA, and
  • The breaker does not exceed the nameplate maximum

the installation is code-compliant.

Why the Breaker Can Be Larger Than the Wire

This is one of the most misunderstood aspects of air-conditioning circuits.

In these applications, installers do not size the breaker to protect the conductor from overload in the traditional sense. Instead, they size it to

  • Handle motor starting current
  • Provide short-circuit and ground-fault protection

Proper ampacity selection using MCA protects the conductor, while Article 440 permits the breaker to be larger.

The nameplate allows a maximum circuit breaker of 30 amps. Under the special rules of NEC 440.22, this section allows the breaker size to accommodate motor starting current, even though electricians typically associate #12 conductors with smaller breakers in general-purpose circuits. As required by NEC 440.6(A), the breaker does not exceed the manufacturer’s marked maximum.

Additional Considerations

Even after meeting MCA and breaker requirements, other factors may require upsizing conductors:

  • Ambient temperature derating
  • Voltage drop on long conductor runs (typically limited to 3 percent)
  • Bundling or conduit fill adjustments

These adjustments affect conductor size, not breaker size.

Final Compliance Summary for Sizing wire for an air conditioner.

For the example shown:

  • Required ampacity: 17.8 amps MCA
  • Selected conductor: #12 copper, 20 amps at 60°C
  • Circuit breaker: 30 amps, per nameplate

This installation meets NEC requirements for air-conditioning equipment.

Key Takeaways

  • Air conditioners follow NEC Article 440, not general wiring rules
  • Always start with the equipment nameplate
  • MCA already includes the 125 percent requirement
  • Termination temperature ratings limit conductor ampacity.
  • Using the 60°C column is often the safest and most defensible approach
  • Breakers may be larger than expected due to motor starting characteristics

Final Reminder for Sizing wire for an air conditioner.

Sizing wire for an air conditioner is not about memorizing formulas. It is about understanding which NEC rules apply, verifying nameplate information, and selecting conductors conservatively and correctly.

When in doubt, verify termination ratings, consult the current NEC, and involve qualified professionals.

Frequently Asked Questions

The following questions address common points of confusion about sizing electrical conductors for air-conditioning equipment. These answers clarify NEC requirements, nameplate values, ampacity tables, and real-world installation considerations to help you understand how HVAC circuits are evaluated and why the Code permits certain conductor and breaker combinations.

Common Questions About HVAC Wire Sizing

1) What size wire do I need for an air conditioner?

The unit’s nameplate Minimum Circuit Ampacity (MCA) determines the wire size. Choose a conductor with an ampacity equal to or greater than the MCA, using the correct NEC ampacity column based on termination temperature ratings.

2) What does MCA mean on an air conditioner nameplate?

MCA stands for Minimum Circuit Ampacity. The manufacturer provides MCA — the minimum conductor ampacity required for that unit’s branch circuit — in accordance with NEC air-conditioning rules.

3) Do I need to multiply by 125% when sizing wire for an air conditioner?

Usually, no. If the nameplate lists MCA, the manufacturer has already applied the 125% requirement. You size the conductor to meet or exceed the MCA.

4) Why can the breaker be larger than the wire for an air conditioner?

Air-conditioning circuits follow NEC Article 440, which allows installers to size the overcurrent device to handle motor starting current and provide short-circuit/ground-fault protection. As long as the conductor meets the MCA and the breaker does not exceed the nameplate maximum, it can be code-compliant.

5) Why does NEC Table 310.16 show #14 copper as 20 amps at 75°C?

#14 copper has an ampacity of 20 amps in the 75°C column, but you can only use that column if all terminations in the circuit are clearly rated 75°C and the wiring method allows it. Otherwise, you must use lower ampacity values.

6) If my terminals are rated 75°C, can I use the 75°C column to size the wire?

Yes—only if breaker terminals, disconnect lugs, and equipment terminals are all identified as 75°C rated, and the wiring method permits it. This requires positive verification, not assumption.

7) THHN/THWN doesn’t appear in the 60°C column—can I still use the 60°C ampacity values?

Yes. THHN/THWN-2 conductors typically have 90°C insulation ratings, but the lowest-rated termination in the circuit limits their ampacity. It is normal and code-compliant to use 60°C ampacity values when terminations require it.

8) What is the most common wire type used for a residential outdoor HVAC condenser?

Most residential outdoor condensers use THHN/THWN-2 copper conductors in conduit, transitioning to a liquidtight flexible whip near the equipment (depending on local practice and installation requirements).

9) Do I need to consider voltage drop when sizing HVAC wire?

Yes. For long runs, voltage drop may require installers to upsize conductors even if the MCA is met. A common design target is keeping branch-circuit voltage drop to around 3%.

10) Does ambient temperature affect the wire size for an air conditioner?

Yes. Higher ambient temperatures can reduce allowable ampacity and may require conductor upsizing. Always apply NEC adjustment factors when conditions warrant.

11) Should I rely on the nameplate or the breaker size to choose wire?

Use the nameplate MCA to choose the wire size and the nameplate maximum breaker to choose the overcurrent device. The breaker size alone is not a reliable way to select wire for HVAC equipment.

12) Is it safe for homeowners to work on air conditioner electrical wiring?

Electrical work can be hazardous. Qualified or licensed professionals should perform or review it in accordance with applicable codes and regulations.

ENERGY STAR Building Rating vs. Energy Use Intensity (EUI)

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ENERGY STAR Building Rating vs. Energy Use Intensity (EUI)

How do engineers, facility managers, and property owners fairly compare energy use between different buildings? The answer lies in ENERGY STAR’s Portfolio Manager, a free benchmarking tool developed by the U.S. Environmental Protection Agency (EPA). While most people recognize the ENERGY STAR label from household appliances—such as refrigerators, washers, dryers, water heaters, and air conditioners—many are unaware that ENERGY STAR also evaluates and rates entire buildings using standardized performance metrics. Two of the most important of these metrics are Energy Use Intensity (EUI) and the ENERGY STAR 1–100 building score.

Portfolio Manager provides a consistent, nationwide framework for measuring, comparing, and tracking building energy performance over time, allowing similar buildings to be evaluated on an equal basis.

Energy Use Intensity (EUI)

One of the primary metrics used to compare buildings is Energy Use Intensity (EUI). EUI is calculated by dividing a building’s total annual energy consumption by its gross floor area:

EUI = Total Annual Energy Use ÷ Building Square Footage

The result is expressed in kBtu per square foot per year, and it allows for quick, normalized comparisons between buildings of different sizes. A lower EUI generally indicates a more energy-efficient building.

EUI Comparison Example

Consider an example where two buildings—an office building and a hospital—each consume 8 million kBtu per year:

  • The office building is 50,000 square feet, resulting in an EUI of 160
  • The hospital is 17,000 square feet, resulting in an EUI of approximately 470

Although both buildings consume the same total amount of energy annually, the hospital uses nearly three times more energy per square foot. EUI reveals this difference clearly, whereas total energy use alone does not.

Table 1 — Energy Use Intensity (EUI) Comparison Example

Purpose: Visually demonstrates why EUI matters more than total energy use alone.

Building TypeAnnual Energy Use (kBtu)Building Size (ft²)EUI (kBtu/ft²·yr)
Office Building8,000,00050,000160
Hospital8,000,00017,000470

Key Insight:
Even with identical annual energy consumption, the hospital uses nearly 3× more energy per square foot, making it significantly more energy intensive.

The Golf Score Analogy

EUI can be compared to a golf score. In golf, the objective is to achieve the lowest score possible. Similarly, with Energy Use Intensity, a lower EUI represents better energy performance. Just as golf scores allow players to compare performance regardless of course length, EUI allows buildings to be compared fairly regardless of size.

Site Energy vs. Source Energy (Why the Difference Matters)

When benchmarking buildings, it is important to distinguish between site energy and source energy, as the two represent very different views of energy use.

Site energy is the amount of energy consumed directly at the building and is what appears on utility bills. It includes electricity used at the meter, natural gas burned on site, fuel oil, and district energy delivered to the building. While site energy is useful for tracking operating costs, it does not account for how that energy was produced or delivered.

Source energy, on the other hand, represents the total amount of raw fuel required to operate the building. It includes the energy consumed at the building plus all upstream losses associated with electricity generation, fuel processing, and transmission and distribution. Because electricity requires significant energy losses before it reaches a building, source energy provides a more complete and equitable picture of total energy impact.

ENERGY STAR uses source energy and source EUI for national benchmarking because it places all energy sources—electricity, natural gas, district steam, and others—on a common basis. This prevents buildings from appearing artificially efficient or inefficient simply due to regional utility infrastructure or fuel mix differences.

Table 2 — Site Energy vs. Source Energy Comparison

Purpose: Clarifies why ENERGY STAR prefers Source Energy for benchmarking.

CategorySite EnergySource Energy
Measured AtBuilding utility meterPower plant + transmission + building
Includes Generation Losses❌ No✅ Yes
Includes Transmission Losses❌ No✅ Yes
Fuel ComparabilityLimitedFully normalized
Used for ENERGY STAR Score❌ No✅ Yes
Best ForUtility cost trackingNational benchmarking

Key Takeaway:
Site energy tells you what you paid for.
Source energy tells you what it really took to produce it.

Benchmarking Similar Building Types

Meaningful benchmarking requires comparing similar building types. Just as it would be inappropriate to compare a professional golfer to a 10-year-old, it would be misleading to compare an office building to a hospital or data center. Instead, ENERGY STAR Portfolio Manager compares buildings within the same category—offices to offices, hospitals to hospitals—using a large national database.

Engineers assist property owners by benchmarking buildings against:

  • Other buildings within their portfolio
  • The national median for similar building types
  • Peer buildings nationwide within Portfolio Manager

This process helps identify underperforming buildings and target energy efficiency improvements where they will have the greatest impact.

Table 3 — Typical Median Source EUI Benchmarks (U.S.)

Purpose: Provides context for what “good” or “poor” performance looks like nationally.

Building TypeMedian Source EUI (kBtu/ft²·yr)
Warehouse (Non-Refrigerated)~80
Multifamily Housing~124
K–12 School~153
Office (All Sizes)~167
Retail (Non-Refrigerated)~160
Hotel~208
Medical Office~237
Hospital~426
Grocery Store / Supermarket~500–600
Data Center600+

Interpretation Guide:

  • Below median → Better than at least 50% of similar buildings
  • Above median → Opportunity for efficiency improvements

ENERGY STAR 1–100 Building Score

For eligible commercial and institutional buildings—including offices, schools, hospitals, retail stores, and multifamily housing—ENERGY STAR provides a 1–100 ENERGY STAR score through Portfolio Manager.

Key points:

  • A score of 50 represents median (average) performance
  • A score of 75 or higher indicates top-tier performance, better than at least 75% of similar buildings nationwide
  • Buildings scoring 75+ may qualify for ENERGY STAR certification

Certification is awarded annually, requires verification by a licensed professional (such as an engineer or architect), and formally recognizes superior energy efficiency. Advanced programs, such as ENERGY STAR NextGen, further recognize buildings with exceptionally low emissions.

Thousands of buildings across the United States have earned ENERGY STAR certification, and owners can benchmark their own buildings or search certified properties at energystar.gov/buildings.

ENERGY STAR Score vs. Energy Use Intensity (EUI)

The ENERGY STAR score and EUI are closely related metrics within Portfolio Manager, but they serve different purposes.

Key Similarities

  • Both normalize energy use by building size (kBtu/ft²·yr)
  • Both rely on national survey data, primarily the Commercial Buildings Energy Consumption Survey (CBECS)
  • The ENERGY STAR score is derived from source EUI, which accounts for upstream energy losses in fuel production and delivery

Differences and Relationship

  • EUI (site or source) is an absolute metric: total annual energy use divided by floor area. While lower EUI typically indicates better performance, raw EUI alone does not adjust for differences in climate, occupancy, or operating hours.
  • The ENERGY STAR score compares a building’s actual source EUI to a predicted source EUI, calculated using regression models that account for weather, occupancy, operating hours, and building characteristics.
    • A ratio of actual-to-predicted energy use near 1.0 results in a score of approximately 50
    • Better-than-predicted performance yields scores above 50
    • Worse-than-predicted performance yields scores below 50

In short, EUI is the foundational intensity metric, while the ENERGY STAR score is a normalized, percentile-based benchmark built on EUI for fair peer comparisons. For building types that are not eligible for a 1–100 score, Portfolio Manager typically defaults to displaying EUI as the primary performance indicator.

Table 4 — EUI vs. ENERGY STAR Score

Purpose: Explains the relationship and differences between the two metrics.

FeatureEUIENERGY STAR Score
Metric TypeAbsoluteRelative (Percentile)
UnitskBtu/ft²·yr1–100
Normalized for Size✅ Yes✅ Yes
Normalized for WeatherOptional✅ Yes
Adjusted for Operations❌ No✅ Yes
Peer ComparisonManualAutomatic
Certification Eligible❌ No✅ Yes (≥75)

ENERGY STAR vs. LEED

ENERGY STAR for buildings focuses primarily on operational energy efficiency, providing performance-based benchmarking and certification. Portfolio Manager also supports tracking of water use, waste, materials, and greenhouse gas emissions.

In contrast, LEED is a broader green building rating system that evaluates energy performance alongside water efficiency, materials, indoor environmental quality, site impacts, and innovation through a point-based framework.

The two systems are highly complementary. Strong ENERGY STAR performance often contributes directly toward LEED credits, particularly in energy and performance-related categories.

Comparing Current Performance to Historical Energy Use

Engineers also use historical energy use data to analyze trends over time. By comparing year-over-year energy consumption and EUI values, they can identify patterns such as seasonal peaks, long-term increases, or improvements resulting from equipment upgrades, operational changes, or retrofits.

This historical analysis helps distinguish between weather-driven variability and true efficiency gains, supports capital planning decisions, and provides measurable evidence of performance improvements. When combined with benchmarking against national medians and peer buildings, historical energy analysis becomes a powerful tool for managing energy costs, reducing emissions, and improving overall building performance.

How to Size Tankless Water Heaters

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How to Size Tankless Water Heaters

How to size tankless water heaters correctly is all about math and realistic assumptions, not guesswork. If you undersize the unit, showers go lukewarm or the flow drops to a trickle when multiple fixtures run. Oversize it, and you may pay more for equipment and infrastructure than you need.

This guide walks through the process step by step so you can confidently determine what size tankless unit you actually need (and then confirm it with manufacturer tools and a professional).

Understand How Tankless Sizing Really Works

A tankless water heater is sized by how much hot water it can produce at a given temperature rise, not by storage gallons.

Three key concepts:

  1. Flow rate (GPM – gallons per minute)
    How many gallons of hot water your fixtures demand at the same time.
  2. Temperature rise (ΔT)
    How many degrees you need to heat the incoming (groundwater) water to reach your desired hot water temperature.ΔT=Tout−TinΔT=Tout​−Tin​
  3. Heater capacity (BTU/h or kW)
    The gas input (for gas units) or electrical power (for electric units) required to deliver that flow and temperature rise.

Manufacturers publish charts or calculators that tell you, for example, “this model can deliver 5.0 GPM at a 70°F temperature rise.”Rheem+1

Your job is to calculate:

Required GPM at peak use + Required temperature rise → Select a model that meets or exceeds both.

Step 1 – Define the Application

Before any math, be clear about what you’re sizing for:

  • Whole-house tankless (most common)
    • All showers, sinks, laundry, etc.
  • Point-of-use (single fixture or small group)
    • Example: one bathroom group, or a remote studio sink/shower.
  • Residential vs. small commercial
    • Commercial may have different diversity and fixture types.

Also consider:

  • Number of bathrooms
  • Typical household size (people)
  • Any high-demand fixtures: large soaking tub, body spray shower, commercial dishwasher, etc.

This will drive what “peak simultaneous use” actually looks like.

Step 2 – List Your Hot-Water Fixtures and Their Flow Rates

Next, list all fixtures that use hot water, plus their typical flow rates. You can find exact flow on product spec sheets or measure with a bucket, but here are common ranges from tankless sizing resources:PlumbingSupply.com+1

Typical residential flow rates

  • Standard shower: 1.5–2.5 GPM
  • Water-saving shower: 1.2–1.8 GPM
  • Bathroom sink faucet: 0.5–1.5 GPM
  • Kitchen faucet: 1.5–2.2 GPM
  • Clothes washer: 1.5–3.0 GPM
  • Dishwasher: 1.0–2.0 GPM
  • Large tub filler: 4.0–6.0+ GPM
  • Body spray/“carwash” shower systems: often 4.0–8.0+ GPM total

Create a simple table like this for your project:

FixtureFlow (GPM)Notes
Shower #12.0Primary bathroom
Shower #22.0Hall bath
Bathroom sink (same bathroom)0.7Brushed nickel low-flow faucet
Kitchen sink2.0High-arc faucet
Dishwasher1.5Spec sheet list
Clothes washer2.0Top-load

Step 3 – Decide What Really Runs at the Same Time

You don’t size for every fixture running at once. You size for a realistic peak scenario.

Examples of reasonable peak scenarios:

  • Family home (3–4 people)
    • Two showers running + one bathroom sink intermittently
  • Small home / couple
    • One shower + kitchen sink or dishwasher
  • Home with large soaking tub
    • Either tub filling or one shower + one sink (not both scenarios at once unless that’s realistic for that client)

For each project, write down one or two “design scenarios” you actually want the system to support.

Example design scenario

Two showers running at the same time, plus a bathroom sink occasionally.

From the table above:

  • Shower #1: 2.0 GPM
  • Shower #2: 2.0 GPM
  • Bathroom sink: 0.7 GPM

Total design flow = 2.0 + 2.0 + 0.7 = 4.7 GPM
(Round to 4.5–5.0 GPM for simplicity.)

Step 4 – Determine Incoming Water Temperature and Temperature Rise

Now estimate your incoming (groundwater) temperature. This varies by climate. Many sizing tools and guides use groundwater temperature maps and typical desired hot water temperature around 120°F.energy.gov+1

You can:

  • Look up your region on a groundwater temperature map from a sizing tool.
  • Use local data (well temp, etc.) if you have it.
  • Use conservative values for winter conditions (coldest likely incoming temp).

Typical incoming temps:

  • Cold northern climates: 35–50°F
  • Mixed / temperate: 50–60°F
  • Warm southern climates: 60–75°F

Most households are happy with 115–120°F at fixtures, even if the water heater is set slightly higher.

Example

  • Incoming water: 50°F (cool climate)
  • Desired outlet temp: 120°F

ΔT=120°F−50°F=70°FΔT=120°F−50°F=70°F

So this home needs about a 70°F temperature rise at their design flow rate.

Step 5 – Calculate Required Heating Capacity (BTU/h)

With flow (GPM) and temperature rise (ΔT), you can estimate the BTU/h required:BTU/h≈500×GPM×ΔTBTU/h≈500×GPM×ΔT

The factor 500 comes from water’s weight and the minutes-to-hours conversion.

Using the example:

  • Flow = 4.7 GPM (round to 4.5)
  • ΔT = 70°F

BTU/h≈500×4.5×70=157,500 BTU/h (approx.)BTU/h≈500×4.5×70=157,500 BTU/h (approx.)

Most residential gas tankless units are in the 150,000–199,000 BTU/h range, so in this example you’d likely be looking at the larger end of that range to comfortably meet 4.5–5.0 GPM at a 70°F rise.

Rule of thumb: For typical homes in cool climates, whole-house gas tankless systems often land in the 180k–199k BTU/h range. Electric whole-house tankless units may require very large electrical service to match this performance.

Step 6 – Match Your Numbers to Manufacturer Sizing Charts

Now that you know GPM and ΔT, you compare that to manufacturer data.

Manufacturers provide:

Look for a table or chart that says something like:

  • 70°F rise → 4.5 GPM
  • 60°F rise → 5.5 GPM
  • 50°F rise → 7.0 GPM

Then compare your requirement:

Need ≥ 4.5 GPM at 70°F rise

Pick a model that meets or exceeds this capacity.

If a model only provides 3.5 GPM at a 70°F rise, it’s too small for our example home. Either:

  • Move up to a larger model, or
  • Decide that your realistic design scenario is less demanding (e.g., only one shower + a small sink at once).

Step 7 – Check Fuel Type and Site Constraints

Even if the math says a certain size will work, your building infrastructure has to support it.

Gas tankless units

Check:

  • Gas line size and pressure – High-input gas models (e.g., 180k–199k BTU/h) often require upgraded gas piping and proper inlet pressure.
  • Venting – Condensing models may allow PVC/CPVC/polypropylene venting and have higher efficiency; non-condensing may need metal venting.Navien+1
  • Combustion air – Especially for indoor installations.
  • Condensate drain – Required for condensing units; you need a place to route neutralized condensate.

Electric tankless units

Check:

  • Available service amperage and voltage – Whole-house electric tankless units may require multiple 40–60A double-pole breakers and heavy conductors.Stiebel Eltron USA+2Stiebel Eltron USA+2
  • Panel capacity – Many homes don’t have enough spare capacity without a service upgrade.
  • Local code requirements for electric water heaters.

If the site can’t support a large enough unit, you may need:

  • Multiple smaller point-of-use units, or
  • A high-efficiency tank or heat pump water heater instead of whole-house tankless.

Step 8 – Decide Between One Large Unit or Multiple Units

If one heater can’t cover everything (or would be very expensive to support with gas/electric infrastructure), consider multiple units:

  • Two smaller tankless units in parallel (“ganged”) to share load. Many manufacturers specifically allow this, and some calculators will flag when ganging is recommended.PlumbingSupply.com+1
  • Separate zones:
    • One unit for “main house” (showers, laundry)
    • One smaller unit for “kitchen/guest wing” or remote structures

This can also offer redundancy and shorter hot-water runs (less wait time, less wasted water).

Step 9 – Adjust for Special Fixtures and Recirculation

High-demand fixtures

If the home includes:

  • Large soaking tub,
  • Multi-head shower or body sprays,
  • Commercial-style kitchen equipment, or
  • Multiple simultaneous uses in a big family,

You must account for those extra GPM in your design scenario, or accept that they can’t all run at once.

Often this pushes designers toward:

  • Two tankless units, or
  • Hybrid solutions (tankless feeding a small buffer tank, etc., in some designs).

Recirculation systems

Tankless units with recirculation:

  • Don’t change the peak GPM the heater must produce,
  • But they do impact control strategy, pump sizing, and standby losses.

Many manufacturers offer built-in recirc pumps or recirc-ready models, with specific piping diagrams and control options that should be followed.Navien+1

Step 10 – Confirm Everything with Manufacturer Tools and a Pro

Once you’ve worked through the sizing yourself, sanity-check your decision using:

Important fine print: many of these tools explicitly state that they’re guides only, and that the contractor or engineer is responsible for the final selection and ensuring code compliance.naviensizing.com+2Gateway ACPro+2

Work with a qualified installer or engineer to:

  • Verify your load calculations
  • Confirm gas/electric capacity and venting
  • Ensure code compliance and proper safety/clearance provisions

Common Sizing Mistakes to Avoid

1. Using number of bathrooms only
Bathroom count is a rough screening tool, but real sizing needs actual fixture flow and realistic concurrency.

2. Ignoring cold-climate conditions
If you size using average or summer groundwater temperatures, you may be under-sized in winter.

3. Not checking gas or electric infrastructure
Discovering after purchase that your gas line or electrical panel is too small is an expensive surprise.

4. Forgetting about special fixtures
Body sprays, big tubs, and commercial appliances are GPM hogs—don’t ignore them.

5. Assuming any “199k BTU” unit is automatically enough
Different models can have different actual delivered GPM at your ΔT. Always check the flow vs. temperature rise table.

Quick Sizing Checklist

Use this as your “don’t guess” checklist:

  1. List all hot-water fixtures and their flow rates (use spec sheets or typical values).
  2. Define a realistic peak use scenario (what’s actually on at once).
  3. Add up GPM for that scenario.
  4. Determine incoming groundwater temperature and desired outlet temperature; compute ΔT.
  5. Use BTU/h ≈ 500 × GPM × ΔT to estimate needed capacity.
  6. Compare your GPM & ΔT to manufacturer flow-vs-rise charts or calculators.
  7. Verify gas line, venting, and electrical capacity can support the chosen unit.
  8. Decide whether you need one whole-house unit or multiple / point-of-use units.
  9. Factor in high-demand fixtures and recirculation if present.
  10. Confirm selection with manufacturer tools and a qualified installer.

Follow that process and you’ll be sizing tankless systems deliberately, not guessing.

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