AHRI 1060: calculate ERV outdoor air with EATR and OACF

How do you calculate net outdoor air through an ERV?

AHRI 1060 rates exchangers; the project ventilation calculation establishes the required outdoor air. Identify the delivery boundary and whether that requirement is actual or standard cfm. Convert an actual-volume requirement using dry-air density at its specified boundary before comparing it with AHRI standard airflow. S1 S2

The correction ends at station 2, the exchanger outlet toward the building. Account separately for downstream duct leakage, recirculation and zone distribution. Exhaust re-entering the outdoor intake is another path that the exchanger rating does not measure. S4

Establish the design target with the outdoor-air ventilation-rate guide before applying corrections. Use the ERV commissioning guide for the broader startup sequence, recovery operation and functional checks. Keep the net-air calculation attached to that commissioning record.

Which four air streams belong in the mass balance?

Number the ports by physical direction: station 1 enters from outdoors, station 2 leaves toward the building, station 3 enters from the building, and station 4 leaves outdoors. AHRI's station convention distinguishes entering supply from leaving supply and entering exhaust from leaving exhaust; map the manufacturer's OA, SA, RA and EA labels onto those directions. S4 S12

The two paths are outdoors → 1 → exchanger → 2 → building, and building → 3 → exchanger → 4 → outdoors. Inside that boundary, supply → exhaust and exhaust → supply transfers can both occur. The two transfer quantities need not be equal.

For a steady-state boundary without external casing leakage, entering dry-air mass equals leaving dry-air mass. On one standard dry-air flow basis, the sum of stations 1 and 3 equals the sum of stations 2 and 4. Moisture transferred through the device or removed as condensate is accounted for separately from dry-air mass.

Balanced room-side flow means stations 2 and 3 match, not that every port equals net ventilation. The net row below is part of station 2, not a fifth port.

Record actual or standard volume, temperature, humidity and barometric pressure. Use the mass of dry air per cubic foot of moist air for normalization, rather than total moist-air density. Adding actual cfm at different densities does not produce a mass balance. The duct static-pressure guide supports the accompanying pressure survey.

Method note: AHRI 1060-2023 §4.9 uses a standard dry-air density of 0.075 lb dry air/ft³. At an Anvilfield-selected local dry-air density of 0.072 lb dry air/ft³, Anvilfield estimates about 979 actual cfm for the worked intake of 939 scfm, using the unrounded intake result. Temperature, moisture and barometric pressure determine local density; altitude alone does not establish it. S12

What does EATR measure?

AHRI 1060-2023 defines exhaust air transfer ratio (EATR) as the exhaust-origin fraction of leaving supply, so 1,000 cfm of gross supply at 5% EATR delivers 950 cfm of net outdoor air. The transferred-flow denominator is leaving supply, not entering exhaust. S2 S12

Rotary carryover transports air held in wheel passages as the wheel rotates between streams. Pressure-driven leakage moves air through available gaps and seals. ASHRAE's 2024 Handbook and Airxchange's application guidance distinguish these mechanisms: purge affects carryover, while fan position and pressure relationships also affect transfer. S4 S5

Sensible and latent effectiveness describe energy transfer, while EATR describes air origin. A supply-temperature rise alone cannot establish the exhaust-origin fraction; use effectiveness for the energy calculation and EATR for the ventilation correction.

In the concentration relationship below, C1 is the outdoor background, C2 is the well-mixed leaving-supply concentration and C3 is the entering-exhaust reference. All concentrations use compatible units. A controlled test needs enough contrast between exhaust reference and outdoor background to resolve the transferred fraction.

Ordinary building CO2 readings do not isolate core leakage when occupancy, other recirculation or concentration differences vary. Likewise, an inert-tracer result does not establish a universal odor or contaminant-transfer percentage: sorption and media behavior can differ by substance. A published zero EATR remains a rating at stated conditions, subject to its test allowance and reporting precision. S4

AHRI 1060-2023 §4.4 specifies an SF6 tracer procedure at rated airflow and pressure differential, laboratory ambient conditions and 20% to 60% relative humidity. A field tracer investigation therefore needs a controlled procedure and sampling plan, beyond routine temperature readings or a flow-hood survey. S12

What does OACF change?

AHRI defines OACF as entering supply divided by gross leaving supply on a compatible airflow basis. It is dimensionless: 1.05 means the outdoor intake is 105% of leaving supply. It does not mean that 5% of leaving supply is exhaust-origin air; that is the separate EATR question. S2 S4

OACF reports the net difference between the supply inlet and outlet. Equal exchanges in opposite directions can leave those flows equal even while exhaust-origin air reaches the building. Consequently, an OACF of 1.00 does not establish zero EATR.

Purge air belongs in the outdoor-intake and exhaust-discharge accounting. When the selected OACF already includes purge, do not subtract that purge quantity again. Record the configuration used to produce the factor so a later wheel or purge adjustment does not leave an obsolete correction in the controls schedule. S5

Which correction equation should I use?

Using AHRI 1060-2023's airflow definitions, required outdoor intake equals required net ventilation multiplied by OACF and divided by one minus decimal EATR. Work with a positive OACF and an EATR fraction below one; at 100% EATR, leaving supply contains no net outdoor air. S2 S4

Use OACF only when crossing between stations 1 and 2. If intake is the measured quantity, first divide by OACF, then apply the outdoor-origin fraction. When selection software already reports net supply, confirm its boundary and avoid applying EATR a second time.

In the method table, e means EATR divided by 100 when entered as a percentage, f means OACF, V is required net outdoor air and Qn is standard dry-air flow at station n. Qact is actual volume, ρda is local dry-air density and ρstd is standard dry-air density. Retain unrounded intermediate values and report final flows to whole cfm.

Check that the selected pair can coexist. For a closed four-port boundary, intake must at least equal the outdoor-origin portion of leaving supply: f ≥ 1 - e. This is Anvilfield's conservation deduction, not an AHRI certification tolerance.

At OACF 0.98, EATR must be at least 2% within that boundary. A zero-EATR pairing signals unmatched ratings, inconsistent density bases, measurement error or an uncounted casing-air path. Resolve the discrepancy before accepting the calculation.

How do EATR and OACF affect fan sizing?

ASHRAE's 2024 Handbook explains that fan capacity follows the fan's location: an upstream supply fan moves entering outdoor air, while a downstream supply fan moves gross leaving supply. Select the exhaust fan by the same physical reasoning. The table shows alternative locations, not a requirement to install four fans. S4

Trace the resistance along each fan's actual path: hood, filter, core, coil, damper and duct. A complete-unit fan table can already incorporate internal resistance; adding those same losses again distorts the selection. Read the manufacturer's available-static definition before adding external duct losses. S8 S9

Keep core pressure loss separate from the cross-stream static pressure relationship that influences transfer. AHRI 1060-2023 defines that relationship as leaving-supply static pressure minus entering-exhaust static pressure. Write the tap locations beside the sign; a positive value without named taps is ambiguous. S12

Airxchange's undated application guidance describes through-matrix purge achieving EATR of 1% or less with favorable supply-to-exhaust pressure relationships on both sides. Its particular example requires an exhaust fan sized at 114% of normal flow. Use that as evidence that purge changes fan duty, not as a universal wheel allowance. S5

Higher corrected flow requires reselecting the exchanger and fans together. EATR, OACF, pressure drop and effectiveness belong to the revised operating point. Convert the standard duties below to actual volume and density at each fan, then use the density-corrected fan curve; the fan-laws guide explains the associated speed and power changes.

Which rating conditions must match the field?

Match model, software version, both airflows, entering conditions, cross-stream pressure, wheel speed and purge. AHRI's certification webpage still lists 1060-2018 as its related I-P standard. Record the certificate's actual edition alongside the application selection; the existence of 1060-2023 does not relabel an older certificate. S1 S3

AHRI 1060-2023's standard-rating envelope uses a supply-flow ratio of 0.80 to 1.25 and a cross-stream differential of -2.50 to +2.50 in. w.g. Its inlet bounds include 35°F to 120°F dry bulb, wet bulb no higher than 80°F, humidity ratio at least 10 grains/lb dry air and relative humidity no higher than 95%. These conditions jointly bound the rating, rather than providing independent operating approvals. S12

AHRI 1060-2018 Table 1 instead spans supply-flow ratios of 0.50 to 2.00 and differentials of -5.00 to +5.00 in. w.g. Preserve the edition on the selection and certificate. A legacy operating point outside the 2023 envelope does not establish a 2023 standard rating; supported application ratings also remain subject to the device's operating limits. S14

The ASHRAE worked example supplies useful correction inputs, but its outdoor wet bulb is 81°F, above the 2023 standard envelope's 80°F ceiling. Keep that psychrometric case identified as the Handbook example. For operating-mode checks, use the economizer and demand-control ventilation guide to identify the minimum-ventilation and bypass commands that need separate records. S4

AHRI 1060-2023 §5.1.4 permits an EATR verification-test result up to one absolute percentage point above its published rating. That certification allowance is separate from instrument uncertainty, the project's transfer limit and the required net ventilation. Include those distinctions when setting the field acceptance margin. S12

Worked example: deliver 850 cfm of net outdoor air

For an 850 scfm net target with the 2024 ASHRAE Handbook's 5% EATR and 1.05 OACF inputs, Anvilfield estimates 895 scfm gross supply and 939 scfm intake. Setting entering building exhaust equal to gross supply gives 895 scfm at station 3 and 939 scfm at station 4. S4

Setting gross leaving supply to the uncorrected 850 cfm target delivers only 808 cfm net, rounded. That is a shortfall of about 43 cfm, or 5% of the requirement. Reset the gross-supply target and check the corresponding intake and exhaust duties, rather than recording the gross reading as proof of ventilation.

Follow the air origin through the device. Anvilfield estimates 45 cfm of exhaust-origin air in leaving supply and 89 cfm of outdoor-origin air sent to the exhaust discharge. Their difference explains the roughly 45 cfm intake-to-gross-supply difference; internal transfers in both directions remain compatible with a closed four-stream balance.

For an unequal-exhaust sensitivity, set entering building exhaust to 800 scfm while holding EATR and OACF at the example values. Anvilfield estimates 845 scfm leaving exhaust and a 95 scfm room-side gross supply-minus-exhaust offset. Obtain revised transfer values at the actual unequal-flow operating point for equipment selection; other exhaust, infiltration and exfiltration also enter the whole-building pressure balance.

Method note: These Anvilfield estimates use an explicitly selected 850 scfm target, adapting ASHRAE's 850 cfm example with its EATR/OACF inputs and AHRI's definitions. The unequal case selects 800 scfm entering exhaust. Calculations assume steady state without external leakage and retain full precision until reporting.

Round required setpoints upward: Anvilfield estimates whole-scfm minima of 895 at station 2 and 940 at station 1, before measurement margin. Reselect the exchanger, coils, fans and motors at the final duties. The DOAS guide covers downstream moisture removal and room-load division.

Whole-cfm values are rounded independently from unrounded calculations; use upward-rounded required setpoints plus the documented acceptance margin.

What leakage is allowed, and why do manufacturers differ?

Keep four decisions separate: typical product leakage, certified performance, the project's specification and the adopted transfer limit. A low EATR does not authorize recovery from every exhaust source. Greenheck's undated FAQ QA10 reports typical wheel EATR of 3% to 5%; its July 2020 discussion distinguishes recovery-device types. S6 S13

Airxchange's 4% to 6% comfort-application discussion and pressure-dependent purge claims describe different configurations from RenewAire's fixed-plate specification. RenewAire's undated HE4XRT guide specification states OACF no greater than 1.02 and zero EATR when its stated static differential is zero or less. Obtain the named taps and sign convention for that selection; the specification sentence alone does not establish equivalence to AHRI's pressure convention. S5 S7

California's 2025 Energy Code provides a jurisdiction-specific example. The exception to §120.1(g)2E permits recovery-device transfer of Class 2 air into Class 1 spaces up to 10% of outdoor intake; the exception to §120.1(g)3B permits Class 3 recovery-device transfer to other spaces up to 5% of outdoor intake. Section 120.1(g)4 prohibits Class 4 recirculation or transfer. These are recovery exceptions to destination restrictions, not general permissions to mix exhaust into supply. S10

The California 2025 edition took effect January 1, 2026; establish which edition governs the permit. Section 120.1(g)6 assigns mixed return or exhaust the highest air class present. Identify classification from the source activity and applicable table, rather than assuming that dilution or a manufacturer's restroom example makes the stream acceptable. S10 S15

The California comparison uses outdoor intake, whereas AHRI EATR uses leaving supply. For the worked case, Anvilfield estimates transferred exhaust air at 4.76% of intake by dividing 0.05 EATR by 1.05 OACF. Record the transfer quantity and comparison denominator separately, then apply the project's exhaust classification and adopted requirements.

Permitted transfer cannot replace required outdoor ventilation. ASHRAE Interpretation IC 62.1-2016-3, approved January 21, 2018, rejects substituting 100 cfm of allowed exhaust leakage for part of a 1,000 cfm outdoor-air requirement. ASHRAE lists 62.1-2025 and its published errata; this interpretation addresses 2016, while the project's adopted provisions establish its requirement. S16 S11

How do you verify net outdoor air in the field?

TAB measures flow and pressure; a matched selection supplies rated EATR and OACF; a controlled tracer procedure measures transfer. Label net flow 'calculated using rated EATR' when that is the evidence. A flow hood does not directly measure the exhaust-origin fraction. Use the TAB report guide for traverse methods and reporting structure. S2 S4

RenewAire's May 2024 HE4XRT manual maps a 0.50 in. w.g. core pressure drop to 3,180 cfm supply but 2,430 cfm return, so equal core pressure drops do not establish balanced airflow. Its §6.4.1 method uses OA-to-SA taps for supply and RA-to-EA taps for return. Use those readings with this model's own pressure-to-flow curves. S9

Confirm the approved unit range as well as the measurement curve. The May 2024 manual lists 1,000 to 4,400 cfm, while the June 2024 HE4XRT submittal lists 1,000 to 4,345 cfm. Use the approved model/configuration selection instead of the largest number shown on an older curve. S8 S9

Record as-found and as-left readings by mode, retaining conditions, ventilation command, density basis, flows, pressures and equipment settings. When a port is inaccessible, identify it as unmeasured. An inferred fourth flow cannot independently prove balance closure; the report must distinguish it from an observation.

Compare measured balance closure with combined instrument uncertainty. A closed balance can coexist with internal exchange; a residual alone does not identify a failed seal.

Anvilfield's sensitivity calculation lowers nominal gross flow by 5% and increases EATR by one percentage point, from 5% to 6%. The worked target then delivers 799 cfm net, rounded; a nominal gross target of 952 cfm is required under those selected lower-bound inputs. The 5% flow reduction is a selected field sensitivity, and the one-point EATR increase also matches AHRI's certification allowance. Neither establishes a universal field measurement tolerance.

For an intake-based net estimate, use the low intake reading, high EATR and high OACF in the lower-bound calculation. Use documented project uncertainties to set margin, then reselect at the resulting flow and pressure. Acceptance compares net delivery with the required target; a general gross-airflow TAB tolerance does not authorize a ventilation shortfall.

• Record system ID, exchanger model, configuration and the project's required net outdoor-air quantity and measurement boundary.
• Attach the applicable code/standard edition, exhaust classification and approved leakage criterion.
• Attach the manufacturer's selection and certificate reference; identify rated versus application conditions.
• Label stations 1 to 4 and fan positions on the as-built sketch.
• Record instrument identifiers, calibration status, zero checks and usable traverse/measurement locations.
• Confirm installed filters, closed access panels, open intended air paths and normal control mode.
• Record Q1 to Q4 where accessible, units, temperature/humidity/barometric information and density conversion; mark inaccessible ports explicitly.
• Record station static pressures and the sign convention, plus filter/core differential pressures separately.
• Record fan speeds, wheel speed, purge setting, bypass position and any frost command.
• Compare the operating point with the selection; obtain a revised selection when the conditions differ.
• Calculate net outdoor delivery, four-stream balance residual and margin above the design target; keep rated leakage separate from measured quantities.
• Verify minimum-flow, recovery, bypass and frost modes required by the approved sequence, recording results separately.
• Correct identified restrictions, leakage or control faults and repeat affected measurements.
• Save the final worksheet, selection printout, instrument records, remaining deficiencies and accepted as-left settings.

Why is net ventilation short when the fan airflow looks right?

Separate a loss of gross airflow from a rise in transferred-air fraction. Record airflow, named static pressures, filter/core pressure drops and fan or wheel commands together. Supply temperature alone does not distinguish restricted flow, changed leakage and normal energy recovery. S4 S9

When totals disagree, normalize density and inspect the measurement locations before assigning the residual to casing leakage. Then inspect the external boundary and repeat affected readings. An internal transfer redistributes air between streams; it does not create a net dry-air source inside a closed four-port boundary.

A failure confined to frost or bypass operation points to the mode's path and commands. Trend airflow, damper state and wheel or fan operation on the same time base, then retest the approved sequence. An odor complaint also warrants checking outdoor intake re-entrainment and the exhaust source; increasing gross supply alone does not resolve those paths. S4 S10

AHRI's education guidance describes exhaust-only defrost that stops the intake blower. Net outdoor delivery stops during that interval. Document how the approved design maintains ventilation through an alternate air path or satisfies its applicable time-averaging provision; normal-recovery airflow cannot stand in for the defrost interval. S2

How often should you recheck the ERV?

Establish corrected net delivery at commissioning and recheck after changes to fans, filters, seals, purge, core, controls or air paths. This is Anvilfield's condition-triggered verification recommendation. Preserve baseline modes, pressures and readings for comparison.

RenewAire's May 2024 HE4XRT manual calls for belt and filter checks 24 hours after startup, electrical/belt/filter checks after 30 days, at least monthly visual filter inspections where differential sensors are absent, and core cleaning at least annually. These are model-specific maintenance intervals, not a universal AHRI leakage-test calendar. S9

The same manual gives a two-month filter-replacement rule of thumb without pressure-drop checking, replacing dirty filters when found. It specifies vacuum cleaning for the fibrous core and prohibits washing or compressed air. Before internal maintenance, RenewAire requires disconnecting power and locking/tagging the disconnect; more than one disconnect can be present. S9

Put the relevant tasks and repeat-measurement triggers into the HVAC preventive-maintenance program. Define access, controls support, recorded operating modes and exclusions in the HVAC maintenance agreement, so the recurring visit produces comparable evidence rather than a new unlabeled airflow number.

What should a verification visit include and cost?

Anvilfield estimates a four-hour site visit with two technicians, two additional person-hours for selection review and reporting, and $1,350 in direct verification cost for the inputs below. The allowance covers ready access to measurement locations and controls. Missing selection records, inaccessible ports, multiple operating modes and controls coordination change the labor scope.

Include selection review, airflow/pressure checks, required mode tests and an as-found/as-left report. Quote tracer testing, repairs, difficult access and repeat visits separately.

Service managers expanding a ventilation verification route receive a CSV or Excel list of buildings with two or more visible rooftop HVAC units within 48 hours through PorchRocket Ground Control.

Method note: Anvilfield estimates direct cost as ten person-hours at a selected loaded labor rate of $120 per hour plus $150 of instrument allocation, dated November 5, 2026. Eight person-hours cover the site visit and two cover review/reporting. Company overhead, profit, travel beyond the stated visit, repairs and specialized tracer testing are priced separately.

Use the HVAC estimating guide to turn that measured scope into the company's selling price. Retain the accepted net result, evidence basis, final settings and deficiencies for the next visit.

Anvilfield estimating inputs, 2026-11-05. Direct cost excludes the separately scoped services shown above.

Citation and update record

Review date: 2026-11-05. Update log: 2026-11-05: technical and editorial review clarified airflow basis, certificate editions, EATR allowance, setpoint rounding and defrost verification; checked calculations, code provisions and manufacturer instructions.

How to cite this page: Anvilfield, AHRI 1060: calculate ERV outdoor air with EATR and OACF, 2026, https://anvilfield.com/field-guides/hvac/erv-outdoor-air-correction-eatr-oacf/.

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Sources checked

• S1: AHRI, Standards 1060 and 1061 landing page, 2023 editions listed. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S2: AHRI, Air-to-Air Energy Recovery Ventilators education page, undated. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S3: AHRI, ERV certification program, undated live page. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S4: ASHRAE Handbook, HVAC Systems and Equipment, 2024, Chapter 26, Air-to-Air Energy Recovery Equipment. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S5: Airxchange, Application & Design, undated. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S6: Greenheck, Energy Recovery Ventilators FAQ QA10, undated page. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S7: RenewAire, HE4XRT Product Specification Guide, CSI 23 72 00, revision not printed in the retrieved text. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S8: RenewAire, HE4XRT submittal, June 2024. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S9: RenewAire, HE4XRT installation, operation and maintenance manual 134775_009, May 2024. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S10: California Energy Commission, 2025 Building Energy Efficiency Standards, CEC-400-2025-010-F. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S11: ASHRAE, Standards Errata index, including Standard 62.1-2025 errata dated August 27, 2026. Primary publisher page reopened with curl, 2026-11-05; HTTP 200. Establishes edition availability; no 2025 transfer clause is inferred from this index.
• S12: AHRI Standard 1060-2023 (I-P), §§3.2, 4.4, 4.7, 4.9, 5.1.4 and Table 1. Publisher PDF download returned HTTP 403; indexed primary text rechecked for definitions, rating bounds, tracer conditions, net flow, density conversion and EATR allowance, 2026-11-05.
• S13: Greenheck, Apply energy recovery to ventilation systems, July 28, 2020. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S14: AHRI Standard 1060-2018 (I-P), Table 1. Publisher PDF download returned HTTP 403; AHRI-authored 2018 text reopened with curl in the Public.Resource.Org incorporated-standards archive (HTTP 200) and Table 1 checked, 2026-11-05.
• S15: California Energy Commission, 2025 California Energy Code summary, December 2024. Primary source reopened with curl, 2026-11-05; HTTP 200.
• S16: ASHRAE, Interpretation IC 62.1-2016-3, approved January 21, 2018. Primary source reopened with curl, 2026-11-05; HTTP 200.