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ERV and HRV effectiveness calculator
Effectiveness says how much of the most an energy recovery unit could move it actually moved. This page divides the change in the supply air by the outdoor to return difference, for temperature, for humidity ratio and for both together, as AHRI Standard 1060 defines it. 60 °F supply with 32 °F outdoor and 70 °F return is 73.7 percent sensible and moves 3,024 Btu/h at 100 CFM. A rated effectiveness can also be run forward to the supply air and the load. The page does not size the ventilation airflow, which the ASHRAE 62.2 ventilation calculator does. For the load of the outdoor air before recovery, see the outdoor air load calculator, and for humidity ratio and enthalpy, the psychrometric calculator.
- Choose readings if you have measured the outdoor, supply and return air, or a rating if you have a rated effectiveness from the manufacturer. Use a temperature-only case for an HRV and a humidity case for an ERV. The page opens on three winter temperatures.
- Give the outdoor and return air, and the supply air after the exchanger or the rated effectiveness, with the supply and exhaust airflow and the EATR. 32 °F outdoor, 70 °F return and 60 °F supply are the opening case.
- The page gives sensible, latent and total effectiveness, the supply air after recovery, the sensible and latent load moved, and the net outdoor air. 60 °F supply on those readings is 73.7 percent and moves 3,024 Btu/h at 100 CFM.
Example result for the default inputs
The readings are outdoor 32.0 °F, return 70.0 °F, supply 60.0 °F at 100 CFM supply and 100 CFM exhaust.
Sensible effectiveness
73.7 %
Supply air after recovery
60.0 °F
Sensible heat moved
3,024 Btu/h
Net outdoor air
100.0 CFM
| Outdoor air, station 1 | 32.0 °F |
|---|---|
| Supply air after recovery, station 2, as read | 60.0 °F |
| Return air, station 3 | 70.0 °F |
| Exhaust leaving, station 4, sensible estimate | 42.0 °F |
| Season | heating, the outdoor air is 38.0 °F cooler than the return |
| Sensible effectiveness | 73.7% |
| Net outdoor air | 100.0 CFM of 100 CFM supply, at an EATR of 0% |
| Sensible heat moved | 3,024 Btu/h added to the supply air, 1.08 x 100 CFM x 28.0 °F |
| Sensible load left on the coil | 1,080 Btu/h to bring the supply air to the return temperature |
- Effectiveness here is the gross figure of AHRI Standard 1060, the transfer actually made divided by the most the smaller airstream could take, worked from the three states as typed. Fan heat, heat gain through the cabinet and leakage between the airstreams are inside a reading, so a supply temperature taken after the fan reads higher than the core alone would give. The sensible recovery efficiency in HVI listings is worked on a different basis and is not the same number.
- At this 38.0 °F difference, a reading error of 1 °F in the supply temperature moves the sensible effectiveness by 2.6 points. AHRI Table 1 allows ±0.5 °F on the average of test readings, so a handheld reading is a rougher number than a rating.
- The outdoor dry-bulb of 32.0 °F falls outside the AHRI 1060 range of standard rating conditions, which is dry-bulb 35 to 120 °F, wet-bulb up to 80 °F, humidity ratio of 10 gr/lb or more and RH up to 95 percent. A rating published at standard conditions does not cover them.
- EATR is 0 percent here, so all of the supply air is read as outdoor air. Rated units list an EATR, and any value above zero lowers the net outdoor air the unit delivers.
- Airflows are read as standard air of 0.075 lb/ft3 as in AHRI 1060. A temperature-only case does not look at moisture, so the latent load an ERV moves is not part of the result.
- Neither the effectiveness nor the load is a rating. Published ratings are measured in a laboratory under AHRI Standard 1060, and the equipment listing and the manufacturer's selection data decide the unit.
For estimation purposes only. Consult a licensed engineer for final designs.
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How effectiveness is worked out
This page reads effectiveness the way AHRI Standard 1060 defines it, as the transfer an exchanger actually makes divided by the most the smaller airstream could take. It works sensible effectiveness from dry-bulb temperatures, latent effectiveness from humidity ratios and total effectiveness from both, and it runs the definition either way. Three readings give an effectiveness, or a rated effectiveness gives the air leaving the unit. A load follows from the airflow, and the net outdoor air follows from the exhaust air transfer ratio. The method is a public standard definition and not a rating, and the page says where it stops.
- Sensible effectiveness
e_s = (supply CFM / smaller CFM) x (T1 - T2) / (T1 - T3)T1 is the outdoor air, T2 the supply air after the exchanger and T3 the return air, the stations of AHRI 1060 Figure 1. 32 °F outdoor, 70 °F return and 60 °F supply give 73.7 percent at equal airflows. With equal airflows the flow ratio is 1.- Latent effectiveness
e_l = (supply CFM / smaller CFM) x (W1 - W2) / (W1 - W3)Humidity ratios W come from the dry-bulb and relative humidity at sea-level pressure. The smaller airstream sets the limit. With 100 CFM of supply and 80 CFM of exhaust the ratio is multiplied by 1.25, so the temperatures that give 47.4 percent at equal flows give 59.2 percent.- Total effectiveness
e_t = (m2 cp |t1 - t2| + m2 hfg |W1 - W2|) / (mmin cp |t1 - t3| + mmin hfg |W1 - W3|)AHRI 1060 Equation C2, with cp of 0.24 Btu/lb F and hfg of 1,061 Btu/lb for rating. Humidity ratios enter in lb per lb so the units agree. Winter ERV readings in the example table give 73.7 percent sensible, 70.0 percent latent and 72.7 percent total. An enthalpy recovery ratio from Equation C8 is shown beside it.- Air leaving the exchanger
T2 = T1 - (Cmin / C2) x e x (T1 - T3)Equation C9 turns a rated effectiveness into a supply condition, and C10 does the same for the exhaust. A 75 percent core at 95 °F outdoor and 75 °F return leaves the supply at 80.0 °F. The same function runs for humidity ratio, and the page turns the result back into a relative humidity.- Load moved
Q_s = 1.08 x CFM x dT, Q_l = 0.682 x CFM x dW (gr/lb)The 1.08 is 60 minutes x 0.075 lb/ft3 x 0.24, and the 0.682 is 60 x 0.075 x 1,061 / 7,000 grains per lb. 3,024 Btu/h moves at 100 CFM and 28 °F of change for the default readings. A field figure of 4840 per CFM per lb/lb for the latent part is 1.4 percent higher.- Net outdoor air
net CFM = supply CFM x (1 - EATR)AHRI 1060 Equation 1. A 5 percent EATR on 100 CFM leaves 95.0 CFM of outdoor air, and the net sensible effectiveness for the default readings falls from 73.7 to 72.3 percent. Leaked exhaust warms the winter supply, so a gross reading overstates the core.
Worked examples
The default case is 32 °F outdoor air, 70 °F return air and 60 °F supply air at 100 CFM each way. Supply air moved 28 °F of the 38 °F available, so sensible effectiveness is 73.7 percent, 3,024 Btu/h is moved, and 1,080 Btu/h is left for the coil. Exhaust leaves at about 42.0 °F. A reading error of 1 °F in the supply temperature moves the result by 2.6 points. Steady readings matter.
Supply temperature and sensible load per 100 CFM at 32 °F outdoor and 70 °F return, for a range of rated sensible effectiveness. Read across.
| Rated sensible effectiveness | Supply air, °F | Sensible load moved, Btu/h |
|---|---|---|
| 50% | 51.0 | 2,052 |
| 60% | 54.8 | 2,462 |
| 70% | 58.6 | 2,873 |
| 75% | 60.5 | 3,078 |
| 80% | 62.4 | 3,283 |
| 90% | 66.2 | 3,694 |
Eight cases below run through the same functions the form uses. Nothing is hidden. Unequal airflows change the answer, so the same temperatures read 47.4 percent at equal flows and 59.2 percent with 100 CFM of supply against 80 CFM of exhaust. A 5 percent EATR on 100 CFM leaves 95.0 CFM of outdoor air. The winter ERV case with humidity reads 73.7 percent sensible, 70.0 percent latent and 72.7 percent total. Latent lags.
| Case | Setup | Result |
|---|---|---|
| Winter HRV, three temperatures | outdoor 32 °F, return 70 °F, supply 60 °F, 100 CFM | sensible 73.7%, supply 60.0 °F, 3,024 Btu/h sensible, net outdoor air 100.0 CFM |
| Same readings, 80 CFM of exhaust | outdoor 32 °F, return 70 °F, supply 50 °F, 100 CFM supply and 80 CFM exhaust | sensible 59.2%, supply 50.0 °F, 1,944 Btu/h sensible, net outdoor air 100.0 CFM |
| Winter ERV, readings with RH | outdoor 32 °F at 80% RH, return 70 °F at 40% RH, supply 60 °F at 48% RH, 100 CFM | sensible 73.7%, latent 70.0%, total 72.7%, supply 60.0 °F, 4,086 Btu/h total, net outdoor air 100.0 CFM |
| Summer ERV, readings with RH | outdoor 95 °F at 45% RH, return 75 °F at 50% RH, supply 82 °F at 51% RH, 100 CFM | sensible 65.0%, latent 60.1%, total 62.1%, supply 82.0 °F, 3,325 Btu/h total, net outdoor air 100.0 CFM |
| Summer, 75 percent core, temperatures only | outdoor 95 °F, return 75 °F, rated 75%, 100 CFM | sensible 75.0%, supply 80.0 °F, 1,620 Btu/h sensible, net outdoor air 100.0 CFM |
| Cold winter ERV from a rating | outdoor 20 °F at 60% RH, return 70 °F at 35% RH, rated 80% sensible and 60% latent, 100 CFM | sensible 80.0%, latent 60.0%, total 74.7%, supply 60.0 °F, 5,503 Btu/h total, net outdoor air 100.0 CFM |
| Winter HRV with a 5 percent EATR | outdoor 32 °F, return 70 °F, supply 60 °F, 100 CFM, EATR 5% | sensible 73.7%, supply 60.0 °F, 3,024 Btu/h sensible, net outdoor air 95.0 CFM |
| Commercial unit, 2,000 CFM at 10 °F | outdoor 10 °F, return 72 °F, supply 55 °F, 2,000 CFM | sensible 72.6%, supply 55.0 °F, 97,200 Btu/h sensible, net outdoor air 2,000.0 CFM |
Standards and data sources
- AHRI Standard 1060 (I-P) 2018, Performance Rating of Air-to-Air Exchangers for Energy Recovery Ventilation Equipment - The definition of effectiveness in 3.5, Equations C1 and C2 with cp of 0.24 and hfg of 1,061, C8 for the enthalpy recovery ratio, C9 and C10 for leaving conditions, Equation 1 for net supply airflow, and Table 1 for the range of standard rating conditions and the supply flow ratio of 0.5 to 2. This page reads the public copy of the standard.
- ASHRAE Journal, May 2014, Energy Recovery (Trane copy) - Quotes the ASHRAE Standard 84 definition of effectiveness, the same wording as AHRI 1060, and the reading of 50 percent effectiveness in ASHRAE/IES Standard 90.1-2010 as a change in outdoor air enthalpy equal to half the difference between the outdoor and return air enthalpies at design conditions.
- Home Assistant Community, Heat Recovery Ventilation Unit (HRU) Efficiency - A forum thread with logged indoor, supply and outdoor readings, worked as 67 percent with the effectiveness ratio and 88 percent with supply divided by indoor. It is a measured example and not a standard.
- Reno House, HRV Energy Recovery Efficiency Explained - A contractor blog that works a 14 °F outdoor and 70 °F indoor day, where 70 percent pre-heats the incoming air to about 53 °F and 90 percent to about 64 °F. It is a commercial source and gives no formula.
Limitations of this calculator
- Effectiveness here is the gross figure of AHRI Standard 1060, worked from the three states typed. It is not the sensible recovery efficiency or apparent sensible effectiveness in HVI listings, which are worked on other bases. The ASHRAE Handbook, HVAC Systems and Equipment, Chapter 26 treats the same equations and is not reproduced.
- Readings carry the accuracy of the instrument. AHRI Table 1 allows 0.5 °F on the average of laboratory readings, and at 38 °F between outdoor and return a 1 °F error in the supply reading moves the answer by 2.6 points. The page refuses a difference under 5 °F, where the ratio is mostly noise.
- A rating is published at stated conditions. AHRI Table 1 covers dry-bulb from 35 to 120 °F, wet-bulb up to 80 °F, humidity ratio of 10 gr/lb or more and RH up to 95 percent, and the page warns when a stream is outside that range. In rating mode a different outdoor condition or airflow can move the real effectiveness away from the number typed.
- Airflows are read as standard air of 0.075 lb/ft3, as in AHRI 1060, and humidity ratios come from relative humidity at sea-level pressure. Elevation, and the density of air at its real temperature, are not corrected.
- The latent load uses 0.682 Btu/h per CFM per gr/lb, from the 1,061 Btu/lb of AHRI 1060. A field figure of 4840 per CFM per lb/lb gives 1.4 percent more. Enthalpy of moist air follows ASHRAE Fundamentals Equation 32, and the page prints the enthalpy total beside the sum as a check.
- Frost control, defrost cycles, bypass dampers, recirculation, fan heat and cabinet leakage are not modeled. A leaving exhaust under 32 °F is flagged, and the rest is left to the manufacturer's data.
- Total effectiveness follows AHRI 1060 Equation C2, which adds the sizes of the sensible and latent changes. Where outdoor air is warmer than the return but drier, the two act in opposite directions, the total reads high, and the page warns.
- This is a calculation from readings or ratings you type, not a rating. Published ratings are measured in a laboratory under AHRI 1060, and the AHRI certified directory, the manufacturer's selection data and a licensed mechanical engineer or contractor decide the unit.
Common effectiveness mistakes
- Dividing the supply temperature by the indoor temperature. Effectiveness divides the change in the supply air by the most it could have changed, which is outdoor to return. A forum thread with logged indoor, supply and outdoor readings gets 67 percent that way and about 88 percent by dividing supply by indoor. The second figure ignores the outdoor temperature.
- Reading the supply air after the fan. Fan heat and cabinet gains lift a supply reading taken downstream of the fan. 32 °F outdoor and 70 °F return with a 60 °F supply is 73.7 percent, and the same air read at 63 °F after 3 °F of fan heat is 81.6 percent. Read it at the core outlet when you can.
- Comparing a sensible figure with a total figure. A sensible figure and a total figure are different numbers for the same core. The winter ERV readings in the example table give 73.7 percent sensible, 70.0 percent latent and 72.7 percent total. Compare cores on the same basis, and an HRV on sensible only.
- Ignoring unequal airflows. The smaller airstream sets Cmin. 32 °F outdoor, 70 °F return and a 50 °F supply read 47.4 percent at equal flows and 59.2 percent with 100 CFM of supply against 80 CFM of exhaust. Use the flows the unit runs at.
- Using a rating outside its conditions. A rated effectiveness belongs to the conditions it was measured at. AHRI Table 1 stops at 35 °F on the cold side, so a 0 °F day is outside the standard rating range, and frost control then changes what the unit delivers. Ask the manufacturer for data at the design point.
- Counting leaked exhaust as outdoor air. Exhaust air that crosses to the supply side does not ventilate. 5 percent EATR on 100 CFM of supply leaves 95.0 CFM of outdoor air, and it also flatters a gross effectiveness. A ventilation calculation should use the net figure.
Common questions about ERV effectiveness
How do you calculate ERV effectiveness?
Divide the change in the supply air by the change it could have made. For sensible effectiveness that is the supply temperature minus the outdoor temperature, over the return temperature minus the outdoor temperature, at equal airflows. 60 °F supply with 32 °F outdoor and 70 °F return is 73.7 percent.
What is the difference between sensible, latent and total effectiveness?
Sensible effectiveness uses dry-bulb temperature, latent uses humidity ratio and total uses both. An HRV moves heat and almost no moisture, so its latent figure is near zero, while an ERV core moves both. The winter ERV readings in the example table give 73.7 percent sensible and 70.0 percent latent.
How do I find the supply air temperature after an ERV?
Subtract the rated effectiveness times the outdoor to return difference from the outdoor temperature. A 75 percent core at 95 °F outdoor and 75 °F return leaves the supply at 80.0 °F. Use a rating mode and the page also gives the supply humidity.
How much load does an ERV take off the system?
Sensible load is 1.08 x CFM x the temperature change of the supply air, and latent load is 0.682 x CFM x the change in humidity ratio in grains per pound. The default readings move 3,024 Btu/h at 100 CFM, and 1,080 Btu/h is left for the coil to bring the air to the return temperature. Both come from the supply CFM.
Why does the page refuse an effectiveness above 100 percent?
Supply air cannot get closer to the return air than the return air itself, so a result above 100 percent means a reading is wrong. The usual causes are swapped outdoor and return readings, a supply reading near a heater and a humidity taken at the wrong dry-bulb. Refusal names the side to check.
What are EATR and net outdoor air?
The exhaust air transfer ratio is the share of the exhaust air that crosses to the supply side. AHRI 1060 Equation 1 takes net supply airflow as the supply airflow times one minus the EATR. 5 percent on 100 CFM leaves 95.0 CFM of outdoor air.
Is this the same as the HVI sensible recovery efficiency?
No. The figures in HVI listings are worked on other bases, and a contractor blog reports that the sensible recovery efficiency allows for fan heat and cabinet losses. This page uses the AHRI 1060 gross definition on the readings you give it. Compare ratings from one program with each other.
How accurate is a field measurement of effectiveness?
It depends on the instrument and on the temperature difference. At 38 °F between outdoor and return, each 1 °F of error in the supply reading moves the result by 2.6 points, and the error grows as the difference shrinks. Read all three temperatures with the same probe once the unit has run steadily.
How is this different from the ASHRAE 62.2 ventilation calculator?
That page finds how many CFM of outdoor air a home needs. This one takes an energy recovery unit as given and finds what it does to that air, which is the effectiveness, the supply condition and the load moved. Use both when sizing an ERV for a house.
Is there a limit on how many times I can run this?
No. This calculator runs in your browser with no daily limit and no signup. Run it as often as you like, for as many units as you have on the list this week.
Used on the same job
Read the air, then check the rating
Take the three temperatures at the same time, once the unit has run steadily, and add the humidity if it is an ERV. The effectiveness and the load then follow, and the rating from the manufacturer is checked at the design point. Simple. For the outdoor air a house needs in the first place, see the ASHRAE 62.2 ventilation calculator. You can also ask the assistant to go over a set of readings.
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