Understanding your result
The headline is the rated unit size and the type, ERV or HRV, with the continuous ventilation rate and the heat recovered on the design day in the sub-line. The bars compare the heat a plain exhaust fan would lose with what the recovery ventilator still loses, the gap being the recovery. The stats give the unit size, the type and the heat recovered at design.
The table gives the reasoning for the type in plain words, the climate and design temperature the recovery is calculated at, the heat a plain exhaust fan would lose for the same airflow, and the typical electrical draw of the unit’s fans. The unit size includes a 25% margin over the continuous rate, and 50% more if you chose a boost mode for bathroom exhaust.
The continuous rate comes from the whole-house ventilation calculator. An HRV in a tight cold-climate house may leave it too dry or too humid depending on the household; the humidifier and dehumidifier calculators size the correction, and the bathroom exhaust fan calculator gives the rates a boost mode must meet.
How we calculate this
The 1.08 is the sensible heat factor for standard air (BTU per hour per CFM per °F). The design temperature is the typical 99% winter design temperature for the climate zone used across the site. The 70% sensible recovery efficiency is typical of residential units tested under the Home Ventilating Institute’s ERV and HRV procedures, whose certified ratings range from about 55 to 85%; use the rating of the model you choose for a final figure. The type recommendation follows the usual guidance from the US Department of Energy and the manufacturers: HRV where winters are cold and dry and summers are not humid, ERV where summers are humid or the climate is warm. The ventilation rates are on the ventilation and exhaust minimums page.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Margin over the ventilation rate | 25% | Filter loading, duct static and balancing; typical installer practice |
| Boost factor | 1.5 × | To meet bathroom intermittent exhaust rates on boost |
| Unit sizes | 70 to 300 CFM | Residential ERVs and HRVs sold in North America |
| Sensible recovery efficiency | 70% | Typical HVI-certified residential units (55 to 85% range) |
| Design temperatures | 50 to −20 °F by zone | Typical 99% heating design temperatures, rounded |
| Fan power | 0.5 to 0.8 W per CFM | Typical residential ERV/HRV fan efficacy |
Assumptions last reviewed October 8, 2026.
The calculator does not model latent (moisture) recovery, which an ERV adds and which matters most in humid summers, the loss of fresh air during defrost cycles, the static pressure of your actual ducts or the fan curve of a particular unit. The guide to whole-house ventilation compares exhaust-only, supply-only and balanced strategies, and the guide to humidity and comfort explains how each affects indoor humidity.
Two worked examples
A cold-climate house
90 CFM of continuous ventilation in zone 5 (0 °F design), summers not especially humid, no boost, 70 °F indoors.
- Rated capacity needed: 90 × 1.25 = 113 CFM → 130 CFM unit
- Type: HRV, for cold, dry winters
- Heat in the ventilation air: 90 × 1.08 × 70 = 6,804 BTU/hr
- Recovered at 70%: 4,763 BTU/hr; still lost 2,041 BTU/hr
On the design night the HRV saves about 4,800 BTU per hour of heat, a little under a third of a ton of heating, for about 65 to 100 watts of fan power. The note reminds the installer to choose a unit with defrost, because at 0 °F an HRV core frosts.
A humid warm-climate house with boost
60 CFM continuous in zone 2 (35 °F design), humid summers, boost mode for the bathrooms, 75 °F indoors.
- Rated capacity needed: 60 × 1.25 × 1.5 = 113 CFM → 130 CFM unit
- Type: ERV, for humid summers
- Heat in the ventilation air at 35 °F: 60 × 1.08 × 40 = 2,592 BTU/hr; recovered 1,814
In a warm climate the winter heat recovery is modest; the ERV earns its place in summer, when it removes part of the moisture from incoming air before it reaches the air conditioner. The boost mode lets it replace two bathroom fans, provided each bathroom has a boost switch and an exhaust pickup.
Where to find your inputs
Ventilation rate. From the whole-house ventilation calculator, or the mechanical ventilation rate on the house’s energy compliance documents.
Climate zone. By county on the DOE map, as for the sizing calculators.
Humid summers. If outdoor dew points in July are often above 65 °F (sticky air), tick the box.
Boost. Tick it only if the unit’s exhaust picks up from the bathrooms and is controlled from them.
Common mistakes
- Sizing to exactly the ventilation rate. Filters load and ducts add resistance; allow a margin.
- Choosing by brand instead of climate. HRV for cold and dry, ERV for humid or warm.
- Kitchen exhaust through the ventilator. Grease fouls the core; use a range hood.
- Unbalanced airflow. An installer should measure and balance supply and exhaust.
- No defrost in a cold climate. An HRV core freezes below about 23 °F without it.
- Never cleaning the filters and core. Clogged filters cut airflow and recovery; check them every few months.
Questions people ask
- What size ERV or HRV do I need?
- One whose rated airflow, at the static pressure of your duct system, covers the ASHRAE 62.2 rate with about 25% to spare. A 2,000 sq ft three-bedroom house needs 90 CFM continuous, so a unit rated about 113 CFM or more, which in practice is a 130 CFM unit. If the unit also replaces bathroom exhaust fans on a boost setting, size for about 1.5 times the continuous rate with the margin, about 170 CFM for the same house.
- What is the difference between an ERV and an HRV?
- Both bring in fresh air and exhaust stale air through a heat exchanger core, recovering 60 to 85% of the heat. An HRV's core transfers only heat. An ERV's core also transfers moisture, so in winter it keeps some indoor humidity in and in summer it keeps some outdoor humidity out. An HRV suits cold climates where a tight house has too much winter moisture; an ERV suits humid climates and houses that get too dry in winter.
- Should I get an ERV or an HRV in a cold climate?
- Usually an HRV in zones 5 to 7 when the house is tight and winter condensation on windows is the problem, because the HRV lets excess indoor moisture leave. If the house is dry in winter, or summers are humid as in Minneapolis or Boston, a modern ERV is the better fit. Both need a defrost strategy below about 23 °F outdoors, which briefly cuts the fresh air delivered.
- How much energy does an HRV save?
- It recovers 60 to 85% of the heat that ventilation air would otherwise carry out. For 90 CFM of ventilation at 70 °F indoors and 0 °F outdoors, the air carries out about 6,800 BTU per hour; an HRV at 70% sensible recovery returns about 4,760 of that, leaving 2,040. Over a heating season in a cold climate that is several hundred dollars of heat, against the 50 to 100 watts the unit's two fans use.
- Can an ERV replace bathroom exhaust fans?
- Yes, if it has a boost mode controlled from each bathroom and its exhaust ducts pick up from the bathrooms. ASHRAE 62.2 allows the bathroom exhaust requirement (50 CFM intermittent or 20 continuous) to be met by the ventilator's exhaust when it can deliver those rates, which is why boost-capable units are sized about 50% above the continuous rate. Kitchen exhaust should still go through a range hood, never through an ERV, because grease fouls the core.
- Does an ERV or HRV need its own ducts?
- Best results come from dedicated ducts that supply fresh air to bedrooms and living areas and exhaust from bathrooms and the kitchen area. A simplified installation ties into the furnace return and runs the furnace blower, which is cheaper to install but uses more fan energy and mixes the fresh air less evenly. Either way the installer should balance supply and exhaust to within 10% so the house is neither pressurised nor depressurised.