# When is an ERV needed?

| Posted in Energy Efficiency and Durability on

How tight does a house need to be at ACH 50, before an ERV or HRV is recommended?

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### Replies

1. | | #1

I would say the dividing line is about 3 ach50. Volume of the house and number of occupants is one variable to factor in.

2. GBA Editor
| | #2

Allen,

1. The question is not really whether or not you need an ERV / HRV; the question is whether you need a mechanical ventilation system. Either an ERV or an HRV can provide mechanical ventilation, but there are other options, including an exhaust-only system or a supply-only system. For more information on all of your options, see Designing a Good Ventilation System.

2. Unlike Doug McEvers, I don't think that your question can be answered with a simple number. There are too many variables. There are factors which increase the need for ventilation that are independent of your ach50 results; these include high occupancy (many family members), humid indoor conditions (for example, due to a damp crawl space), or occupant behavior like tobacco smoking.

You can determine whether you need a mechanical ventilation system using common sense. Signs that your house probably needs a mechanical ventilation system include dripping windows during the winter and indoor odors.

3. | | #3

Thanks Martin. All good points.

4. | | #4

I took Allen's question on its face and gave a simple answer for a complex question. A home with a volume of 28,000 cubic feet @ 3 ach50 would have a natural air change of .176 in MN. This would be a natural air change of 82 cfm, should be plenty of ventilation under normal conditions.

Indoor relative humidity can be a measure of air quality but all conditions should be considered.

5. GBA Editor
| | #5

Doug,
You wrote, "This would be a natural air change of 82 cfm, should be plenty of ventilation under normal conditions." The problem with calculations of "natural air change" is that these calculations reflect annual averages, not actual air change rates. During the winter, when the stack effect is strongest, air change rates will be much higher than average; the same is true on windy days. On calm days during the spring and fall, this home could have a very low air change rate.

There may be reasons why homeowners might want a mechanical ventilation system, even if they have a "natural air change rate" that averages 82 cfm.

6. Expert Member
| | #6

The indoor air humidity isn't really a very good stand-in for indoor air pollution levels or air quality. Even though high humidity in winter is one indicator insufficient ventilation rates, it's a lousy indicator of when the ventilation rates are actually high enough to purge other indoor pollutants. In very tight houses with low-VOC contents and obsessive use of exhaust ventilation during cooking using humidity as the indicicator can sort of work, provided nobody uses aerosol sprays, insecticides (any type), or smokes. Duty-cycling the ventilation to a known ventilation rate is probably a better approach.

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