# Understanding HRV Energy Use Calculations

| Posted in Energy Efficiency and Durability on

I am trying to understand the last column on the below sheet. I can see that the “last but one” column value is arrived at by subtracting the “Total Annual Energy Use (KWh @ COP 1.0)” from “Recovered Heating Energy (KWh/yr)”. But I am trying to figure out how the last column values are being arrived at and also what the last 2 columns mean in layman terms. Thanks.

## Join the leading community of building science experts

### Replies

1. | | #1

You can't compare ERV and HRV directly like, also, interesting that 180 cfm is considered poor while 101 cfm is considered great! That sheet seems to be designed to sell something. I wonder what!

2. | | #2

Here's my interpretation.

The last column = [(Recovered Heating energy / efficiency) / COP] - (Tot. Ann. Energy Use @ COP=3).

Which actually makes some sense! Basically the original energy use was the energy used to heat up the full air flow. Since the COP 3 is assumed, then it takes 3x less electric energy to warm that air up. this table basically shows that when COP gets large, items like the HRV save less and less.

But now the second to last column doesn't make sense - it should use the same formula as above - and it doesn't (or so it seems to me).

And another problem. When calculating the Net Energy Saved, the basis shouldn't be compared to the minimum airflow of the HRV, it should be the ASHRAE Standard 62.2 that 3-bedroom home. That is, the folks in the home without an HRV will ventilate at the minimum rate.

3. | | #3

Thanks for the input, Bill. I am still at a loss. If we take the first row values and apply to the formula you mentioned, it would be:

2561 / 3 = 854. Then 854 - 767 would be 87. But it shows 249 in the last column.

4. | | #4

you forgot to divide by the efficiency!
try this:
[(2,561 / 0.84) / 3] = 1016. Then 1016 - 767 = 249

5. | | #5

Thanks for the correction, Bill. I wonder if there's a similar analysis available that takes the issues you mentioned into account; standard exhaust-only ventilation vs HRV/ERV that takes into account the efficiency of the home heater as well.

6. GBA Editor
| | #6

Venkat,
Q. "I wonder if there's a similar analysis available that takes the issues you mentioned into account; standard exhaust-only ventilation vs HRV/ERV that takes into account the efficiency of the home heater as well."

A. There is. The analysis was performed by John Semmelhack, and the results were published on GBA. Here is a link to the article: Are HRVs Cost-Effective? See Images #2 and #3 on that page for detailed data.

7. | | #7

Thanks, Martin.

• |
• |
• |
• |