Do HRVs recycle some of the indoor air
nvman
| Posted in General Questions on
Is it common for HRVs to recycle some of the indoor air rather than exhaust it?
Years ago, I had installed a Venmar HRV and wondered why I could smell cooking odours from a supply vent in the basement even though I had dedicated ducting.
I called Venmar about it and they told me that they included some recycled air in the fresh air supply so that the fresh air would not feel so cold.
Is this still common? Do all manufacturers do this?
Years ago, I had installed a Venmar HRV and wondered why I could smell cooking odours from a supply vent in the basement even though I had dedicated ducting.
I called Venmar about it and they told me that they included some recycled air in the fresh air supply so that the fresh air would not feel so cold.
Is this still common? Do all manufacturers do this?
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Replies
I know that's true of our Venmar HEPA 3000.
Some do. There's an article on here somewhere about it. The reason for recycling, more often called recirculating, is not for comfort but to protect the core from frosting up. Some brands of ERV claim to not need any means of frost protection, relying entirely on moisture removal from the air stream. This is not an option for an HRV. Other brands use either use resistance or ground source heating methods instead of recirculating. The vast majority of brands for sale in North America use recirculating, and resistance heating is actually not allowed by code in some locales.
Trevor,
A sort of basic question: Does that mean that when commissioning, the measured air-volume has to be adjusted to take into account the amount of recirculated air to see if the unit meets the required ACH?
Good question, and I don't know. It's possible for the manufacturer to configure the HRV to maintain the level of incoming fresh air and simply add the recirculated air to it. I don't know if any of them do this, but I doubt it.
As part of the design process that HRAI specifies, you are supposed to take the "Very Low Temperature Ventilation Reduction Factor" from manufacturer's literature or from HVI's product directory and using the temperature chart from the HRAI Ventilation design manual, find your low temperature ventilation factor and increase your ventilation rate by this factor. This factor ranges from 1.0 to 2.0. Of course, this means you are over ventilating the rest of the year...
The increase is meant to compensate for reduced fresh air delivery as a result of defrost recirculation and the reduction of air flow caused by the frosting of the HRV core until defrost.
It's not really something you could compensate for during commissioning, since the recirculation only happens during defrost mode. The "Very Low Temperature Ventilation Reduction Factor" is the percent reduction in airflow rate of the supply and exhaust systems at the end of the 72-hour test compared with operation under non-frosting conditions. The final airflow rate is taken as the average of the last 60 hours of the test. This would be very difficult to determine on site!
Thanks Trevor and Yupster. Useful info as always.
Thank you for the info.
Aaron
Sounds like it would also make such an HRV not suitable as a bathroom exhaust fan replacement (due to odors).
Most HRVs have both a low and high speed setting. One way of addressing reduced ventilation due to defrost recirculation is to run the HRV on the high speed setting for an appropriate period of time.
For bathrooms, a recirculating HRV will still reduce moisture, but it just spreads the moisture through the rest of the house until the recirculation cycle is complete. Odors can be addressed using an activated carbon filter. However, I believe that best management practice is to install a timer. The timer overrides the recirculation function.