Which of the following is not a defrost system for heat recovery ventilators?

In the context of heat recovery ventilators (HRVs), defrost systems play a crucial role in maintaining efficiency during cold weather conditions. The primary purpose of a defrost system is to prevent frost build-up on the heat exchanger, which can impede functionality.

Directing furnace air to the HRV is not a recognized method of defrosting. Typically, this option would involve utilizing warm air from a furnace system to keep the heat exchanger clear of frost. However, this method does not align with the operational principle of HRVs, which aim to recover heat from outgoing exhaust air to pre-condition incoming fresh air without significantly altering the source of that air.

In contrast, the other methods listed are common practices for defrosting in HRVs. Recirculating exhaust through the fresh air side effectively warms incoming air while helping to defrost the unit. Running house air through the fresh air side can also be an effective way to maintain temperature and minimize frost. Shutting off the fresh air fan temporarily allows the HRV to defrost by pausing the incoming air flow when conditions necessitate it.

The rationale for identifying directing furnace air as not a valid defrosting method lies in the fundamental operation of HRVs, which emphasizes energy recovery and air exchange