As structures become more airtight, simply relying on outdoor air finding a way inside for ventilation won’t cut it anymore. Mechanical ventilation is recommended for optimal indoor air quality (IAQ), and under that umbrella, balanced ventilation is the best choice. This can be achieved via an Energy Recovery Ventilator (ERV) or a Heat Recovery Ventilator (HRV). 

 

 

 

HRVs Need Frost Control

With an HRV, if the outgoing air has enough humidity and the incoming air is cold enough, frost will form in the core. Further, HRVs have a drain pan and condensate line to remove excess liquid, and both of these are susceptible to icing. In general, HRV cores will ice up when outdoor temperatures drop to the low 20s (°F).1 When frost does occur, this restricts airflow and impairs the ventilation process.

There are several frost-prevention and defrosting strategies for air-to-air heat recovery systems that HRVs can employ:

 

Frost-Prevention Strategies-

Preheat coil to heat up outside air: Preheat frost control is a preventive strategy that can be used with any air-to-air heat recovery device. The objective is to prevent frost from occurring within the heat exchanger, while maintaining 100% or continuous ventilation. Heat recovery is reduced, because the difference in temperature between the preheated outside air and the return air has decreased. Heating coils (electric, steam or hot water) are duct-mounted or integrated into the unit in the outdoor airstream so that the entering outdoor air temperature is preconditioned to a temperature above the frost threshold. While preheat typically has higher upfront costs, it can result in significant operating savings in climates where frost control is required for a long period of time.

 

Face and bypass damper:

Face and bypass frost control is a preventive strategy that can be used for flat plates, heat pipes and rotary wheel exchangers. As the outdoor air becomes colder, face and bypass dampers upstream of the heat exchanger modulate to reduce the amount of outdoor air flowing through the heat exchanger. This reduces the amount of heat recovered and keeps the exhaust temperature above the frost threshold. With this strategy, there’s no interrupted ventilation and no depressurization of the building, eliminating the potential for combustion appliance backdraft into the occupied space. The lower leaving supply air temperature would potentially require post-conditioning, or some type of terminal reheat within the space to ensure that occupants remain comfortable under extreme conditions.

 

Performance modulation:

The heat exchanger can be controlled to ensure that frost does not form in the return air by reducing the effectiveness and, as a result, suppressing the frost threshold. For example, wheel rotational speed can be reduced, runaround pumps can be slowed, heat pipes can be equipped with control valves or tilted. Since the heat exchanger is recovering less heat, more heating is required in the process downstream.

 

Defrosting Strategies:

Exhaust defrosting: Exhaust-only defrost is one of the most cost-effective and simple strategies to implement. It periodically defrosts ice forming on the heat exchanger by shutting down the supply fan to remove the source of cold air, while using the warm exhaust air to heat up the exchanger. When the unit goes into a defrost cycle, the exhaust fan continues to operate, the supply fan is deenergized and the outdoor air damper closes. This method is most commonly used with flat plate or heat pipe heat exchangers and is ideal for source-control applications where continuous exhaust is required. One drawback of this method is that ventilation is interrupted when the supply fan shuts down during the defrost cycle, which may not be acceptable as the equipment may not meet IAQ requirements as defined by ASHRAE Standard 62.1. This also creates negative indoor pressure, resulting in infiltration.

 

Recirculation defrost:

Recirculation defrost is also cost-effective and simple, commonly used in light commercial stand-alone air-to-air heat recovery systems that aren’t used as a primary ventilation system. Ice formation on the heat exchanger is periodically defrosted by shutting down the exhaust fan, by closing the outdoor and exhaust air dampers and by opening a recirculation air damper to remove the source of cold air. When the unit goes into a defrost cycle, the supply fan remains on to recirculate building exhaust air back into the occupied space. The exhaust air goes through the heat exchanger and provides defrosting in the absence of cold outdoor air. A drawback of this method is that ventilation is interrupted when the exhaust fan shuts down during the defrost cycle. This may not be acceptable in all applications and may not meet IAQ requirements as set out in ASHRAE Standard

62.1. It may however be acceptable in more moderate climates where freezing conditions occur only during unoccupied hours for a few hours per year.

 

ERVs and Frost-

 

ERVs Don’t Need Frost Control 99% of the Time

 

ERVs are much less susceptible to frost than HRVs for several reasons. The first is the fact that ERV cores transfer humidity in a gaseous state and avoid liquid condensate, thus eliminating the need for drain pans and condensate lines. Therefore, the areas in HRVs most susceptible to icing aren’t even present in ERVs.

Along those lines, because ERVs also recover humidity via an enthalpy core, frost formation is curbed. This is due to ERVs’ ability to transfer moisture between the two airstreams. Subsequently, this process prevents frost because any liquid in the core is in the form of bound water, which is more difficult to freeze. If any frost does form in the case of extremely high indoor humidity and very low outdoor temperatures, it thaws out quickly.

 

For example, a typical residential ERV will have up to an 18°F lower frost threshold than a similar HRV at 70°F and 30% relative humidity (RH) indoor conditions.7 In general, ERV cores may not develop icing problems until outdoor temperatures drop to the low teens (°F).8

In extreme climate conditions, ERVs are considered to be the optimal choice when compared to HRVs. This is because when HRVs are in situations with a large temperature gradient between airstreams, and at least one of those airstreams is even slightly humidified, condensation is sure to occur in the unit. The result can be mold and impaired IAQ. This isn’t the case with ERVs. 

 

See below for details and graphs of this information. If you have questions, create a ticket using the red tab on the left or the submit ticket at the top of the page, select commercial sales. Thanks!

 

-All of the above information comes from Renewaire document below.....