Why Cores Frost
In cold climates the warm, humid exhaust can condense and freeze on the cold intake side of a plate core, blocking airflow. Defrost strategy is the difference between a recovery core that runs all winter and one that fails when needed most.
Common Strategies
The main approaches are a bypass that reduces recovery as intake nears freezing, pre-heating the intake air before the core, recirculating a warm exhaust fraction, and using a frost exhaust that vents a little warm air to keep the surface above freezing. Rotary wheels use a purge and modulation to limit frost.
| Strategy | Trade-off |
|---|---|
| Bypass | Simple, slight energy loss |
| Pre-heat | Reliable, uses energy |
| Recirculation | No extra heat, mild mixing |
| Wheel purge | Effective, moving part |
Choosing
For most buildings a modulated bypass with a temperature setpoint is enough and cheapest to run. Pre-heat or recirculation suits severe climates or where uninterrupted recovery is critical. The right choice keeps effectiveness high while preventing ice buildup.
Outcome
A correct defrost strategy lets the core recover energy through the whole heating season without freeze shutdown, protecting both comfort and the energy saving that justified the investment.
Sizing and Selection Checklist
| Climate | Strategy |
|---|---|
| Mild cold | Modulated bypass |
| Severe cold | Pre-heat or recirculation |
| Wheel duty | Purge and modulation |
| Control | Temperature setpoint |
Common Pitfalls
Specifiers sometimes omit defrost entirely in cold sites and face icing shutdowns. Others use pre-heat without a setpoint and waste energy. A fixed bypass that never modulates over-recovers and causes complaints.
Frequently Asked Questions
Which defrost is cheapest to run?
A modulated bypass with a temperature setpoint is usually the lowest-cost option for typical buildings.
Do wheels frost like plates?
Wheels can frost too, but purge and modulation limit it; severe sites still need a strategy.
Summary
A correct defrost strategy keeps the core recovering all winter without freeze shutdown, protecting the energy saving.
Next Steps for Defrost
Start by mapping the site climate degree-days, because that sets how aggressive the defrost strategy must be.
- Record the coldest intake design temperature to size the defrost margin.
- Pick the simplest strategy that prevents icing for your climate.
- Set the bypass or pre-heat control to a temperature setpoint, not a fixed state.
Correct defrost keeps the core recovering through the whole heating season without freeze shutdown.
Cost, Payback and Upkeep
Defrost adds little capital, usually a bypass or pre-heat control, and its payback is the protection of the winter saving: without it the core ices and stops recovering when heat is needed most. The upkeep is a control check, not a hardware burden.
- Capital: bypass damper or pre-heat coil with temperature control.
- Payback: preserves the winter recovery, effectively free given the risk avoided.
- Upkeep: verify the setpoint and bypass action each heating season.
A correct defrost strategy keeps the core productive through the whole cold season.
Key Takeaways
Defrost is cheap and protects the winter saving: map the climate degree-days, pick the simplest strategy that prevents icing, and set control to a temperature setpoint rather than a fixed state. Without it the core ices when heat is needed most.
- Match strategy to climate severity.
- Prefer a modulated bypass where possible.
- Set a temperature setpoint, not fixed state.