A heat pump supplies heating and cooling efficiently, but it must still condition every cubic metre of outdoor air a building brings in. An energy recovery ventilator shrinks that load first; the heat pump then handles only the smaller remainder. Together they form the backbone of low-carbon ventilation design and are increasingly specified together in new construction.
How they share the load
In winter, cold outdoor air entering at -5°C is pre-warmed by the ERV to roughly +10°C using exhaust, so the heat pump lifts temperature only the last 12°C instead of 27°C. In summer the ERV precools and partially dehumidifies, so the heat pump compressor runs less. The ERV does the "free" recovery; the heat pump does the precise final lift. This split is what makes very high ventilation rates affordable in tight, efficient buildings.
Sizing and control sequence
- Size the ERV for the full design airflow and the heat pump for the post-recovery load, not the raw outdoor condition.
- Add an ERV bypass for shoulder seasons when recovery would over-condition the space.
- Match the heat pump's variable capacity to the varying ERV-outlet temperature rather than a fixed design point.
Interaction with heat-pump defrost
Air-source heat pumps periodically defrost, briefly shedding cooling into the space. A well-tuned ERV reduces the steady heating load so the defrost event is a smaller fraction of total demand, smoothing comfort and cutting peak electrical draw. In all-electric buildings this peak reduction can also lower capacity-based utility charges.
| Step | Without ERV | With ERV (75%) |
|---|---|---|
| Winter lift | 27 K | 12 K |
| Heat-pump load | 100% | ~44% |
| Summer relief | none | Partial precool |
Why it pays
Cutting heat-pump duty by more than half at design conditions also lets you specify a smaller unit and reduces peak electrical demand, which matters where capacity charges apply. For a 10,000 m³/h system this routinely saves tens of thousands of kWh per year. See the ERV guide, efficiency, AHU recovery and retrofit options.
Controls integration and verification
Treat the ERV and the heat pump as one system from the controls side, sharing a BACnet or Modbus point list so the heat pump sees the post-recovery supply temperature and trims its capacity accordingly instead of reacting to raw outdoor conditions. During commissioning, verify the bypass opens in mild weather so recovery does not over-cool or over-heat the space, and confirm the compressor defrost cycle still leaves a comfort margin once the steady load has shrunk. Re-verify seasonally, because a setting that is correct in January may fight the load in April as outdoor conditions shift. Where the building is all-electric, record the peak-demand reduction alongside annual kilowatt-hours, since capacity charges often dominate the business case more than the energy saving alone.
Specification checklist
- Size the heat pump on the post-recovery load, never on raw outdoor conditions.
- Add an ERV bypass so shoulder-season free cooling is not blocked.
- Tune the heat pump's variable capacity to the ERV outlet temperature.
- Confirm the defrost cycle still leaves comfort margin after recovery.
- Weigh peak-demand savings, not just annual kWh, in the business case.
- Commission the two controls together, not as separate packages.
Specifying a unit for your project?
EN159 builds plate, rotary, heat-pipe, run-around coil and counterflow air-to-air exchangers. Send airflow (m3/h), inlet temperatures and available space and we will size it.
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