Most “failed” heat exchangers have not failed at all — they have quietly lost 20–40% of their recovery to a fixable problem. Here are the modes we see most, in order of frequency.
1. Fouling (the usual suspect)
Dust, oil mist or biofilm coats the plates and blocks both heat transfer and airflow. Symptom: rising ΔP and falling supply temperature. Fix: pre-filters sized right, and a cleanable core where the exhaust is dirty (see cleaning & maintenance).
2. Frozen / cracked core
In cold climates the cold side freezes; repeated freeze-thaw can crack plates or warp a wheel. Symptom: winter recovery collapse. Fix: frost control — preheat, exhaust bypass, or a defrosting wheel. Run the frost calculator to set the threshold.
3. Blocked condensate drain
Slime or debris plugs the drain; water backs up and can reach the supply side. Symptom: standing water, odour, sometimes leakage. Fix: trap access, periodic flush, biocide in wet duties.
4. Seal leakage / carryover
Worn gaskets or a damaged wheel seal let supply leak into exhaust (or transfer moisture you did not want). Symptom: lower net effectiveness than rated, odd humidity behaviour. Fix: gasket replacement; for wheels, seal inspection during service.
5. Corrosion
Acid, salt or solvent exhaust eats aluminium. Symptom: pinholes, eventually stream crossover. Fix: upgrade to stainless or coated polymer (see materials).
6. Bypass and imbalance
Leaky casing or unbalanced fans let air skip the core or push the building positive/negative. Symptom: good core, poor result. Fix: leak test the casing; balance supply = exhaust flows.
7. Fan / motor drift
A slowing fan drops airflow below design, so recovery (and comfort) fall together. Symptom: lower total flow, higher ΔT across the core than expected. Fix: verify delivered CFM against the rated value.
A 5-minute health check
- Measure supply-out temperature vs the rated effectiveness — big gap means trouble.
- Read ΔP on both sides — above spec means fouling.
- Check the drain is clear and the seals intact.
- Confirm flows are balanced.
Recovering poorly?
Send us the symptoms and duty — we will tell you whether it is fouling, frost or bypass, and what to do.
Contact us