When an HIU plate heat exchanger needs replacing
The classic symptom is hot water that is hot at low flow and cool as soon as the tap is opened properly. That is a fouled DHW exchanger: with scale on the plates there is not enough surface area left to heat the water at full flow. On the heating side, a fouled exchanger shows as radiators that never reach temperature even though the primary is at design. We confirm with temperature readings across both sides before recommending replacement, because a blocked strainer produces similar symptoms and costs nothing to fix.
- Hot water cool at high flow rates
- Long delay before hot water arrives
- Heating not reaching temperature with correct primary flow
- High primary return temperature indicating poor transfer
- Visible scale or debris on removal
