Signs an HIU control valve has failed
A seized-shut valve gives no heating or no hot water with a perfectly healthy primary supply. A valve stuck open gives uncontrolled, often scalding temperatures and a hot unit even with no demand. A passing valve is subtler — the flat is warm when the heating is off, the HIU casing is warm all summer, and the network return temperature stays stubbornly high. Debris from the primary network is the usual cause, which is why strainer condition always gets checked alongside.
- No heating or hot water with correct primary temperature
- Scalding or uncontrollable outlet temperature
- Unit warm with no demand
- Valve not responding to actuator drive
- Debris or scoring on the valve seat
