Furnace runs but radiators remain lukewarm or cold, especially on upper floors

How to Fix / Troubleshooting

Safety: Let system cool to a safe temperature before bleeding radiators. Protect floors from water spills.

Bleed all radiators starting from the lowest floor upwards until water flows without air.
Check the system pressure and top up if needed after bleeding.
Clean the Y‑strainer or dirt separator on the return line if fitted.
Verify the three‑way mixing valve position; ensure it is not stuck in a bypass or cold position.
Confirm the circulation pump speed setting is appropriate for the system size.

If circulation remains poor, a technician should assess pump sizing and system balancing.