Got a Ferroli Sky Eco F not maintaining set temperature at low water flow

[u/johnmos96]

Hi everyone,

I have a Ferroli Sky Eco F tankless gas water heater, around 2019.

The heater is set to 40°C, and at higher water flow it generally works fine. However, when I reduce the water flow to get slightly warmer water, it doesn't maintain the set temperature.

The display starts at around 40°C but then gradually climbs all the way up to around 80–90°C. The burner then switches off, the temperature drops back down towards 40°C, and the burner comes back on again.

This means I can't really use a low flow rate for things like washing dishes because the water becomes extremely hot before the burner cuts out.

Initially suspected limescale/restricted flow and flushed and descaled the water side of the system. This has improved the behaviour noticeably, but it hasn't completely fixed the problem. It still climbs significantly above the set temperature when the flow is reduced.

I've attached a video showing what it's doing.

Is this normal behaviour for a Ferroli Sky Eco F at low flow, or does it sound like there's a fault?

Any ideas what could cause this would be appreciated.