I have a Viessmann Vitodens 100 WBIA open vent system boiler (not combi) 24KW Natural Gas running CH only with a dozen rads, so a high heating load. About 10 years old. Until recently it worked perfectly and the system is unchanged.
I run the boiler thermostat at just above the 60C mark on the dial and an external room stat controls the temperature in the conventional way. Nothing clever.
Recently, it only fires at two bars max on the modulation scale even from cold and therefore the system is taking a long time to heat up to temperature, but it does reach 62C eventually. Even if I turn the boiler dial up to max 80C, it only goes to three or four bars for a short time and then back down to 2 bars. It is condensing fine but only operating on low power.
The Grundfos 15-60 circulating pump is new. I have a MagnaClean filter fitted with Fernox X-100 Inhibitor in the system and it does not trap much on the magnet when I clean it. I have flushed the system with Fernox.
The boiler temperature thermostat tests at correct resistance on multimeter. I have tried pushing the ‘Reset’ button on the boiler, but no change.
There are no Fault Codes being displayed.
Any advice what to look for or how to test for fault would be appreciated.
Many thanks
I run the boiler thermostat at just above the 60C mark on the dial and an external room stat controls the temperature in the conventional way. Nothing clever.
Recently, it only fires at two bars max on the modulation scale even from cold and therefore the system is taking a long time to heat up to temperature, but it does reach 62C eventually. Even if I turn the boiler dial up to max 80C, it only goes to three or four bars for a short time and then back down to 2 bars. It is condensing fine but only operating on low power.
The Grundfos 15-60 circulating pump is new. I have a MagnaClean filter fitted with Fernox X-100 Inhibitor in the system and it does not trap much on the magnet when I clean it. I have flushed the system with Fernox.
The boiler temperature thermostat tests at correct resistance on multimeter. I have tried pushing the ‘Reset’ button on the boiler, but no change.
There are no Fault Codes being displayed.
Any advice what to look for or how to test for fault would be appreciated.
Many thanks