I have a Viessmann Vitodens 222-F FS2B 26kW Storage Gas Boiler installed in 2014.
The boiler has developed a problem recently where it short cycles but only when hot water is used.
When hot water is turned on (on any tap in the property) the boiler remains off for about 45 sec. At the beginning the water temp from the tap is very close to the set cylinder temperature of 50C. In about 45 sec the boiler fires up and runs up to 75C, turns off, goes down to 65C, turns on, runs up to 75C and so on. At the same time water temperature from the tap gradually goes down from 50C to about 35C over the course of 5mins (I haven't tested whether it goes further down after that). When the tap is closed the boiler continues the same short cycling pattern for 5-10 mins before going back to steady state where it runs continuously at the same lower temp. The boiler has two pumps - presumably separate pumps for CE and DHW.
What I've tested:
1) With hot water tap opened and the boiler firing I measured the difference between the temp of cold water inlet pipe and DHW outlet pipe and it was around 25C (hot water outlet at 36C and cold at 9C). I was hoping that this test will determine whether plate heat exchanger is free of any blockages. I believe delta T across incoming cold and outgoing hot terminals of the plat of 35C or more suggests the plate heat exchanger is not blocked, but I'm not an expert. In my case it is less than 30C so the result is not conclusive.
2) The secondary pump serving the cylinder seems to be fine - i.e. it has not seized or anything but I can't attest to how effective it is.
I'd appreciate any thoughts/pointers at potential culprits and ways to diagnose.
Thank you
The boiler has developed a problem recently where it short cycles but only when hot water is used.
When hot water is turned on (on any tap in the property) the boiler remains off for about 45 sec. At the beginning the water temp from the tap is very close to the set cylinder temperature of 50C. In about 45 sec the boiler fires up and runs up to 75C, turns off, goes down to 65C, turns on, runs up to 75C and so on. At the same time water temperature from the tap gradually goes down from 50C to about 35C over the course of 5mins (I haven't tested whether it goes further down after that). When the tap is closed the boiler continues the same short cycling pattern for 5-10 mins before going back to steady state where it runs continuously at the same lower temp. The boiler has two pumps - presumably separate pumps for CE and DHW.
What I've tested:
1) With hot water tap opened and the boiler firing I measured the difference between the temp of cold water inlet pipe and DHW outlet pipe and it was around 25C (hot water outlet at 36C and cold at 9C). I was hoping that this test will determine whether plate heat exchanger is free of any blockages. I believe delta T across incoming cold and outgoing hot terminals of the plat of 35C or more suggests the plate heat exchanger is not blocked, but I'm not an expert. In my case it is less than 30C so the result is not conclusive.
2) The secondary pump serving the cylinder seems to be fine - i.e. it has not seized or anything but I can't attest to how effective it is.
I'd appreciate any thoughts/pointers at potential culprits and ways to diagnose.
Thank you
