Boiler stability

[daveaspy]

I have a Worcester Highflow 440CDi and have added some temp sensors to the flow/return pipes for monitoring purposes (or geeky interest - whichever ). Here is a graph from last night...

Dark blue - flow, light blue - return

It is currently set to 60C flow temp which it holds very well until there is a disturbance in the system like a tado TRV opening up a radiator. It seems to initially handle that but then leads to the flow temp going over the 60C and the boiler turns off. When it comes back on a few minutes later, the flow temp rises but sometimes cycles on and off a few times before stabilising. I'm wondering if this is down to the lag in the system - it takes a while for the return to start rising after the boiler fires, is the boiler looking for that rise in return and giving up too soon? Are there any parameters that can be tweaked? {edit: looking at the service manual there doesn't seem to be any return temp sensor so it must control off the flow temperature)

 

[Swwils]

Is it possible that your TRV requesting heat is the only one? The boiler will have difficulty modulating the output for such a small heat demand.

Try the same thing but with the whole system requesting heat.

 

[daveaspy]

good point, there are a few rads without TRVs - towel rails, one in the room with a wall stat, but those are all quite small radiators. I'll see what effect having more radiators on has

 

[Swwils]

What are you using to log the temps out of interest?

 

[DP]

Check each radiator glow and return and adjust the preset valve to attain the differential of 15 degrees
You could aim for 20 even
You will probably find the towel rails have hardly any drop between flow and return

The boiler will be looking at the flow temperature and modulating the flame. Overshoot may cause the burner to shut down and stay off for predetermined period

 

[daveaspy]

What are you using to log the temps out of interest?

I have a microcontroller (ESP32) that has a couple of temp probes connected to it and attached to the flow and return pipes (as well as a pressure sensor on the flow pipe, and also a connection to a current transformer so I can measure electrical power usage). It sends the data over wifi (by MQTT) which ends up in node-red which displays them in a pretty graph.

 

[daveaspy]

Check each radiator glow and return and adjust the preset valve to attain the differential of 15 degrees
You could aim for 20 even
You will probably find the towel rails have hardly any drop between flow and return

The boiler will be looking at the flow temperature and modulating the flame. Overshoot may cause the burner to shut down and stay off for predetermined period

yeah I might have another round of balancing. I went around last year trying to get about 15 drop but it was really hard to get a big drop while still actually getting flow through the radiator, and as rads turn on and off that all goes to cock anyway

 

[SpecialK]

I'm having a similar issue with my vaillant 831 and it cycles as it doesn't like smaller loads - I am trying to run a loop of UFH without the rads connected.
Can you turn down the cycling time on the bosch? It may also be possible to work out your radiator load and tell the boiler what it is...