Faulty return temperature sensor?

[MrTom]

Hi there,
I've got an Ecotech 630 heating a 3 zone UFH system, 7 radiators (TRV's on all bar the towel rail) and 250L unvented hot water cylinder. All controlled by a VRC700 with weather compensation.

I realise it's probably an oversized boiler for the set up (a previous plumbers choice, not mine) and have scaled down the heating capacity accordingly, but the boiler starts cycling a lot even when heat is called for by the rads and UFH i.e. TRV's still open or room stats are calling for heat. There isn't a boiler cycling problem when the cylinder is heating.

In trying to reduce the amount of boiler cycling, I've been checking parameters d40 (flow temp) and d41 (return temp) and most of the time d41 is only ever 2 or 3 degrees less than d40, regardless of whether the cylinder (~65c) or the UFH (~43c) is calling for heat. This small difference occurs within a few minutes of the boiler firing up.

The whole system has been balanced and the temperature drop across each element (rads and UFH) is always around 10c, so I'm starting to wonder whether the return temp sensor might be giving the wrong readings? And would this then effect the operation of the boiler?
Or, could the bypass valve within the boiler be set incorrectly and hot water from the flow outlet is passing to the return when it shouldn't be?

Any thoughts/advice welcome and thanks in advance!

 

[ScottishGasMan]

Theres not an awful lot you can do when boiler is oversized, the sensors are fine (the boiler tests these and will automatically detect an issue with the sensors - and the system for testing it works well)

if the return heating pipe is cool then yes, the bypass in the boiler will be letting water recirculate back causing the return to show its hot.

the bypass can be adjusted down, however this in turn will reduce the flow rate through the heat exchanger which is not healthy for the boiler.

 

[MrTom]

Theres not an awful lot you can do when boiler is oversized, the sensors are fine (the boiler tests these and will automatically detect an issue with the sensors - and the system for testing it works well)

if the return heating pipe is cool then yes, the bypass in the boiler will be letting water recirculate back causing the return to show its hot.

the bypass can be adjusted down, however this in turn will reduce the flow rate through the heat exchanger which is not healthy for the boiler.

Thanks ScottishGasMan - I wonder if the bypass might need adjusting then.
Take your point about reducing flow, but surely if some elements are still calling for heat there is still somewhere for the heated water to ‘go’ as it were, therefore flow is still maintained? Particularly if system is balanced with respect to temperature drop over elements? Or am I missing something?
Would adjusting the pump speed make a difference here? Can’t remember the exact boiler parameter, but have currently set it to 100% as leaving it on ‘auto’ meant it never changed for 15% which I thought might cause problems with flow through the heat exchanger...