Vaillant ecoTECplus418 cycling on start up

[sniktun1]

IF starting up say a 5kw UFH from cold that normally runs with flow/ return temps of 45C/37C and assuming a boiler flow temp of 65C, then the flow through the boiler should be 8.96LPM with a dT of 8C, once up to temperature then the flow through the boiler will only be 2.56LPM with a dT of 28C, the remainng 6.4LPM will be recirculated through the UFH manifold so can understand if the boiler was tripping on a high dT with UFH up to temp but difficult see it tripping on UFH start up except the TMV is stuck in one position?

Not sure, it shouldn't be. the blending valve on the UFH is brand new, I assume it is thermostatic. I've got it set to 45°C flow currently so as hot as I can get it really.

I guess I can get the pump datasheet so find out what the flowrates are at the different speed settings, & set it to the above. Honestly 7 poxy radiators and an UFH system in a house should not be this difficult. I design controls for commercial building, but the boilers at least do what we ask them, rather than these domestic pre-set routines.

 

[Johntheo5]

Is there a ABV (auto balancing valve) installed?, if so it may be opened too much. The grundfos boiler pump vanes may be choked with deposits especially if you have a open vented system, you should close the pump isolating valves, remove the 4 stud bolts, remove the pump head and clear out the impeller vanes with a tie wrap, also backwash through the pump flow & Return ports.
If the pump is choked with hard deposits then you might consider installing a 6M DAB Evosta 3, or, like mine, a 6M Wilo Yonos Pico, both these pumps show the system flowrate in M3/hr and also the pump power (W) in watts, very handy for trouble shooting.
Its not clear to me at any rate if the UFH can be run on its own.

 

[Johntheo5]

You may possibly have a boiler return sensor problem since a 65/25C, 40C, dT, post 15, will certainly cause the burner to trip for boiler HEX protection, I think it doesn't like anything higher than 25C/28C dT.
Is the 3 way MPV operatig correctly and going fully to the CH position?, does the boiler operate ok on cylinder heating only? Maybe postt a photo of the UFH manifold & pump.

 

[jackthom]

Deleted

 

[Harry Bloomfield]

Just a thought - are there two pumps in the system? Any chance one pump might be fitted and pumping the wrong way, fighting the first?