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?

 

[Johntheo5]

Could possibly be something like the above.
The 418 has only a pressure loss of 1.38M at its rated flow of 774LPH, 12.9LPM @ 20C dT so a Grundfos Alpha2 or even Alpha1, should have plenty of reserve head to circulate through both the Rads & the UFH with both on together. The Alpha2 comes in both 35W and 45W sizes.

 

[sniktun1]

I'll have another look tonight. You may be onto something there if the return temp sensor is out of calibration or something (the boiler is around 10 years old now).

I'm going to turn it on later and before the boiler fires up, see if the flow & return temps are exactly the same. If the return temp sensor is reading lower than genuine, might be the cause of all these issues. Start with the simpler problems first and work forward!

If not - i'll look more into the above. The pumps are definitely the right way round - Theres no ABV. The main grundfos pump is brand new (just fitted a few months ago so won't be clogged),

 

[Johntheo5]

Does your alpha pump not show the flow rate and power?

 

[sniktun1]

Does your alpha pump not show the flow rate and power?

Just the power - can probably work out flowrate from the datasheet if needed

 

[Johntheo5]

Thats a "first class aid to navigation".
Check if a Alpha1 or Alpha2 then look down on the top of the pump housing, you will or should be able to see one way ot the other whether its a 35W pump or a 45W pump, if you cant figure out the flowrates from the pump curve(s), just post them here, plus the mode and setting you gave the pump set to.

 

[sniktun1]

Thats a "first class aid to navigation".
Check if a Alpha1 or Alpha2 then look down on the top of the pump housing, you will or should be able to see one way ot the other whether its a 35W pump or a 45W pump, if you cant figure out the flowrates from the pump curve(s), just post them here, plus the mode and setting you gave the pump set to.

This is what I have - 34W

https://product-selection.grundfos.com/uk/products/alpha/alpha1/alpha1-15-5060-130-99415672?pumpsystemid=2520079087&tab=variant-specifications

 

[Johntheo5]

Ok, can you now post a photo of the pump front showing which LEDs are illuminated, if its running at max speed, III, third led from left then, at 34W, its producing a flowrate of at least 0.65m3/hr, 10.8LPM, which should not be a problem if its the same at whatever combination of CH, UFH & HW you are running, if not, post the pump power in watts for each or any combination.

 

[sniktun1]

Will get tonight/tomorrow. It's pump speed 3 - so running flat out - I know it says 21W when running the UFH & CH together which is what i'm after. I wanted to use the max constant pressure setting but it's not fast enough for what I need.

 

[Johntheo5]

Its not the pump thats the problem here, if you look at the very bottom pump curve, then look horizontally to the right and where 21W inersects with the full speed curve then look vertically down and you will see that the pump is circulating virtually nothing, to prove it, just shut the pump outlet valve for 5 secs or so and see does the power change, what does it show on UFH only and DHW only?

 

[sniktun1]

Its not the pump thats the problem here, if you look at the very bottom pump curve, then look horizontally to the right and where 21W inersects with the full speed curve then look vertically down and you will see that the pump is circulating virtually nothing, to prove it, just shut the pump outlet valve for 5 secs or so and see does the power change, what does it show on UFH only and DHW only?

It's similar, I think it still said 20-21W when just on the DHW & UFH (I'm a bit limited on my running modes from how the pipework is connected, I currently can only run the UFH with either the DHW or CH. I can't run the UFH by just itself).

Well, tell a lie I could go round and close all the TRV's on the radiators, that would give me just UFH then. Don't think it's needed for this testing though.

I didn't get a chance last night to check the boiler sensors - i'm hoping thats the issue.