Vaillant ecoTECplus418 cycling on start up

[Johntheo5]

I'd be surprised if its a faulty boiler sensor, now.
How is the system pressurised?, if a open vented system see where the vent and cold feed are teed in, or if a sealed system, where the expansion vessel is teed in, you might indicate it on your schematic.
Obviously, of course, if a OV system, ensure the feed & expansion cistern contains water or lf a sealed system, that the the EV is pressurised to 1/1.5bar.

 

[sniktun1]

I'd be surprised if its a faulty boiler sensor, now.
How is the system pressurised?, if a open vented system see where the vent and cold feed are teed in, or if a sealed system, where the expansion vessel is teed in, you might indicate it on your schematic.
Obviously, of course, if a OV system, ensure the feed & expansion cistern contains water or lf a sealed system, that the the EV is pressurised to 1/1.5bar.

it's an OV system fed from a tank in the loft. Although I am ditching that next year and going straight off the mains with a pressure regulating valve.

 

[Johntheo5]

The normal way of doing this is, from the boiler (flow), you have the vent, then no mre than 150mm away, the cold feed and then the pump inlet (suction). The cold feed can often get blocked/partially blocked where its teed into the system OR you may have a combined vent&cold feed like mine, try and see what you have and sketch it.

 

[sniktun1]

Done more messing around - I think it's purely down to how long it takes to get the return temp up. Boiler only really starts firing when the return temp is ~25°C delta to the flow setpoint (roughly might be +/- couple dregrees either side). I've been testing running the system on the UFH only and take anywhere from 1.15hr's - 1.45hr's to get enough return temp to run the boiler constantly. If I try to start the UFH + Radiators together, takes like 3-4 hours.

With the natually colder return temp from the UFH it just takes way longer to do this startup routine. am guessing it is fixed in the startup routine for efficiency wanting to optimise the condensing part of the boiler.

Only way around this I can see is to stop this "efficiency startup" routine. Have attached the screen it just sits on for most of the startup - it's on the "radiator only" setting. i'm wondering if the "hot water only" setting would ignore this startup and just run the boiler for me. Thats what i'm going to try next.

 

[Johntheo5]

Can you take a photo looking down on top of the pump front, the power etc is stamped on the (black part of the housing top, this is to ensure that the the pump is a 6M pump, likee the one you have the link to, if it is, then 20/21W will circulate practically nothing even at full speed (III), you can see for yourself from the attached.

 

[Johntheo5]

OR/And, just shut the pump discharge (outlet) valve fully for say 5 secs, and see if the watts W change.

 

[Johntheo5]

Done more messing around - I think it's purely down to how long it takes to get the return temp up. Boiler only really starts firing when the return temp is ~25°C delta to the flow setpoint (roughly might be +/- couple dregrees either side). I've been testing running the system on the UFH only and take anywhere from 1.15hr's - 1.45hr's to get enough return temp to run the boiler constantly. If I try to start the UFH + Radiators together, takes like 3-4 hours.

With the natually colder return temp from the UFH it just takes way longer to do this startup routine. am guessing it is fixed in the startup routine for efficiency wanting to optimise the condensing part of the boiler.

Only way around this I can see is to stop this "efficiency startup" routine. Have attached the screen it just sits on for most of the startup - it's on the "radiator only" setting. i'm wondering if the "hot water only" setting would ignore this startup and just run the boiler for me. Thats what i'm going to try next.

On start up all the boiler is looking for is that the flow temperature does not exceed SP+5C, (70C in your case), otherwise the burner will trip and the boiler will just keep doing repeated recycles. The (all) gas boiler(s) fire up at ~ 65% of rated output so 11.7kw, ideally, the boiler shouldn't exceed the SP,65C, assuming a flowtemp of 35C, (dT 30C), at start up then the flowrate will have to be, 11.7*860/60/30, 5.6LPM, Vaillants then maintain this 65% output for 60S (unlike other boilers which modulate after 10secs or so) so practically speaking you should have a flowrate of ~ 10.0LPM.
The minimum output of the 418 is ~ 5.0kw, assuming the flowrate is 5.6LPM, then the dT (at 5.0kw) is 5.6*860/60/5.6, 14.3C which you do seem to to be getting eventually.
These boilers don't have any "efficiency startup routine" but if the dT is ~ > 28/30C, just after start up then they will more than likely trip the burner and cause repeated recycling.
This is why I keep harping on ad nauseam about the pump flowrate, so you might, at the very least, just shut the discharge valve briefly and note the pump power, 21W will, theoretically, only result in a flowrate of, at most, 2.0LPM on speed III.

 

[DP]

The boiler is shutting down because the differential between flow and return is too great
Series 4 were notorious for this and the plumbing need3d to be correct else at times a LLH was required to calm the boiler down
Pump often is the issue where system resistance resulted in boiler cycling like it has been mentioned

 

[sniktun1]

Yep, confirmed - it's the return temperature being too low from the UFH system on startup. Just takes hours because the boiler won't run and am guessing is more setup for radiators Everything else is fine - not sure what I can do about this?

Would a low loss header before the UFH system help? if the boiler isn't runiing for 95% of the time during this startup routine to get the Delta, not sure a LLH would do much.

There isn't another setting in the boiler so it know's theres an UFH system with lower return temperatures, is there?

Once it's up to temperature it runs perfect. Very frustrating

 

[Johntheo5]

I still think you have a circulation problem.
The boiler fires up at 60/65% of its max output, say worst case of, 18*65%, 11.7kw, to avoid burner trip due to a high dT of a assumed 25C (but its probably ~ 30C) then the circ flow through the boiler must be, 11.7*860/60/25, 6.71LPM, you said in one of your posts that you are getting 65C/25C, dT 40C, on start up with both systems, so the circ flow can only be, 11.7*860/60/40, 4.2LPM, its thermodynamically as simple as that.
Just confirm again, but preferably take a photo of the top of the pump showing its output (either 35kw or 45kw) also post a photo of the LEDs with the pump running (and pump power W)

I wouldn't rush off just now and install a LLH but it would get the dT down if its primary flowrate is greater than the secondary flowrate as it will be recirculating some of the boiler flow (hot) water to mix with the cold return water from the system (via the LLH secondary side) to reduce the dT.

 

[Johntheo5]

Just to emphazise,
If you have the 45W Alpha1 then it can't have been set to its highest fixed speed 3, (CC3) as the power with a closed valve is 33W, you are seeing 21W, so if its a 35W model then still circulating practically nil at 21W as its closed valve power is 20W on (IF) max speed 3.