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The problem is that its turning down far too much, it should only be turning down on Auto to a speed that doesn't result in the residual head falling below 100mb, you can see from the attached, this corresponds to ~ 50% ("Minimum with Auto"), with my system corresponds to a flowrate of 10.5LPM...

The available flowrate at 15% output based on a Vaillant boiler's HEX and my rads/system dT is (only) 2.8LPM, depending on the number of rads and output you can haveanything up to 5.8kW with 9.5/11.5 "kW of rads", say 6 or 7 rads but the (residual) head available is only ~ 0.07M, 7mB, not a hope...

Another reason I configured this spreadsheet is that someone with the latest Vaillant 624 couldn't get his boiler to control on flow/return temperature spread dT, settable between 10C and 20C, it doesn't, by design, allow the pump speed to fall any further once the residual head falls to 100mB...

No, its a bit generic but still I would think quite accurate, I just based it on a (Vaillant) HEX dP of 3.1M at 20LPM and a (my) rads losses of 3.44M at 20LPM, the rad calcs obviously have no effect on the residual loss, have a look at the spreadsheet which I put together some time ago and am...

I’ve been asked numerous times to explain the residual pump curves published in some Vaillant Ecotec Boilers MIs, I’ve looked at them several times but afraid I can’t make any sense of them. Someone on here might explain them or/and post similar curves from their (Vaillant) HP circ pump curves...

Asking for someone to check their Boiler.;)

Can you access the fan impeller, if so, use a screwdriver to see if it will rotate without much effort. (Ensure boiler switched off)

Some one might have a look at the pump speed, d.015, in their boiler menu if they have Auto selected in the pump settings, (last version of the 624, not the present), Someone is getting a constant 15% in Auto which is, theoretically, not allowed. It looks like ~ 40% is the minimum speed in Auto.

Just start up any zone and lokk at and post a snapshot of the LEDs. IF its running with one solid green followed immediately by two solid yellow LEDs then its running at full speed which can lead to excessive noise, while its running, just press the setting button briefly to change settings...

It's the blank plug,1, immediately below the adjusting screw, with the boiler off put something underneath the plug and slacken it right back until oil dribbles out, then retighten it, do not do this if the oil tank is below the level of the boiler. You could also use the vacuum connection plug...

Why are these heaters rated at 240V? The supply voltage in the UK has been, nominally, 230V for years, that heater will only give 4.95kW at its proper 230V rating, all the above flowrates should be X by a factor of 0.958.

You could also run that UPS3 on either CC1, one sold green LED at 4.2M which will be sufficient for most systems, or, certainly, CC2, one solid green followed by one solid yellow LED at 5.2M which will satisfy 99% of systems, should have no need to run at CC3, one solid green followed by two...

I had one of those EPH stats, they certainly switch around the temperature setting on the dial

Not a great test but just fill that pocket very very slowly with the immersion upright and look for any signs of dampness anywhere along the tube and especially at the bottom.

Thanks, can you see signs of corrosion on the end of the tube/pocket where its crimped, its difficult to see in photo 3. Is that tube touching any part of the (heating) element?.

Did you notice any corrosion or whatever in the stat pocket? One migh think that faulty wiring + electrolytic action causing a current flow to earth might cause the RCD to trip at 35ma but maybe a far less current will do the damage, but may require a copper stat pocket. Did the plumbers...

What was the pressure this morning when you fired up the boiler?, check it again in the morning before firing the boiler.

Unlikely, but possibly one or both filling valves left open. What is the boiler pressure now now and is the system hot or cold and is CH on or off?.

Correct, I got mixed up with the switched live colour of blue.... so 230V constant between the brown and the blue (N) on the left, irrespective of the stat setting, and 230V between the switched live, blue on the right and blue (N) on the left with the stat calling for heat and 0V when not...

If you can use a M.meter, check for 230V between the brown and the blue on the far right, ter 2, while turning the dial up and down.