Vaillant 828E boiler cutting out with central heating on

[tom-225]

Hi all, this is my first post here and I am grateful for any help.

I have the titled boiler in my house that I moved into in May. My problem began with two radiators that had issues, one is stone cold and the other gets only luke warm. I have bled these radiators and I am getting pressure out of them so the flow is there. The other 5 boilers in the house get red hot.

I looked at the boiler and noticed this had zero pressure, so I filled this to 1.4 bar. Tested again and still the same issue to the two rads. I then went around and isolated the 5 working rads to see if I could get anything to the other two but this is where I ran into another issue. The heating on the boiler display gets to 73 degrees pretty quickly (<1 min) and then cuts out, with a code s.7 for pump overrun. The boiler then drops temperature and starts up again, and keeps repeating this procedure (i guess infinitely). I have a tado smart thermostat and this looks like it is permanently calling for heat.

I have then gone around my house and opened every radiator valve and the boiler starts up fine and does not cut out, providing heat to the 5 radiators but not the other two.

Any help is greatly appreciated, thank you

 

[cross thread]

Check lockshield valve on rads are open if thermostatic valves remove the heads and check the pin goes up and down.

 

[tom-225]

would this cause the boiler to be short cycling? Surely you could isolate a radiator and the boiler would still work?

Thank you

 

[Johntheo5]

The challenge with gas boilers is that they fire up at ~65% of rated output before modulating down but some Vaillants maintain this high firing rate for up to 60 secs by which time the flowtemperature may exceed the setpoint+5C and burner trip, the circ pump then runs (pump overrun) for the anticycle time after which the burner refires, you require a high circulation rate which you may not be getting with those two rads only on, also the minimum output of the boiler is 10.4kw which 2 rads will certainly not emit but the boiler should definitely run for a few minutes before the boiler temp rises to its burner trip point.

What does the boiler temperature fall to before refiring??, the lower the better, say 35C or so, to help get the boiler away in that 1 minute or so.
Also check d parameters d.00 & d.01, d.00 can be set to limit the max output (range rating), so may help if you set that to its lowest setting, possibly 10/11kw, also check d.01, the pump overrun time and note its setting, this can be increased if the boiler temp is still high (ish) on burner refiring.

 

[tom-225]

It’s now escalated that even with all radiators open the boiler is short cycling, it gets to around 70 quickly and then drops 40 ish before restarting.

Thank you for the help!

 

[Johntheo5]

The boiler itself may now be air locked since you topped it up from zero to 1.4bar, there should be a AAV (automatic air release valve) in the boiler or maybe accessible from the boiler top, these are generally/often shut off after commissioning, is the boiler running OK in DHW mode?.

 

[tom-225]

Perfect thank you, yes it is completely fine for hot water

 

[Johntheo5]

I take it that the boiler ran fine again after top up, with 5 rads opened up, but boiler cycling now again?, if so, strange, except air has built up somewhere in the heating system. If there is a magnetic filter installed you could try venting that. Even though I dont't think think this will cure the problem, maybe note and the reduce d.01 to minimum as a temporary measure.

 

[tom-225]

Yes for a short time (5 mins) and I presumed it was ok. And then when I next went to put the heating on, prepared to live with those two rads off for the time being, it is permanently cycling

 

[Johntheo5]

If the burner is tripping at 73C then the boiler target (setpoint) temperature should be, 73-5, 68C?, suggest increasing the setpoint to its max of 82C, (burner shouldn't trip until 87C), if the boiler does then run constantly, can you note the flow/return temps, d.40/d.41 and also d.00, part load setting.

 

[tom-225]

Thanks a lot John, I will do that this evening and post back

 

[tom-225]

Ok so I have turned boiler to max on CH

This help at 78 for a couple minutes and then went up to 82 and cut out, it then cooled to 62 and fired back up, getting to 82 in around a minute, when it then cut out again, and the cycle begins.

Another thing to note is the boiler pressure started at around 1 bar when off, and keeps creeping higher and higher, to the point where it was 2.4 bar and I then turned it off.

D.00 is set at 15
D.40 is 78
D.41 is showing no numbers but instead -

Thank you

 

[Johntheo5]

I wouldn't be too worried as long as the boiler pressure doesn't exceed 2.7bar, you topped up to 1.4bar but its now down to 1.0bar??.

The burner shouldn't cut out until it reaches Targettemp+5C, it doesn't modulate until it reaches the Targettemp and normally will overshoot by 2 to 3C but will never get away if its tripping at the Targetemp, any modulating boiler can't have the burner cut out at the Targettemp, what was the Targettemp previously?, suggest resetting it to say 70C, also check d.02, the anticycle time, I would like to see the flowtemp down to ~ 30C on burner refire as this will give a better chance of modulation before burner trip, the anticycle time varies with Targettemp, see below, if d.02 is set to 15C then the actual anticycle time is only 2.5mins at a targettemp of 70C and only 1.0mins at 75C so suggest setting the Targettemp to 70C and d.02 to 45mins, this should give a actual anticycle time of 5mins to get the flowtemp down to 30C ish. If your MIs show a different chart to the one below, just set the anticycle time to give you ~ 5min anticycle time at 70C.

 

[tom-225]

Thanks a lot for the info, definitely know your stuff!

It’s down to about 0.8bar this morning so something more is going on with this. I need a specialist really, you don’t live near to Halesowen do you lol, or know of anywhere I can find someone who is reputable, failing that I might just bite the bullet and get the boiler replaced.

 

[Johntheo5]

The EV may be failing or losing its precharge air pressure, the boiler pressure relief valve (PRV) may have lifted at 3.0bar and is now leaking, you should be able to check this, you should find a small bore copper pipe sticking out (pigtail) of the wall behind the combi, top the boiler up again to 1.4bar, and see see if any leakage.