Vaillant boiler always exceeds set temperature

[kilodelta]

Thanks. I brought up the installer menu and it asks for a code. Do I need to enter the value “17” in order to check the diagnostics code? I’d prefer a read only menu so I don’t screw anything up.

When the boiler engineer did the annual check last year he did the service but we’ve always had low gas pressure. Like every time a gas safe engineer comes by they called Cadent. This time, there was a team in the area and they move the gas meter outside and fixed the problem that day.

I started noticing the 5C cycle issue in the next couple of weeks. Could the service and/or gas pressure issue be the cause of issue? I called the engineer and he said it was normal, as above.

Should I open all the TRVs to full heat? I did dial them back.

 

[John.g]

I think so yes, 17 is the passcode, you can then just look at the appropriate parameters without changing anything.
If the 5C cycle issue is coming up while the boiler is firing for > a few minutes then the heating demand is lower than the minimum boiler output and it will cycle. (normal)

 

[Deleted member 120897]

Yes 17 is the installer code.

 

[kilodelta]

You can look at these and change, if required, page 13 of the attached, suggest you have a look and note the following parameters, page 28, and post back.

d.00 Heating maximum output kw
d.02 Maximum burner anticycling time in heating mode. mins

D.000 = 30kw
D.002 = 20 minutes

 

[John.g]

You have 7 rads?, so a max output of ~ 14kw, d.00 should be reduced to say 16kw IMO.
The 20 min anticycle setting is a actual time of 4.5 minutes with a SP temperature of 65C so probably OK.

 

[kilodelta]

You have 7 rads?, so a max output of ~ 14kw, d.00 should be reduced to say 16kw IMO.
The 20 min anticycle setting is a actual time of 4.5 minutes with a SP temperature of 65C so probably OK.

Is this something I can change myself or should I call an engineer?

 

[John.g]

Change it yourself, just follow the instructions.

 

[kilodelta]

Change it yourself, just follow the instructions.

Thanks. Finally, this is a system boiler so it powers 7.5 radiators but also a 150L unvented cylinder when the hot water is on. It’s the same circuit ie we don’t use the hot water function of the boiler. Does that change the equation?

 

[John.g]

No really, the main purpose is to see does it stop the cycling, it can always be changed afterwards.

 

[kilodelta]

Thank you. I had my annual service done and the engineer advised exactly as you said @John.g. The setting is now at 16kw.

 

[kilodelta]

@John.g This is still happening 🤯 Is the boiler supposed to regulate its temperature so that it maintains flow temperature? If so, why isn't this happening?

If I only have 6 radiators on, can I reduce the the KW from 16 to something even lower?

 

[John.g]

There can be two reasons for this, one is that the boilers minimum output is greater than the heat demand from the rads, in which case the burner will cut out when the flow temperature exceeds the boiler setpoint temp by 5C. The other reason is if the burnercuts out within a minute or two of firing up if the boiler hasn't modulated down fast enough. You can try I creasing the boilerflow temperature to 75C or more and yes you can kw from 16kw to say 8kw, might help

 

[kilodelta]

Thanks! I’ve noticed something different. The issue only seems to happen when the Nest has the hot water on. The hot water goes to an unvented cylinder.

When the radiators are on, the 64C is maintained. When they are off but the unvented cylinder is heating, the boiler turns on, goes straight to 70C, cuts off, then repeats over and over again. is the unvented Cylinder broken? I thought if the water was on but it was at temperature, it would trigger the boiler to go on.

 

[John.g]

The HW (unvented cylinder ) temperature setpoint temperature should always be set lower than the boiler water temperature, ideally by 10C lower but certainly at least 5C lower.

 

[ShaunCorbs]

Reduce the boiler output to 6 or the lowest you can do

 

[kilodelta]

The HW (unvented cylinder ) temperature setpoint temperature should always be set lower than the boiler water temperature, ideally by 10C lower but certainly at least 5C lower.

The attacher is on the cylinder… it’s set to 65C (I think it always has) and this is higher than the boiler flow temperature. This should be dialed down 5-10C?

 

[ShaunCorbs]

The attacher is on the cylinder… it’s set to 65C (I think it always has) and this is higher than the boiler flow temperature. This should be dialed down 5-10C?

Boiler should be 65 cylinder stat wants reducing to 60

 

[kilodelta]

Still happening with the boiler set to 64 and the cylinder to 50.

What is the expected behaviour? Is the boiler supposed to turn off when the cylinder signals it’s at temperature (but the Nest indicates it’s still on, waiting to signal
To the boiler that it needs to heat its water?)

Should I call an unvented cylinder engineer or should I try reducing the boiler to 8KW?

 

[ShaunCorbs]

Nest will still signal it needs heat as they don’t talk to each other the stat controls the temp

 

[kilodelta]

So as long as the Nest has the hot water on, the boiler be on to heat the water in the cylinder? What happens when the cylinder’s water reaches the cylinder’s set temperature?