Vaillant boiler always exceeds set temperature

[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?

 

[ShaunCorbs]

Once the nest is set for water on eg calling the cylinder stat takes over and controls the temp

 

[John.g]

Suggest running off say 10 to 15 litres of HW with tap opened for 5 minutes or so, then programme the CH + the HW to come on together then switch off the CH only after 3 or 4 minutes and see if the boiler continues to fire until the cylinder is fully heated.

 

[kilodelta]

Thanks for all your help everyone. The house is thankfully warm and we still have hot water so I can’t complain at the moment, even if it doesn’t seem to be operating as it should.

I dropped the power to 8kW to see how things runs.

I assume it’s safe to play with the power output?

 

[ShaunCorbs]

Yes as it’s just settings will soon tell you no if it doesn’t like one

 

[V1ks44]

Do you have the spec for the ufh / what flow rates do the ufh flow meters show with everything on / calling ufh wise ?

Attaching photo of the UFH located under my stairs, i cant rem for 100% the loop lengths of the pipe run. Kitchen is split in into 2 circuits i know one was about 70m and the other was about 30-40m. Same with the living room they were about 75m each.

I have been playing around with the Flow rates recently but at present they are set to around 3l and pump speed to 3. I have recently tried to have the rates at around 2l with the pump speed on 2 but the kitchen seem to take a very long time to warm up due to higher heat loss because i have very large 3 panel bifold doors.

Let me know if u need any other info...ill try my best to get this..

 

[V1ks44]

Suggest changing the Kw from Auto to 6kw, also the anticycle time d.002 might have to be increased from its present settiung of 20 mins, you should see a table somewhere that gives the actual anticycle time as it looks at the target flow temp and the set anticycle time to give the actual time.

Would doing this have any negative impact on the boiler?

Attaching the photo of the table for anticycle times, so at 60c as set currently would it be?

 

[ShaunCorbs]

Works out to around 6 kw on the ufh maybe

You want a difference of 7 degrees between flow and return on each circuit when warm so alter the flow meters to achieve this if you can

 

[John.g]

No, it shouldn't, 60C+20C(time) = anticycle time of 6 minutes, 60C+25C=7.5minutes & 60C+30C=9minutes.

I would increase that (d.002) initially to 25, a 7.5 min anticycle time, as it might reduce the starting temperature low enough to get the boiler away without exceeding the target temperature.

 

[V1ks44]

Works out to around 6 kw on the ufh maybe

You want a difference of 7 degrees between flow and return on each circuit when warm so alter the flow meters to achieve this if you can

yes thats what i have been trying recently to get, i dont know what my return temp are to be honest is there a way to check this?

when i checked the diagnostic menu on the boiler d.41 (i think it was) it was showing me like 55c, im not entirely sure if this is the return from the manifold or the whole system? as it seemed very high because when i touch the return copper pipe (one on the left in the pic i uploaded) its not very warm even when the UFH has been on for few hours. it could be due to the way the pipes are laid (single serpentine).

I dont have a temp gauge on the return part of the manifold, im certain the difference is higher than 7c because the UFH pipe layout is Single serpentine (this was a total mess by the plumber who did it and i didnt know anything about ufh at the time to correct anything). however i cant change this now so im just trying to find a way to get this best as i can. I will increase the flow rates and see if this changes.

What are your thoughts on the suggestion by John.g to change the boiler to 6kw and increase the Anticycle time. Im thinking to give this a try tomorrow morning and see how the system behaves...

 

[V1ks44]

No, it shouldn't, 60C+20C(time) = anticycle time of 6 minutes, 60C+25C=7.5minutes & 60C+30C=9minutes.

I would increase that (d.002) initially to 25, a 7.5 min anticycle time, as it might reduce the starting temperature low enough to get the boiler away without exceeding the target temperature.

Thanks John.g - i will give this a try tomorrow morning and report back.

I know a buffer tank is an option but i just dont have space near the boiler to fit this as this would solve the anticycle issue and allow the boiler to run in fully condensing mode.