Is it normal for Vaillant boiler (18kwh) to have flow temperature at 48c after 1 hr of running?

[Samsg30]

Hi,

I have a vaillant system boiler of 18kwh size and even after it's on for more than an hour, the flow temperature shown on the front panel is at around 48c with gas burning very slowly whilst whole house is cold.
Every few minutes (not timed the interval), gas burning intensity increases which pushes flow temperature very quickly to 70-75c (the max limit set) and it comes down back to 48c within seconds.
Then after a couple of minutes, the regular flow temperature rises by 1c to 49 and repeat the same cycle as above. But even after continuously running boiler for several hours, I don't see boiler flow temperature reaching to above 55ish C

Practically, after running heating for 2 hours, radiators are just warm when the actual flow temperature settings should allow it to stay at 75c to make them hot enough to heat up the house.


Is this a normal way of functioning of vaillant boiler or you think there's some fault?

 

[Johntheo5]

Definitely a fault I would think, could be anything from the boiler flow temperature sensor to a PCB fault.
Can you access the "d" parameters?,
Post the Vaillant make/model and age.

 

[Samsg30]

It's "ecotech plus 418" and I can access the d menu.

Looking at the the flow (d.40) and return (d.41) temperature, there seems to be the difference of 12c

 

[Harry Bloomfield]

What controls are you using for the heating system, are they Vaillant's own controls?

 

[Samsg30]

I have got basic controls not specific to vaillant
The timer is "honeywell central heating timer st6400c1003" like below image

The thermostat is "Honeywell manual thermostat" like below image:

It has a 3 port valve from Corgi
Grundfos pump set at highest pump settings

All the radiators have drop pipes as it's a bunglow so red pressure cylinder, pump, pipe work etc is all in the loft and pipes are dropped from the ceiling to the rooms where radiators are.

 

[Harry Bloomfield]

Has this system ever worked properly/ is this a new problem?

There is a setting in the boiler, for maximum flow temperature, that might be set at 50/55C. Do you have a copy of the manual?

 

[Johntheo5]

Has this system ever worked properly/ is this a new problem?

There is a setting in the boiler, for maximum flow temperature, that might be set at 50/55C. Do you have a copy of the manual?

Thats d.20 I think but only applies to system boilers.

 

[Johntheo5]

It's "ecotech plus 418" and I can access the d menu.

Looking at the the flow (d.40) and return (d.41) temperature, there seems to be the difference of 12c

Check
d.0 Heating part load
d.5 Target temp
d.17 0 = set to flow temp control. 1= set to return temp control. (default 0)

I read somewhere that changing d.0 to Auto and back to set kw, 18.0 or lower, and vica versa somrtimes gets rid of the above problem for unknown reasons.