VAILLANT 831 not reaching temp

[Coldone]

It seems to stay hot with the tap half open
But Iv not run it for long

 

[CBW]

The reason I asked about the preheat was to determine something else. Hot water on a combi depends on a few factors, the incoming cold temperature, flow rate, pressure and sometimes some other factors

 

[Johntheo5]

If it does stay hot at a moderate flow but runs hot/cold at a high flow then it may be a fouled PHEX (plate heat exchanger), I think these boilers also have a canoe filter in the primary flow somewhere which may be partially blocked.
It may not tell you anything but look at d.40 while running on DHW,
Also try running it on minimum DHW temperature, 35C at the different flowrates.

 

[Coldone]

Thanks I’ll give it a try tomorrow

 

[Coldone]

Ok so canoe filter seems fine
when the hot water is run the display goes up to about 60 then comes back down with the flame still on
It goes all the way down to 41
Then it flicks between 70 and the water temp eg 39

If I press I s.53 is shown

 

[Coldone]

If I turn the gauge down to 50 it does seem to hold its temperature better

 

[Johntheo5]

Thats interesting as S.53 can mean the flow/return dT (primary side) is excessive, you might repeat the test and look at d.40 & d.41, I'm assuming that the sensors, on both, are positioned to read "correctly" on both CH & HW.. Normally, if the PHEX is blocked then the primary flow temperature might rise to > 85C before burner trip. Of course it might be the diverter valve or something else, there are lots of post with your problem, see "Similar threads" below.

For interest I have some numbers here from a WB boiler taken with both a fouled PHEX and a new replacement. The HW supply was fine (55C) at moderate flowrates but when the flowrate increased then the primary flowtemp exceeded 85C and tripped the burner.
Fouled PHEX. mains in/out 19.0C/55C, primary flow/return 68C/50C DHW flowrate 4.5LPM, boiler output 11.3kw.
New PHEX. mains in/out 19.6C/55C, primary flow/return 64.5C/49.5C DHW flowrate 9.9LPM, boiler output 24.06kw.

 

[Johntheo5]

If I turn the gauge down to 50 it does seem to hold its temperature better

Is that at all DHW flowrates?.

 

[CBW]

S53 is usually a circulation issue.

 

[Coldone]

Is that at all DHW flowrates?.

Yes

 

[Coldone]

That’s interesting
The plate heat exchanger has been off and cleaned
May be worth replacing

 

[CBW]

May be worth replacing

Expensive cost if not though.....

 

[Coldone]

True….
Although I’m close to changing the full boiler with everything that’s gone on with it recently

 

[Johntheo5]

Just check d.40 & d.41 on DHW duty, might point to something.

 

[Coldone]

D.40 = 58
D.41 = 50