Vaillant boiler not hitting target temp

[Pjweaver]

Hi - hopefully someone will be able to help

Have a Vaillant boiler which generally works fine but never seems to reach target radiator temp. Hot water supply to taps etc is fine.

For example it’s about 1 degree outside today. Thermostat is set to 23 and triggering the boiler. Despite this the boiler is only heating the rads to around 40 degrees C and the gas burner symbol on the LCD display seems low whereas surely it should burning harder to hit the required temp?

Not an expert but this doesn’t seem right.

Any suggestions much appreciated. Pics of current temp, desired temp and boiler activity attached

 

[Dan Robinson]

Search the forum for range rating Vaillant boilers.

 

[Pjweaver]

That’s great. Much appreciated. I thought this was the case and had read briefly about this before. One more question though. The thermostat is a cheap salus one and thought it just sent simple on/off trigger to turn boiler on or off until it reaches required room temp. How does the boiler know what to change flow temp to? Eg low flow temp for 21 degree room temp or higher if we whack the thermostat right up?

 

[John D v2.0]

press the top right button by the radiator symbol on the boiler a couple o ftimes. Check the flashing figure, it should be 50 to 60+ for a condensing boiler
PS I'm not a registered gas fitter

 

[Wildcat1961]

Check D.00 isn’t set to ‘auto’.

 

[Pjweaver]

press the top right button by the radiator symbol on the boiler a couple o ftimes. Check the flashing figure, it should be 50 to 60+ for a condensing boiler
PS I'm not a registered gas fitter

Thanks. Know how to adjust what it should be but just wondered how the boiler knows to heat low if I’ve asked for 23c on the thermostat as opposed to heating high if I’ve asked for higher

 

[Pjweaver]

Check D.00 isn’t set to ‘auto’.

Thanks. Been reading about this on another site. Under installer menu? Seems it’s a bit of a know issue. Normally gradual slow heating seems fine but a real cold snap here at the moment. Woke up this morning and thermostat is saying 18.5 despite thermostat being set to 23 overnight. I’ve changed the setting from auto to 20kw to see if this changes anything. In a 4 bed Victorian house with single glazing and a variety of drafts from doors/vents so understand there will be a certain amount of heat loss but 50c flow temp just isn’t cutting it despite me changing to 65c on the boiler. Do you think 20kw will be enough to sort it? Can always flick back to auto once we’re back in Spring

 

[Dan Robinson]

In this weather it might not be possible for your radiators to get to 23.

 

[John D v2.0]

Thanks. Know how to adjust what it should be but just wondered how the boiler knows to heat low if I’ve asked for 23c on the thermostat as opposed to heating high if I’ve asked for higher

It doesn't, a standard thermostat modulates the boiler by switching it on until the temperature is reached, and then off until it drops.
It's your responsibility to make sure the heat output is enough to get there.

Sounds like your rads are not sized for a condensing boiler in your current house. If so you'll need higher flow temps or change to bigger rads. Or morew insulation.

 

[Wildcat1961]

Thanks. Been reading about this on another site. Under installer menu? Seems it’s a bit of a know issue. Normally gradual slow heating seems fine but a real cold snap here at the moment. Woke up this morning and thermostat is saying 18.5 despite thermostat being set to 23 overnight. I’ve changed the setting from auto to 20kw to see if this changes anything. In a 4 bed Victorian house with single glazing and a variety of drafts from doors/vents so understand there will be a certain amount of heat loss but 50c flow temp just isn’t cutting it despite me changing to 65c on the boiler. Do you think 20kw will be enough to sort it? Can always flick back to auto once we’re back in Spring

Various settings (appliance configuration) can/should be changed in order to suit the system Ie: pump/anti cycle time etc etc.
If you’re not sure then leave it.