Valliant boiler never reaches flow temperature set on boiler

[ThatUKDiyGuy]

Hi everyone

I have a Vaillant thermoCompact 637E. If I set the flow temp on the boiler dial to say 67 it only gets up to 61/62 on the display. If I set it to say 72 it sits around 65/66. I've balanced the system and aimed for around 10-12 deg differential on the rads. When checking the differential at various times on the boiler diagnostics it seems to range between 10-15 deg.

Is this normal behaviour for the boiler display to always be 5-6 deg less than the value you set?

I have an infrared thermometer and it seems to suggest something around 70-74 deg when measuring flow side of rads. I know this maybe answers my question but I am curious to understand if this behaviour is right.

Many thanks

 

[D_Hailsham]

IR thermometers accuracy depends on the surface it is pointing at. They are calibrated for a black surface. If the pipes coming out of the boiler are unpainted copper but the rad pipes are painted, this would account for the difference in temperature readings.

Wrap the pipes in some black insulating tape and then take readings.

You will get a better reading if the IR thermometer is almost touching the pipe. This is because it collects over a fairly large area (like a torch beam) and averages the results.

 

[ThatUKDiyGuy]

What I have been doing is measuring in the bottom corners of the (white) rads and aiming for 10-12 deg drop. Seems to work pretty well - I accept it won't be super accurate.

Ok. I'll need to get some black tape though to do what you suggest.

In the meantime, does the boiler behaviour seem correct?

Thanks

 

[Agile]

An IR thermometer is totally useless for meaningful heating measurements. You need a contact thermocouple thermometer!

Its quite common for many boilers to never reach the set temperature. Thats not very important because you just set the temperature to give the desired flow temperature.

Tony