Getting the Room Temp Right

[Al Growler]

Have switched from a conventional boiler to condensing and have noticed the difference in the room stat temp to required temp. I initially calibrated room stat to make sure that was OK.

At the moment, I have the boiler coming on when temp reaches .5 deg below desired temp. (20.5). After the temp in the room has dropped to this level it takes the boiler around 20 mins to get up to 21 and then goes off. However the temp keeps rising after this point up to more than 22, which is a bit too hot. (using a digit thermom to confirm temp)

At the moment it seems I either live with that, or turn room stat down so boiler doesn't come on until the room is colder and goes off before it reaches desired temp.

Any other suggestions would be gratefully received.

 

[mogget]

I don't know what room stat you have but if it has a TPI mode, try that, otherwise you could change it for a stat with TPI control. They still tend to overshoot a little but tend to settle down to a stable temp.

TPI(time-proportional-integral) is essentially very slow pulse-width-modulation for the boiler.

 

[muggles]

What boiler and roomstat do you have?

 

[Al Growler]

Thanks for the quick replies. Boiler is a Vaillant Ecotech Plus 418. Stat is just a standard wired Honeywell.

 

[mogget]

Honeywell DT90e if it isn't already.

 

[Gasguru]

See if the stat has a Neutral connected...measure the voltage to confirm it's also connected at the boiler end. The Neutral connection allows operation of the "anticipator" resistor...it heat up during stat demand and helps prevent the overshoot.
If the stats mounted on a stud wall ensure the cable entry through the plasterboard is completely sealed...the tiniest draft will thwow the stat out.