A question related to CO2 FGA failing on minimum rate.

[DTBoilerDude]

I am going back to a Worcester Greenstar 30 CDi Classic that's failing a CO2 emissions test on minimum rate only. I do adjust the gas valve but there is zero effect. I haven't mixed up the "min" and "max" adjustment screws.

What could be causing this boiler to fail on minimum only? It's perfect at max rate, heat exchanger is clean (fan pressure test) and everything else is OK.

 

[ShaunCorbs]

Have you cleaned the hex ?

 

[DTBoilerDude]

Have you cleaned the hex ?

no, I did a fan pressure test and it was OK, above -4.1 mbar, do you reckon it could be that?

 

[DTBoilerDude]

Have you cleaned the hex ?

Ah... I might have figured it out. The air/gas adjustment screw isn't located on the gas valve for this make/model! There's a screw beside the HEX... I will check this.

 

[ShaunCorbs]

That’s the max adjustment eg behind the plastic grey plug

 

[DTBoilerDude]

That’s the max adjustment eg behind the plastic grey plug

Yea, behind the plastic plug. I'll make adjustments tomorrow and see what happens. I'll clean the HEX. Or I'll just call WB? Completely baffled :-( not like I haven't adjusted Air/Gas before.

 

[ShaunCorbs]

Which did you adjust for high ?

Tbh would clean and fully service the boiler first before making any adjustments

 

[DTBoilerDude]

Nothing, I put the adjustment key in to adjust what I thought was the low. 1/4 turn, no change. 1/4x2 more. No change, so turned it back to where it was and started looking at the boiler baffled. Maximum rate was OK. I thought the little 2.5mm screw just above the the adjustment screw on the valve was the minimum rate ffs. I'll do this tomorrow. Lesson learned. READ the manual regardless. The application for air/gas ratio is different on this, it's the first time I've encountered this.

 

[ShaunCorbs]

Centre behind the Allen bolt is low it’s touchy so 1/16 at a time and let it settle over a few mins for adjust it another 16th

 

[DTBoilerDude]

Centre behind the Allen bolt is low it’s touchy so 1/16 at a time and let it settle over a few mins for adjust it another 16th

sorted , perfect readings. Yes, the 1/16th turns is actually still crazy heavy on the adjustment believe it or not!!! But when you adjust the max you have to adjust the min and likewise. It was just very sensitive to the adjustments. That's the first air/gas ratio I've had to call the manufacturer.