Greenstar 28i junior DWH problem

[Darren Jackson]

Hi, I have a Worcester greenstar 28i Junior combi mk111, that works fine in CH mode but locks out in DWH after 1-2 mins, Slow Flashing blue (light mostly on) with flashing reset showing, I have re pressurised the expansion vessel as it was flat and Pressure is reading about 1.2bar now and holding. The fan was pulling minus 1.2mbar, so cleaned out the hex including the burner and it is now pulling a healthy minus 7.2mbar, there wasn't much left of the electrodes when I took it out to clean the hex. So I fit new ones along with a new seal of course. But alas still locks out under DHW demand. changed the high limit stat still no joy. No water leaks. wiring harness and connections are good and pump runs good with no unusual noises and the flue is clear. So I bit the bullet and changed the control board... Guess what... still locks out when hot tap running. So if anyone has any suggestions I would love to hear them. (me thinks maybe plate exchanger now)??? after that I,m stumped.

 

[ShaunCorbs]

Gas pressure on full load ?

 

[townfanjon]

HW thermistor , maybe or blocked plate

 

[Last Plumber]

DHW Flow switch/sensor ?
Have you checked that ?

 

[kb76]

As above check plate and thermistor, never had a faulty thermistor on one of these but always a first. The other thing to check is the rubber flexible pipe behind the fan...known for splitting. Had this and been fine on heating but when fan ramps up on hot water, air/gas mix is incorrect and will lock out. Can you analyse it in HW before it locks out?

 

[Darren Jackson]

Gas pressure on full load ?

Yes mate.

 

[ShaunCorbs]

Yes mate.

What was it ?

 

[Darren Jackson]

DHW Flow switch/sensor ?
Have you checked that ?

Yes mate I have thanks and all checks out ok

 

[Darren Jackson]

As above check plate and thermistor, never had a faulty thermistor on one of these but always a first. The other thing to check is the rubber flexible pipe behind the fan...known for splitting. Had this and been fine on heating but when fan ramps up on hot water, air/gas mix is incorrect and will lock out. Can you analyse it in HW before it locks out?

Ah not tried that will try to check the air gas mix before it locks out and check the tube at the back, I normally analyse after about 10mins use, but not got that time as it locks out well before.

 

[mfgs]

That's an overheat lockout isn't it? Check for blockages and the three sensors. Although normally when its a plate problem it will pass down the flow, have you checked for that?