Boiler shutdown

[Backboiler]

Hi guys, hope some bright person on here can shed some light on the cause of this problem.
I did a gas safe check on a Ravenheat Primary AAA on Wednesday and had to replace the gas valve yesterday due to it passing on lockout. As usual, before working on an appliance I checked the basic operation to ensure it fired up on demand etc. System is Y plan open vented. On completion of replacing the gas valve I switched on CH and HW demand and the boiler ran perfectly. After half an hour I switched off CH demand but left the HW demand on. After 10 minutes the boiler suddenly shut off. The kind of shut off that sounds wrong. The lock-out light did not illuminate. According to MI's this warning lamp should light on flame failure or overheat. I know the bulb works in the lockout warning lamp as it illuminated on flame failure while re-checking safety device. Before doing anything else I re-energised demand for CH but the boiler was dead. Still with demand for CH only I switched the boiler's main switch off and then back on and it fired up. I switched on the demand for HW but within 30 seconds the boiler stopped again. Reset again and tried on CH only and it ran normally. After half an hour I switch demand for HW and it stopped again! I'm thinking flow problem on DHW; either restriction in primary pipes, possibly the 3 port valve is not opening fully on HW demand only, or maybe faulty Overheat stat (but why doesn't the lamp illuminate?) I don't know if it is anything to do with the way the pump is wired as the boiler needs pump over-run. The Mi's shows the pump wired directly back to a dedicated pump connection on the boiler when using Y plan. The pump does over-run but this appears to be taken care of in the control centre.
Can anyone help solve the problem please?

 

[andyg0507]

i take it this has only happend on finding that the g/valve was passing? or was this fault apparent before?

 

[Backboiler]

i take it this has only happend on finding that the g/valve was passing? or was this fault apparent before?

Tenant is a kid of 19 years old who tells me it didn't do it before I replaced the gas valve but you never know. There is nothing you can do to cause this fault by replacing a gas valve. I set the max and min gas pressures spot on.

 

[buckley plumb]

if you think its the 3 port valve try removing the head and manualy moving also to eliminate any electric faults remove sw live and put a constant live to boiler sw live to try it

ant

 

[andyg0507]

3 port is what id be looking at as already said above, programmer is also an option, as per your diagram back boiler does the programmer have a heating only option??

 

[Backboiler]

if you think its the 3 port valve try removing the head and manualy moving also to eliminate any electric faults remove sw live and put a constant live to boiler sw live to try it

ant

Thank you Ant. It's a metal Honeywell valve so I don't think you can remove the head can you? I will move the lever over manually and will try the constant live though. I will have to make sure there is a demand for heating and/or hot water for the pump to run though.

 

[Backboiler]

3 port is what id be looking at as already said above, programmer is also an option, as per your diagram back boiler does the programmer have a heating only option??

Yes, it has a three port mid position valve mate

 

[buckley plumb]

should be able to remove the head remove the cover and then the 2 corner screws be careful some older ones did not have removable heads

ant

 

[Backboiler]

should be able to remove the head remove the cover and then the 2 corner screws be careful some older ones did not have removable heads

ant

Will do. I guess if it has no screws then it can't be removable eh?

 

[buckley plumb]

no if it has 4 screws then its not removable without draining

Port valve diagnosis and repair - YouTube

 

[GrahamM]

The changeable head has the raised dimple on top.