Boiler overheating

[Dicksonkj]

Hi, wonder if anyone can help with the following issue.
I have a Worcester Bosch CDI 30 Greenstar boiler with a DAB 40-70/130 pump, 2 times 2 port valves one ch, one dhw. When the room stat reaches temperature the boiler temperature increases rapidly and can display U9 value which I believe is 109 degrees c. The boiler doesn’t lockout and the temp gradually comes down. I’ve noticed that the pump stops at the same time as the room stat reaches temp, not sure if it should run on? There is a bypass valve fitted a USV 16/22 which is set to zero which I think is fully open which doesnt seem right? Any advice gratefully received. Thanks.

 

[RandomGrinch]

Here's a post about setting the bypass valve:
https://www.diynot.com/diy/threads/...sv16-22-differential-valve-for-bypass.427511/
But, no, what you have doesn't seem right

 

[Dicksonkj]

Thank you for the link that’s really useful and I will give that a go. Would I be right in thinking that with the valve fully open I’m loosing some heat to the rads and also that the water going back to the boiler via the valve will make it too hot for the boiler to condense?

 

[Gasguru]

The pump must be wired to the boiler for the overrun to operate and the bypass set to allow sufficient flow during firing and pump overrun...set to say 2 meters to start with.

 

[Dicksonkj]

Thanks for your help. So would 2 metres equate to the number 2 setting on the bypass valve?

 

[Gasguru]

Probably but if the pumps not wired to the boiler and therefore not operating during overrun then the bypass isn't going to help much.

 

[Dicksonkj]

Ok thanks, just wondered if the bypass being open all of the time would make the boiler less efficient as it might stop it condensing?

 

[Londoner2]

Pump could be full of air, need to bleed pump?

 

[Dicksonkj]

Thanks, it runs pretty quiet but I will take a look at that too.

 

[DP]

The pump must be wired to the boiler for the overrun to operate and the bypass set to allow sufficient flow during firing and pump overrun...set to say 2 meters to start with.

Was going to ask if pump is connected to the boiler and there is permanent live feed to the boiler. Not uncommon when a heat only WB or another is fitted ( that required pump over run) and electrical requirements are ignored and same connections as existing boiler are used. Result is overheating at the end of the demand.

 

[DP]

Ok thanks, just wondered if the bypass being open all of the time would make the boiler less efficient as it might stop it condensing?

Is an auto bypass not fitted?

 

[DP]

Is an auto bypass not fitted?

Reread your post. Wire the pump back to the boiler and check for permanent live feed to the boiler
With bypass fully open, boiler is not struggling but is when MV shuts down and pump power is removed. Residual heat in the block spikes the heat exchanger temperature hence you see overheating.

Bypass fully open will effect flow/ return differential

No air in the system else boiler would struggle when MV open

 

[Dicksonkj]

Thanks DP that’s really helpful, the pump is not currently wired to the boiler so I will take a look at that. I’ve adjusted the bypass valve and that’s reduced the return temp to the boiler. I’ve also noticed that when the boiler stat is showing 78degrees I’m only getting 63 degrees at the outlet pipe from the boiler to the pump, that seems like a big drop?

 

[DP]

78 is the target temperature. It will take time to get there.

Bypass should technically only open when MV close and pump runs under power from the boiler in order to remove heat from heat exchanger

 

[Dicksonkj]

Thanks again, that all makes sense.