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The valve is working fine mechanically, it will move back to the off position once you release the lever. If you want to keep the valve opened manually, (used for refilling the system) then you just latch in the lever by pushing it upwards after opening the valve. Follow one of the leads back...

Go to "Worcester Bosch Greenstar 8000 System Boiler Issues" in buildhub.org.uk for the continuing saga. The above assumed HEX dP is correct for your boiler.

Does the boiler start up if you toggle that button on? (both zone valves OFF)

I can't see the zone valve but if its a Honeywell then opening the valve manually with the lever will not close the auxiliary (end) switch contacts to fire the boiler, others, including EPH will fire the boiler if you open them manually.

A rad running at its rated T50 output requires 1.43LPM/kW, flow/return 75C/65C but you will get 50% output at a flowrate of 0.34LPM with flow/return temps of 60C/39C, might be worth measuring the flow through the outlet of the disconncted TRV but remember the flowrate will be a good bit less...

Your measured flow temp of 59.4C (dT 14.7C, flow 9.79LPM) gives the best fit for a residual head of 1.5M, strange how the target&indicated temps are relatively so far away from it, anyhow, as long as the boiler stays firing whenever on its minimum output, not a problem.

You have a positive head pump which requires a minimum flow both to start it and stop it, it probably has 2 flow switches, one on the hot and one on the cold, pump probably requires ~ 1 to 2LPM to start it. If you try and start it with the shower mixer to cold what is the flowrate (pump off) and...

OK, thanks. Why (if you are) are you running with such a low (residual) head of 150mb, 1.5M, it's far easier to balance with a higher head, also, I would think most systems would require at least 200mb to run properly.

Can you try the TRV side again with the boiler/circ pump off, the flow from ech side then should be almost the same except that the flow side should be ~ 10 to 15C hotter than the return side., if both flows fairly equal, run the pump but only on speed1 and repeat the test, if OK try speed2, if...

Is the flow temperature reaching the now target temp of 70C? If so, and the return temp is 60C, dT10C, then (at a 20C room temp) the rads will output 87.% of their T50 rating, if the return is 50C, dT20C then 74.8% of the T50 rating, if the return is only 40C, dT30C, then only 62.9% of the T50...

Can you have a look again sometime when the boiler is running steadily at the target & indicated flow temps and ideally, those or any other readings above.

Can't remember how I did it now but your boiler will have a permanent live and a switched live, the switched live will be from terminal 10, the boiler then tells the pump to run/overrun (not like the schematic version, below), when all the zone valves close then there is no voltage at terminal...

Thanks Tony, It must be a characteristic of WB boilers to drop the pump speed to 20% on overrun. If happy with that 150mb setting then just set the minimum pump speed to say 43% and see if the pump speed only falls to this (43%) on pump overrun, if so, set the ABV to slightly < 0.15,say "0.13"...

The LLH makes your trouble shooting easier as you can rule out boiler HEX fouling as playing any part of the problem. Also, the UPS3 is a giant of a 6M (6.4M) circ pump at 60W as against 40W for most of the "others". You can see, below that it will circulate over 20LPM (1.2m3/hr) and still...

Just had a scan through some fairly recent posts of yours and see it seems that you have a LLH??, if so, what is the boiler make/model/circ pump, internal/externally installed? and what is your secondary (rads etc) pump make/model? Would definitely replace that TRV on that new rad.

If the horizontal right (from the Surrey Flange) is going to the Shower pump/valve and the vertical is the vent, then installed correctly? but the vent pipe should be sloping upwards with 45deg elbows until it connects to the vertical. What T are you referring to?.

IF the pump is mapped to one of the constant residual pressures then if set to 150mb the minimum pump speed should not fall below 46.3%, if set to 200mb, it should not fall below 53.4%, if set to 400mb. it should not fall below 75.6%. It appears to be mapping to the thermal output of the boiler...

Worth checking out that (if) ABV, these will bypass a lot of water if debris trapped under the seat ofrif incorrectly set, check the setting. When the boiler is running normally the pipe downstream of the ABV should be cold.

What make/model/age, of Boiler?. is there a ABV installed in the system? I see from another thread that the boiler is 25 years old, has the pump ever been changed or at least had its head removed to check for impeller or port fouling?, is it internal or external to the boiler?

Are you sure the pump speed is down to 10% at 3.0kW minimum boiler output, if so, then its maximum head is 7.0mb on pump overrun with a pumphead/reshead/flowrate of 6.2mb/5.3mb/1.5LPM, this will result in a boiler dT, of, 3.0*860/60/1.5, 28.7C.