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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.

Didn't "get" your Sat post until about 2 hours ago. OK, first thing is, regarding re pump pressure rise against a closed valve, all centrifugal pumps will only rise to the max head at 100% with a closed valve, a 700mb pump will not rise above 700mb at 100%, since head is proportional to...

Do these tests with the boiler running, if the rad return is teed back into the main flow, then, when testing with the TRV shut and return + vent fully open, the return pipe should eventually get very hot, if pipework installed correctly then, there should be a stronger flow through the vent...

It's possible that it's installed inadvertently as a "one pipe" system, shut off both rad valves then open the vent fully, open the inlet (TRV) fully and note the intensity of the jet of water from the vent into a container, shut off the inlet and repeat by opening the return fully and compare both.

Yours should look like the attached, (if a Surrey flange). Take a photo if possible, and post.

If a Surrey flange then the HW is taken from the side and the vent is on the top. If a Warix type then the HW is taken from the Top and the Vent from the side. Post the exact type you have.