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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.

They shouldn't pass anything if shut, not even 0.2LPM, what's the point otherwise?, you may as well have that gate valve suggested above.

Have you got a actual pressure gauge for doing these tests?, if so, then run the tests by measuring the flowrates at different pressures and ABV settings, including shut, index 0.6, by running it into a bucket for say exactly 30 seconds, then measure this with a 1 litre container and X 2 to give...

The DU144, according to AI, is Drop Tight.

Using the first pump map of 150mb...... The pump head at 50% speed and even using a 800mb pump = 800x0.5^2, 200mb, a new HEX loss at the calculated flow rate of 8.5LPM is ~ 28mb leaving a residual head of 200-28, 172mb to circulate water through the rads and the ABV, no way should the ABV be...

Take all the readings, same as in post #101, please.

I don't know how (or if) the calculation for residual head is done to give the proper pump speed/output but quite easy to calculate the pump head from a known speed. For example if the pump speed is 46.3% then the pump head (at the pump) is 700×0.463^2, 150mb and the residual head will be a...

When you start the boiler normally then you should see the pressure gauge rise by around o.2/0.3bar, if it does, then possibly a faulty "pressure sensor" or a failing pump. When it's firing for say 15 minutes or so note the flow and return temperatures D40& D41 in the boiler menu.

Big improvement with new HEX but again, same as other WB with a new HEX, a 3C dT between Target/Flow temps. Burner cuts out at Targettemp+5C so a worry that this may happen if you switched off a CH zone as the boiler now has only 2C to play with, under steady state conditions, the target/flow...

Yes, but the dowstream pressure o ly has to fall by a tiny amount, maybe 0.1bar for it to start lifting. Have you installed a new one?? G

MI = Manufacturers Instructions. Can't understand d why the don't use the previously used real flow sensor and a additional HEX sensor which doesn' play any part in the flowtemp control but will flag a alarm/trip if it does exceed a certain temperature.

The drop tight has a soft "rubbery" seat. Caleffi give a very good description/picture of how PRVs work.0

Thanks Tony Won't go through my dreary calcs just now but you may have a very high either HEX or system resistance. You appear to have a calculated flowrate of 6.75LPM at 1.82M system head ((assuming a clean HEX) which equals only 8.7LPM at a 3M head, my system which is fairly typical of a lot...

Terminal 10 should supply SL (switched live) on the boiler and PL (pump live) on the boiler should supply the pump, when term10/ SL has 230v then PL (pump live) also has 230V from the boiler's L (permanent live) and the PCB, when the programmer is switched off or all the zone valves close there...

Have you got zone (motorized ) valves, if so, then, assuming CH only on, if the timer is on and calling, then it supplies 230V to the roomstat, if the roomstat is also calling, it opens the zone valve and a auxiliary (end) switch closes which then supplies 230V to the boiler, the boiler then...

They don't build them like they used to, I changed a circ pump on a 20 kW SE Firebird for a neighbour about 6 years ago and found that the circulation was the wrong way round and had been for about 9 years, he had found that he had to keep the boiler stat dial setting well down to keep a...