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If you have turned down the target (rad) temperature by more than the flowtemperature, (which is normally at the targettemp) -5C then the burner will trip and will not refire again until the anti cycle time has elapsed, under normal conditions this only happens if the heating (rad) demand is...

No, but if it jumps up when you press the start button or very shortly afterwards then that would point to the pump actually starting, don't know what increase the boiler needs to restart but would have thought ~ 0.2bar or so.

That immersion appears to have a separate Hi limit stat attached to the Immersion boss?, Are they still making Immersions with this set up, most of the new ones seem to have it incorporated in the control stat?.

Is that your old EcoTec Plus 831?. When (if) the circ pump starts then you should see the pressure increasing by 0.2/0.3bar, also ensure the pressure is ~ 1.3/1.5bar, cold.

I think both types come with integral stats, the top mounted certainly does, the one below is a dual element immersion where the (one) stat switches either element by switching the neutral, the stat pocket length is normally only the length of the short ~ 11ins element so unlikely to foul on the...

Is that (they) top mounted Immersions?, if so, the stat pocket is renewed each time you renew the immersion as its part of it.

Can you reply to post #9.

If a negative positive head pump then requires I think > 1LPM to close the flowswitch so measure this trickle from the shower head with it in place.

I would suggest that getting a, say, single 2kW rad to operate properly with a 37kW Vaillant is a challenge, to say the least. The boiler will refire (or fire) at ~ 65% of its max output, 37*65%, 24kW and the flowtemp must be kept below the targettemp+5C for up to a minute to avoid the boiler...

Your WB RI boiler should/will have a external circ pump?, can you post details of its make/model/mode/setting and/or a close up photo of its LEDs while running, may be able to reduce its head which might reduce noise level.

The circulation flow rate through the coil may be too fast, this can be slowed down by installing a (normally) gate valve on the coil return and then throttling this to say a 1/2 to 1 turn open, it may, but only my, help. A corrugated coil is a coil with corrugations, gives better heat transfer...

I think the older type 937 have a minimum output of ~ 12kW, enough to satisfy the max whole house heating requirement for many, if so. 12kw minimum, then not a hope IMO of running it, economically or otherwise, with one rad in circuit.

Megaflo suffer from air baffle break up, don't know about the others. OSO cylinders do use a internal air bubble but no air baffle, also, the dip tube can be removed and a external expansion installed which gives ~ 10/15% increase in HW storage.

Easiest way to avoid errors is to change one wire at the time, remove one old...replace with one new etc.

If you are happy the the valve head grey& orange wires are terminated correctly then depending on the valve manufacturer, simply opening the valve manually and latching it open will fire the boiler, a Honeywell won't, but a EPH or Danfoss will, not sure re Myson or any others.

This schematic may help, may have been posted allready above??.

I did say to remove the valve head grey and orange wires before testing, maybe should have said "remove from the terminal block"

I take it the (D/stairs) valve is opening but not firing the boiler?, if so then even though a new valve head I would confirm that its micro switch is closing by removing the valve head grey&orange wires and with the M.meter on resistance check for continuity between them, should read maybe...

Pumped over warm??, are you saying that hot water is being pumped up and over the vent?, if so, blockage somewhere because its impossible (U tube) for that to happen in a combined feed & vent system. What is your present pump's name/model/mode/setting? and what pump are planning to replace it...

Thats a giant of a 6M (6.4) pump as "good" as a 8M DAB Evosta2or3, I thought that speed2 would be a bit on the noisy side allright so absolutely no downside of running on speed1 as it probably still circulates more than the selectric on speed3. You're lucky you didn't run it on speed3!!. Have a...