The motor is a bit of a puzzle :
Old wiring appears to be three-phase, but the current motor, which looks fairly new, is single-phase.
The motor is numbered 1.541TPCB3, which doesn't appear to correspond with the parts listed at http://www.tecmotors.co.uk/ though TPC is one of their designations.
The exact same motor is listed at http://www.engineersmate.com/products/SINGLE-PHASE-MOTORS-230V/1-5KW-4-POLE-FOOT-MOUNT-230V-1PH-PERM/P239654
The wiring diagram inside the connection cover appears to show a capacitor-start motor with a centrifugal switch, but there is no obvious switch. It would be reasonable to assume some of the internal wiring is to a switch, but the connections shown are open circuit. Furthermore, if they were to a switch, it would turn the motor off once it got to speed.
I'm guessing that it's not a centrifugal switch, but a thermal fuse. So the mains input is actually on Z2/U1 as shown on Tec's data sheet, but goes via V1/V2 to match the cover diagram. The fuse has blown for reasons unknown.
Possible solutions :
Short, expensive solution - get a heating engineer to fix it.
Less expensive solution - buy a replacement motor from the boiler manufacturer and restore the wiring to original
Other possible solutions
- buy another motor from engineersmate and fit it, hope it lasts longer than the previous one.
- take the existing motor to a motor repairer (there's one near Old St) and ask for an expert opinion on it
- buy a 1.5kW (or bigger ? perhaps it's undersized) single phase motor with the correct shaft / pulley and fit it.
Things we've checked :
Looking at the motor connections on the cover diagram, it's apparent that mains power must flow into V1 for any effect at all. But this connection has a high resistance to all other pins. It's either a burnt out winding, or possibly a thermal fuse. The capacitor appears to be fine
Things we haven't checked :
There is a wiring diagram on the Tec website for capacitor-run motors which does not use the unconnected V1 pin. Perhaps this is the correct wiring ? The motor might still work if connected that way, but with no thermal protection. This would not normally bother me, except that it's already blown once and might overheat again. And we want it to run automatically for long periods inside a gas boiler in a screwed-together cabinet in a locked room, so no bodgery please.