Wickes 2000 (Halstead Quattro?) Help!

[croydoncorgi]

Hang on a minute....
Are the thermistors on the Quattro wired in series, like on the (ghastly, dreadful) Ferroli design?

If not, I'd be VERY surprised if both went short-circuit at the same time.

The Halstead fault-finding flowchart for the Quattro only mentions the DHW thermistor right at the end of the line, and only in the context of checking why the burner pressure won't reduce, not why the boiler fails to start!

In fact, the DHW and CH NTCs are identical and (according to M&I) in a 'dry pocket', so it would be dead easy to take them both out, compare their resistance at room temperature (should in the region of 15kOhms, then if the values are the same, put them back swapped over, in case one has gone 'intermittent' (which also happens). If the values are significantly different, they probably ARE wired in parallel and this would account for your problem.

 

[naffedoff]

The thermistor on the DHW outlet pipe reads around 20 Ohms and the other reads around 20 kOhms. Does this mean they are wired in parallel then?

 

[croydoncorgi]

No - it means that the one reading 20 Ohms is stuffed (unless the pipe it's on is quite hot).

 

[naffedoff]

The pipe its on is stone cold. I don't understand though if it is this thats causng the fault why we lost the hot water first then the heating later. Nevertheless I will replace it and see what happens. Do you know of any sites where I can buy one from as they are only sold in pairs on the Halstead site?

 

[croydoncorgi]

If they're sold paired, there's probably a reason. Like: if one goes, the other one will probably go soon too. Or (more importantly) they may actually need to be matched pairs because of the circuit they're in.
Assume Halstead mostly know what they're doing.

 

[naffedoff]

I took your advice and bought a pair, the only trouble is I can't get one of the buggers off! The screws are stuck solid and its behind the pump so difficult to get at. However, I've changed the one on the DHW outlet pipe and hey presto we have hot water but no CH. As I couldn't get the other one off I just trying plugging the wires on to the new one but still got no CH. Do you think then that there could be another fault causing the loss of CH? Also, the new thermistors read 15kOhms where as the old one I've so far had to leave in place reads 20kOhms. Could this cause any problems?

 

[croydoncorgi]

Don't forget the thermistor resistance is affected by their actual temperature...
Are you seeing 20Kohms with the boiler cold? or very cold? Sounds very high resistance to me.

If you fitted and connected the new HW thermistor, AND also connected the new CH thermistor but without actually attaching it to the boiler, you SHOULD then get the CH working - in fact 'running away' because the boiler won't see the temperature of the CH Flow going up. If HW is OK but CH really does nothing there's another fault to be found.

Ah - I'm reminded looking at the M&I that the thermistors are the nasty ones held by 2 small screws - not the better type which screw into a 'dry pocket'. My suggestion is try to find the other fault and then decide (based on whether the boiler works OK or not) whether to do more surgery to get the CH sensor out.

 

[naffedoff]

Following the fault finder in the manual it tells me to check the wiring on the pump over-run thermostat. What kind of voltage should I be getting here? Also, is the heatsink paste I cleaned off critical to the correct operation of the boiler?

 

[croydoncorgi]

I think I can see how you got there (section 8.2 of the M&I?) but I think it might be a Red Herring. If you've got HW, the pump and fan MUST work and (I'd have thought) the pump overrun stat must be OK too.

It's a while since I worked on one of these and I've never seen this particular fault.

Since the wiring from the overrun stat all goes back to the same plug on tghe PCB, it's hard to work out the link between 'faulty wiring' there and the fan not getting power - I'd have thought the boiler SHOULD restart into either HW or CH mode from pump overrun or from cold, so the position of the stat should be immaterial. It WOULD be worth checking the wires, however. Isolate the boiler and (gently!) pull the plug off the board and disconnect the other ends, and check for end/end continuity on all three wires.

Since the pump o/stat and the power connections to the pump are next to each other on the board, I'd expect to see mains voltage on two out of the three wires when the boiler is ready-to-run. (But it might be low-voltage sensing....)

 

[naffedoff]

All is OK with pump o/stat wiring. Any ideas where to go next?

 

[naffedoff]

Anybody?

 

[naffedoff]

It seems I've been deserted . Thanks to everyone for their help so far.

 

[croydoncorgi]

Not actually deserted...
Just that without the actual boiler to play with I can't (safely) advise you what to do next.

If you've (still) got hot water, it's very odd that the CH won't work at all and I still can't quite see why the pump overrun stat should be implicated for CH and not for HW. Have you actually tested the pins on the PCB connector for the pump o/run stat for voltage with the plug removed? (I'm not very happy suggesting you run the PCB in this state.... your risk.)