Baxi 105 HE - No DHW unless CH is on. No CH except pump.

[Karl Austin]

Hi,

Another Baxi 105 HE issue, been reading the threads on here all afternoon!

Issue started with lukewarm/intermittent DHW, which I know from previous experience and fixing of, would be the diverter valve/pressure assembly. I replaced the two diaphragm, the two nuts (one with integral o-ring, one with separate - replaced), greased it all up etc.

On putting it back together I'm left with a situation where:

1) If I turn on a hot tap, I can see the pressure pin move and trigger the microswitch, but we get no hot water, but a flashing 60 degree "Low pressure or pump problem" light.

2) If the CH is triggered by the controller, I can see the diverter valve pin pop up and trigger the microswitch, the pump runs, but the boiler doesn't ignite. Flashing 60 light.

3) If the controller calls for CH to go off, the pump just keeps running - not the normal pump overrun, but much longer - I got bored after 5 minutes.

4) If I turn on a hot tap whilst CH is on then the boiler ignites, fan kicks in, and we get HW. Soon as the tap goes off, the boiler goes off, followed by the fan, but the pump carries on as per CH only.

Boiler Pressure: 1.4bar
Pressure Increase: 0.5bar (when boiler ignites)
AAV: Open
Pump Bled: Yes (via big silver screw)
Rads Bled: Yes

Any help much appreciated, as it's driving me nuts now!

Thanks in advance.

 

[Karl Austin]

A bit more diagnostic. It isn't the primary temperature sensor, that's reading the correct resistance.

I think it's the PCB, as I'm not getting 240v where I should be on A2 5&7 or A4 3&6.

 

[ianmcd]

sounds like the pump proving microswitch is not making properly on CH, the pin is pushing out but not far enough, turn the CH on and manuslly push the microswitch lever in with a screwdriver and see if the boiler fires

 

[Karl Austin]

Assuming you mean the one on the diverter manifold, the pushrod is coming up and pushing the switch to the extent that it engages.

 

[ianmcd]

Assuming you mean the one on the diverter manifold, the pushrod is coming up and pushing the switch to the extent that it engages.

well remove the two red wires on the switch and isolate the spades, set your multimeter to continuity and check that the switch is open ciruit then turn the heating on and check that the switch goes to closed circuit, do not check with the switch wires still connected

 

[Karl Austin]

Both switches functioning correctly. Looks like the pushrod isn't actually coming up when asking for DHW.

 

[ianmcd]

Do you mean the pump proving switch (the small diaphragm housing on the top of the diverter valve )

 

[Karl Austin]

Yep, that's the one.

 

[ianmcd]

if it comes out for the heating, and fully engages the micro switch then the boiler should fire for heating

 

[Karl Austin]

Current situation:

CH = Pump proving switch gets activated. Pump comes on, no fan, no ignition. 60 light until boiler is reset, which stops the pump overrun.

DHW = Pressure differential switch gets activated. No pump, no fan, no ignition. 60 light.

CH + DHW = Pump proving switch gets activated, pressure differential switch gets activated. Pump comes on, fan comes on, boiler ignites. We get hot water.

Have tried a different PCB, same result.

 

[ianmcd]

well try the test I already asked you to do in #5, do it exactly as I described , just because the pump proving pin is coming out, does not mean it is actually making the switch

 

[Karl Austin]

Apologies if it wasn't clear, but that's what I did to test in #6. I took the connectors off and tested them in place.

 

[Gasguru]

You haven't got the microswitches mixed up?
Green wires on front DHW switch, red wires on primary flow switch at back...

 

[Karl Austin]

Well bugger me, don't I feel a proper idiot now? I would have sworn on my life that I'd put the switches back in the right way round. Good job I didn't, I'd be dead now!

Thank you very much