Half-working combi system diagnosis?

[hamishallan]

Hi all,

I am completely new to this area so please be gentle and if you use any jargon, please include a brief explanation!

We have a combi boiler (Baxi Bahama 100). Domestic Hot Water works fine. Central Heating doesn't work at all. The CH circuit pipes running into and out of the boiler are both cold. The expansion vessel and internal piping gets warm, as the boiler ignites about every 6.5 minutes for about 6 seconds and then shuts itself off. This happens regardless of whether the boiler is in DHW-only or DHW+CH mode.

My first question: it seems that DHW is heated indirectly, through an exchange sink, from the CH circuit. A 3-way valve is used to switch the circuit being pumped through the heating element between a short loop (via the heatsink) and a long loop (via the radiators). Is this correct?

If so, given that the short loop seems like it's getting hot okay, it seems likely that the 3-way valve is stuck in the DHW position. This valve is a Honeywell VC6012, pictured below. As you can see, there is a white plastic switch on its right hand side. It can be pressed in a short distance, but springs out again when released (you can see the spring in the photo). It also looks as though it should slide along the groove, but it does not do so willingly (I have pressed as hard as I dare), whether I try it pushed in or sprung out.

Should I be able to slide the switch? If so, does this mean the actuator is all gunked up inside? Is it possible to open it up and fix it myself? If not, how difficult is it to fit a new one myself? (I have very little plumbing experience.)

Or am I barking up the wrong tree? I still don't quite understand why the boiler ignites at all when the system is in DHW-only mode and there is no DHW being drawn.

Thanks very much in advance for any insight,
Hamish

Update: I found the tech specs for the actuator at http://www.honeywell.se/energi/vceng.pdf.

There are 3 wires connecting it to the logic board: brown, blue and black. I probed them for AC and found that black is always live and brown is never live, regardless of whether I'm running hot water, whether the external timer box is calling for central heating, whatever. This means that the actuator is behaving correctly, always sending water to port A (the DHW loop). So do I need a new logic board?

 

[ollski]

create a ch demand and run a hot tap, actuator should sit in hw, turn off hw inlet isolator and it should move to ch.

 

[hamishallan]

create a ch demand and run a hot tap, actuator should sit in hw, turn off hw inlet isolator and it should move to ch.

Ollski,

Thanks for your response.

1) Do you mean that the white slider switch I described should physically move -- i.e., I can see the movement of the valve behind the actuator motor by the movement of the white switch?
2) Should I be able to move it manually myself?
3) If it doesn't move and I can't move it myself, how can I tell whether it's the actuator or the control board at fault?

Thanks,
Hamish

 

[Agile]

There are 3 wires connecting it to the logic board: brown, blue and black. I probed them for AC and found that black is always live and brown is never live, regardless of whether I'm running hot water, whether the external timer box is calling for central heating, whatever. This means that the actuator is behaving correctly, always sending water to port A (the DHW loop). So do I need a new logic board?

You have used good logic above.

Unfortunately you seem to have run out of logic with your last posting!

Since the actuator is apparently behaving correctly, the fault is either with the PCB or more likely with the components which tell the PCB what demand for heating is present.

Check those and you will find the fault!

Tony Glazier

 

[ollski]

Hi Hamish

1)yes
2)No
3)If it doesn't behave as it should test voltage on the main board connector X3 with the hot water tap and heating on.Testing across the brown and blue wires turn off the hot water inlet isolator and make sure you see mains voltage.If you don't you're looking at the pcb, if you do then diverter valve.

 

[hamishallan]

You have used good logic above. Unfortunately you seem to have run out of logic with your last posting!

True. I sort of thought that perhaps the valve had failed as well as the PCB, but the later in the day it got, the more muddled my thinking became!

Since the actuator is apparently behaving correctly, the fault is either with the PCB or more likely with the components which tell the PCB what demand for heating is present.

Thank you so much for pressing that point! I thought I had reasoned that all the demand components were okay, but this morning I tried disconnecting the DHW flow switch and lo and behold! Central heating (no hot water of course!) Of course, in hindsight, that was the most likely thing -- that's why the boiler would ignite even when in DHW-only mode, but I'd somehow discounted that in my mind because I'd have expected an overheat lamp to be lit if that was why it was cycling -- but now I figure that's normal operation for DHW, i.e., if the flow is slow enough that there isn't enough heat dissipated on the heat exchanger, the burner gets switched off until it is needed again. Also, I don't think I mentioned that the pump was on constantly... now I know why!

Is a DHW flow switch the sort of component you can open up and lubricate to get it working? And to replace it, do I just switch off the DHW inflow, turn on a hot tap until the water drains as much as possible, then just unscrew the black plastic cap pictured below? Simple as that?

Check those and you will find the fault!

It's so nice to be warm again!
Thank you again,
Hamish

 

[hamishallan]

Hi Ollski,

Thanks! See above -- it turns out the flowswitch is to blame. I intend to get in touch with Baxi because their troubleshooting flow diagram doesn't have a branch -- but could easily incorporate one -- to check for this kind of fault.

Best wishes,
Hamish

 

[mchu6am4]

Sorry to hijack the thread but how do i open one of these units, had it a leak and need to dry out the inside