Turbomax 242E hard to light when using DHW.

[chrismcc]

I have a Vaillant Turbomax 242E (might be 282E, not sure) which is very difficult to get going when drawing hot water. First let me say that when there is a demand for central heating it seems to work perfectly. When turning on a hot tap though, nine times out of ten, the boiler will go through its start up routine, light and about half a second later will cut out. The diverter valve is almost new (the fully brass type) and has also had a new diapraghm as a precautionary measure. I can also see the arm on the right operating the microswitch and can also hear the click when the microswitch makes. That part all seems to be OK and works every time a hot tap is turned on, even if it's not very much. What seems a bit odd though is that the orange indicator light for the hot water will sometimes come on when a tap is opened and sometimes not. It is also sometimes very dim and seems to flicker, except when all is working well and it's bright. I thought that this light was controlled by the microswitch and therefore had to be either on or off! I have developed two ways to get the boiler to stay lit, one is to keep turning the hot tap on and off until it stays lit, the other is to leave the hot tap running and turn the boiler on and off using the green switch. However, even using these methods it will sometimes just cut out on its own. Another oddity is that if I fiddle about with everything it will sometimes work OK for a while. I called in the heating guy who installed it and he scratched his head for two hours, went through the fault finding charts and ended up giving me a new microswitch. The boiler worked for half an hour then started playing up again. Have to be fair to the guy though he didn't charge me a penny for the 2 hours but I think in the back of his mind he knew he hadn't solved the problem. Now I'm stumped and would welcome any suggestions as to what the problem could be.

Chris.

 

[Rob]

Sounds like an electrical fault. If the heatings working id put it down to the circuit board possibly thermistor!

 

[chrismcc]

Rob884,

Thanks for your reply, would that be the NTC temperature sensor or the O/H thermostat? I don't seem to have a thermistor specifically mentioned in my manual.

 

[namsag]

It is not unusual for the microswitch to be faulty. I would start there and check connections back to the board

 

[chrismcc]

Thanks namsag. I have removed the microswitch and operated it manually and it always seems to work just fine. Would a faulty switch cause the water light to flicker? I will try all the connections back to the board though.

 

[simond]

I have something in the back of my mind regarding a water pressure switch on these models causing seemingly unrelated problems with DHW. But I can't remember the detail, have to check the manual........

 

[Agile]

You must be thinking of the system flow switch!

Its the black thing just above/behind the pump. Very likely its only intermittently making!

We assume the DHW microswitch is always making when there is a DHW demand? That is the boiler always starts its pump when the diverter moves.

Tony

 

[namsag]

The switch you are on about by the pump is the differental pressure switch if this was faulty the same fault would appear when central heating is demanded.

Chrismcc you say when you operate the microswitch manually it works ok.. do you mean by demanding HW and pressing the arm on switch down with a screwdriver or something.. HW demand light does get its power after microswitch has been made

 

[chrismcc]

Namsag, I watch the arm on the diverter depress the microswitch arm and can hear it click (when the kids aren't screaming in the background). I have also taken it out but left it connected then depressed it manually just to be sure and it still works fine. What I can't fathom is this. The boiler starts normally when a hot tap is turned on but then cuts out after half a second or so when the microswitch is in position. However, when a hot tap is not turned on but I have the switch out but still connected, exactly the same thing happens, boiler starts then cuts out.

 

[chrismcc]

Also, I've just remembered, why would the HW demand light appear dim or flicker sometimes. Could I just have a dodgy connection somewhere?

 

[ChrisR]

The system pressure switch does have variable output. To eliminate it though, put the pressure up toi 1.7 or so - things should be different.

The microswitch:
They started off pretty and turquoisy, later ones are black with a little (terracotta?) ring round the button.
But you can't just put the new type in place of the old, the carrier arrangement is slightly different.

SO you could have a replacement old type microswitch (Or an old one from the fellah's bag), which is as cr@p as the one it replaced. Easy enough to check it - remove and short the beggar out.

 

[chrismcc]

Yes, I remember the old turqoise microswitch, it got very crudded up by a leak from the old type diverter valve, the one with the plastic base. This microswitch came with the new valve and is black and adjustable in position via a small crosshead screw. I might just replace it anyway and definitley eliminate it as a source.

 

[Agile]

The switch you are on about by the pump is the differental pressure switch if this was faulty the same fault would appear when central heating is demanded.

Although this is probably not the fault in this case, thats not always so.

The resistance of the hydraulic circuit is different in DHW usage and can have a diffferent response in CH/DHW.

In this case he has ( probably ) disproved that because he has apparently operated the DHW flow microswitch whilst leaving the diverter in the CH rest position.

Tony

 

[Rob]

Although this is probably not the fault in this case, thats not always so.

The resistance of the hydraulic circuit is different in DHW usage and can have a diffferent response in CH/DHW.

Tis true. Just repaired a Eurocombi which had HTG but no HW. It was indeed the pump flow switch

 

[chrismcc]

Thanks for all the assistance guys but I think I've solved it. Shorted out the switch as advised by ChrisR (it is one of the newer black switches) and the boiler works fine. So, replaced the switch and all is OK. Took the switch to work and gave it to one of the electricians to look at. He put a meter on it and found that when the switch was depressed untill the click was heard there was a small current flow through it initially but this dropped almost to zero straight away. When pressing the lever down even further the current flow went up substantially. I have no idea why this is but it seems to bear out what was happenening to me. When I was pushing the switch down with my finger or when it was fitted and the diverter valve was doing it, it just wasn't going down far enough. Enough current initially to fire the boiler then a residual to keep the light going at a flicker. I don't know if this is right and there needs to be a certain level of current through the switch to keep everything going put I can't come up with any other explanation.