Vaillant Combi VCW-Sine 18 T3WF H

[MacII]

This is one for the mature and experienced Vaillant heating engineer!

Symptom:

28 year old Vaillant Combi VCW-Sine 18 T3WF H in excellent working order. Heating and Hot water work correctly. However, the unit has developed a minor tapping/knocking noise after HOT WATER demand is closed. The noise occurs for approximately 3 secs just prior to the burner shut off.

The tapping is coming from the DC solenoid. The DC Solenoid is not switching off cleanly. As the PCB/Electronic board reduces the voltage to the DC Solenoid it appears to get to a level when the DC Solenoid oscillates on/off for about 2 - 3 seconds. This causes the tapping noise.

The heating works well. It shuts off the DC solenoid perfectly when it reaches the set temperature ( 1 thro 8 on temperature dial). There is no tapping. This means that the PCB Electronics which modulate the voltage to the DC Solenoid is working correctly. (The NTC sensor and heating temperature rheostat voltages at the inverting and non inverting inputs of the op amp when equal drive the op amp into saturation and reduces the output voltage to zero (which switches off the voltage to the DC solenoid). The Vaillant installation and servicing manual states the voltage across the DC Solenoid should be between 15 - 5 vdc during operation but only suggests switch off should be about 5vdc. The voltage to the DC solenoid actually reduces to about 3.1vdc before shutting off for both Hot Water and Heating (This I believe is not uncommon and not the problem).

My question to the forum is:

Does anyone know the PRIMARY mechanism used by the Vaillant Combi VCW-Sine 18 T3WF H to switch off the DC solenoid after Hot Water demand is closed WHEN heating is switched ON? Or possibly know the answer to solving the tapping problem?

Thanks for any help.

 

[Agile]

I know that you have mostly written clearly and described the problem fairly clearly but I still dont know if the problem is after DHW with heating or after DHW without CH as well.

As far as I recall it goes straight from DHW to CH without turning off the flame.

What I can say is that the flame should cease immediately the DHW demand stops ( UNLESS the DHW preheat is activated or the CH is on ) and not 3 sec later.

The switch off is by removal of the voltage not a gradual decline.

Have you measured the supply rail voltage and ripple level?

There is also a mistake in your description of the op amp circuit.

Nor do you say if the step modulation is operating. I suspect perhaps not as you seem to think that it continually modulates.

Tony

 

[Bunnyman]

Jesus agile u complicate things ! To us oldies from Vaillant this is known as the machine gun lol! Change the microswitch on the solenoid 126215 job done!

Untitled

• Bunnyman
• 26 Mar 2010
• 499

 

[Agile]

When the OP has been investigating the detailed operation of the PCB I have to expect that he has first checked the simple microswitches.

But you are probably right though.

I still suspect that the boiler is NOT operating correctly in other respects as well. But until each fault is fixed in turn that will not be seen.

Tony

 

[Bunnyman]

Tony all the described faults are being caused by the micro switch it is the 3 wire one an is trying to switch from the the fixed pot on the hot water to variable on the heating

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• Bunnyman
• 26 Mar 2010
• 499

 

[Agile]

It seems that I will have to check up of the purpose of that microswitch.

I have always thought that it switched windings on the solenoid after it was first activated.

Until I find a fault on something then I dont have any need or the time to investigate exactly how they operate.

With the declining number of Sine18s left it becomes less likely that I will ever go to many faults any more which are not just diverter valves.

I did go to one with a failed high resistance sensor a couple of weeks ago, first time I have found that though.

Tony

 

[MacII]

Tony,

Many thanks for the reply. It is much appreciated.

I will answer each of your questions and then offer my thoughts on the problem for you to corroborate or challenge.

To simplify correspondence I have cut and pasted your questions below along with answers.

Q1. "I know that you have mostly written clearly and described the problem fairly clearly but I still don’t know if the problem is after DHW with heating or after DHW without CH as well. "

A1. The HOT Water supply (without heating on) set for INSTANT DHW switches the DC Solenoid on/off correctly. This is because it relies on the water pressure differential switch to activate/deactivate pump and burners (providing Flow Switch stuffing box DHW micro switches are closed). The solenoid switches off as soon as the hot water demand is closed (no tapping) as there is no preheat required.

• The HOT Water supply (WITHOUT HEATING ON) but set for IMMEDIATE DWH causes the DC Solenoid to oscillate on/off at close of Hot Water demand.

• The HOT Water supply (WITH HEATING ON) set on either INSTANT or IMMEDIATE DHW supply causes the DC Solenoid to oscillate on/off at close of Hot Water demand.

The tapping only appears to occur when the combi has to maintain water temperature after close of DHW demand either to provide IMMEDIATE DHW or continue with Heating.

Q2. "As far as I recall it goes straight from DHW to CH without turning off the flame."

A2. Yes and NO. This is dependent on the rate of change produced by the thermostatic element inside the flow switch. It needs to close the HEATING micro switches in the stuffing box (top of Flow Switch) to bring the HEATING under the control of the temperature dial setting.
If the thermostatic element does not change rapidly enough to break the DHW micro switch and close the HEATING micro switches during the overrun period then the burners shut down for a short time until the element has expanded sufficiently. Interestingly it is the transitional or preheat period prior to the burner shut down in advance of the HEATING micro switches closing when it creates the tapping noise.

Q3."What I can say is that the flame should cease immediately the DHW demand stops ( UNLESS the DHW preheat is activated or the CH is on ) and not 3 sec later)."

A3. Correct. Flame switches off immediately when no preheat is required (INSTANT DHW Only). See answer to Q1 above.

Q4. "What I can say is that the flame should cease immediately the DHW demand stops ( UNLESS the DHW preheat is activated or the CH is on ) and not 3 sec later."

A4. Sorry should have been clearer. The 3 seconds I refer to is the approximate time the tapping caused by the DC Solenoid switch off occurs for. The preheat period is significantly longer during which the fames modulate down as the temperature of the circulatory water heats up.

Q5. "The switch off is by removal of the voltage not a gradual decline."

A5. Only when DHW is set for INSTANT without HEATING. The voltage to the DC Solenoid gradually declines from 15Vdc as the temperature of the internal circulatory water increases (Preheat and Heating Only). This continues until a point where the two inputs of the op amp are equal and the output goes to zero which forces the power transistor (2N3055) to switch off. There is always a small quiescent voltage (if you use a digital voltmeter)but can be considered negligible.

Q6. "Have you measured the supply rail voltage and ripple level?"

A6. Need more clarification on the question. Do you mean ripple level on the dc voltage supplied to the DC Solenoid? If so not sure it is relevant as the switch off of the burners works OK for Heating.

Q7. "There is also a mistake in your description of the op amp circuit."

A7. Probably tried too hard to minimise the basic function of the 761 Op Amp. Did not think it relevant to include a description on transitional state and how it modulates the voltage down prior to saturation returning a zero output. If my understanding of Vaillant's application of the Op Amp is incorrect I would appreciate some guidance...

Q8. "Nor do you say if the step modulation is operating. I suspect perhaps not as you seem to think that it continually modulates."

A8. The step modulation is working correctly. Yes certainly for the Heating. I will need to check it on pre heat/overrun.

Sorry for the lengthy explanation above but hopefully the Q and A approach makes it easy to understand.

I have ruled out the PCB circuitry. I have a spare working unit and it produces the same results.

The Flow Switch has been recently replaced (about three months ago). However the tapping symptom has only been happening for the last month. The complete reconditioned flow switch and stuffing box (micro switches on the top) was supplied by Allspares.

The flow switch stuffing box micro-switches appear to be working correctly. They are activated/deactivated by the thermostatic element within the flow switch. The flow switch micro switches have been checked and adjusted cold per Vaillant manual.

My thoughts at this time is the thermostatic element in the reconditioned flow switch may not be reacting fast enough. It is certainly moving sufficiently to activate the micro switches but it is the rate of change I am suspicious about.

I don't know but I think the preheat period should circulate the system water long enough for the DHW demand micro switch in the Flow Switch stuffing box to break. This switches off the pump and burners. In normal operation I don't think it is reliant on the PCB circuitry for switch off.

The PCB circuitry only switches off the burners during preheat if the DHW micro switch does not break and internal water temp increases to a set level measure against the NTC voltage. This is only a guess and hence my original question which after our correspondence modify to:

What is the mechanism used by the Vaillant Combi to switch off the burners after preheat. Let forget about the HEATING.

I am tempted to return the reconditioned Flow Switch as it is guaranteed for 1 year. However, given that it moves and activates the micro switches I suspect the company may say it is OK. If I know that the preheat switch-off is definitely controlled by the rate of change of the thermostatic element I suspect they may provide a replacement under warranty.

So that's the full story...It is a bit like the chicken and the egg. What comes first (the break of the DHW micro switches on preheat or some other switch off via the PCB)?

I have asked Vaillant technical services for the answer but to date no one has replied. I think they may be struggling with finding an expert due to the combi's age.

I am certainly impressed by the amount you have helped people. You obviously know what you are talking about.

Again thanks for the reply.

Bill

 

[MacII]

Jesus agile u complicate things ! To us oldies from Vaillant this is known as the machine gun lol! Change the microswitch on the solenoid 126215 job done!

Untitled

• Bunnyman
• 26 Mar 2010
• 499

I have taken the DC Solenoid microswitch off and checked it. Seems to work fine. However, if this is a common fault known as "Machine gun" I will order a new one.

Only thing which does puzzle me is it doesn't "Machine Gun" when burners are switched off on HEATING. I would have expected the "Machine Gun" effect to be consistent on either shut off (other than DHW INSTANT).

Thanks for the guidance. Only a few pennies to replace so I am game for it.

Cheers

Bill

 

[Bunnyman]

Look agile is good but does tend to over complicate things! I am a german factory trained Vaillant engineer,!, if u ask for advice please take it! This is a known fault the reason it chatters as said before is it is trying to switch from the fixed pot,stat on hot water to the variable pot stat on heating, once it finally makes contact an stops chattering it is the fine modulating on the variable heating stat IT IS NOTHIN TO DO WITH WAX STAT IN FLOW SWITCH u can't check the switch it won't showon a meter

 

[Agile]

Bunny, who has replied, repairs only Vaillant and sees a lot of them. He seems convinced the solenoid microswitch is to blame. But my previous expectation was that it was a power saving tapping on the coil.

I have considerable difficulty following your text because you are not using terminology that I can easily follow.

There are two components that are involved. The DHW flow switch at the base and the diverter valve at back left. It would help me if you could edit it using those terms!

The only thermal element is in the diverter valve, that operates switches on the top which terminate the preheat when its hot.

The DHW flow switch is only a diaphragm operated pin.

NOW I have a problem. Bunny's considerable experience of fixing these says its just the microswitch and I have to bow to his knowledge of fixing these.

But your obvious time spent would probably have diagnosed the microswitch as the fault early on and my previous understanding is that its only associated with the operation of the solenoid and that the change from DHW to CH control is on one of the DHW flow switches.

My comment on the op amp is just that with equal inputs the output will in theory be at half the supply voltage. In practice the high gain will not allow that condition unless there is a lot of feedback applied. So for the output to be saturated the inputs must be unequal. Not really relevant to the problem though.

If unusually Bunny were not right in this case then I would question the speed at which the flow switch pin extends and retracts. That should be virtually instantaneous! I suspect if its slow to retract then that may be part of your problem.

So my question would be does the flow switch pin retract quickly? Do both flow switch microswitches "click" together as it retracts? If not then can you adjust them so they do?

Please take heed of what Bunny says, he is usually ( always ) right !

I must apologise that my detailed knowlege of the operation of this model is not complete. Thats because I only see a very few of them, perhaps 2-3 a year. Many have multiple faults but the customer only wants to pay to get them working again and thats a diverter valve repair in most cases so I just dont have time to investigate the finer details of their operation.

It seems that I need to spend more time examining how these work. Interesting perhaps but rather academic as any further knowledge will not help me in the future. In any case if I encounter a new fault all I do is work out how that part of the design operates. I do like industrial history, steam engines, old aeroplanes etc. so perhaps I should spend some time more with them.

By chance I am currently dealing with one which is owned by a judge whose father was a boiler engineer and went on a Vaillant course on them when they first came out. Probably rightly he chose law instead of boilers!

Tony

 

[MacII]

It seems that I will have to check up of the purpose of that microswitch.

I have always thought that it switched windings on the solenoid after it was first activated.

Until I find a fault on something then I dont have any need or the time to investigate exactly how they operate.

With the declining number of Sine18s left it becomes less likely that I will ever go to many faults any more which are not just diverter valves.

I did go to one with a failed high resistance sensor a couple of weeks ago, first time I have found that though.

Tony,

From what I can see the DC Solenoid microswitch does a couple of things. When the DC Solenoid is activated the microswitch initiates the circulation pump via the PCB relay by removing the -ve suppplied on E12. It also applies a -Ve to E6 on the PCB. I don't have a complete circuit diagram for the PCB but it looks like it influences the dc bias output on E5 which in turn changes the voltage presented on E10 (via the Heating closed contacts of the Flow Switch which brings into play the variable stat). If the flow switch Heating microswitch is not made it fixes the dc voltage presented on E9 to the OP amp via E10.

So Bunnyman is correct in the fact that the microswitch does have an influence on the bias on the op amp and could therefore influence the preheat switch off. However, it looks like it should cause the Heating switch to exhibit the same problem.

Goodness now my head is aching...I am going to go with the Bunnyman idea to change the DC Solenoid microswitch based on the "machine gun" theory...

Thanks to both of you....

Bill
Tony

 

[MacII]

Tony,

Wont do the terminology conversion as it looks like Bunnyman has the answer......I think you and I must have the same "investigatory" gene. I don't like guess work and always like to know how things work.

Yes you are correct. More knowledge on the old VCW Sine 18T is not going to help us but I have to hand it to the Germans they know how to engineer things. I always think the Vaillant combi looks like a part from an Apolo Moon rocket......Fascinating.

Again thanks for the help and time in correspondence. It is appreciated.

Bill

 

[Agile]

Now I am more confused!

There is a mechanical interlock that prevents the gas valve opening until the pump is running. The pump pressure at a venturi operates the diaphragm which retracts the pin above the solenoid.

In that case I dont see why the microswitch needs to turn on the pump!

Just possibly its a German idea that even though the pump is already running having that device ensures the pump stays running when the gas valve is already on! Vaillant seem to have liked over complicated boilers.

It would be so much easier for me if I could see/test a working boiler! From an early age I have always liked taking things apart to see how they work. Started at about 6 with a concrete sundial in neighbour's garden. As I turned it over to look underneath it was too heavy and fell and broke in two. I was spanked for that but it did not deter me from taking things apart.

From memory thats only a standard microswitch and probably available in Maplins if not already in your junk box.

Tony

 

[MacII]

Now I am more confused!

There is a mechanical interlock that prevents the gas valve opening until the pump is running. The pump pressure at a venturi operates the diaphragm which retracts the pin above the solenoid.

In that case I dont see why the microswitch needs to turn on the pump!

It would be so much easier for me if I could see/test a working boiler!

Tony

Tony,

For clarity there are two pins...One is as you say above the DC Solenoid (on Differential Valve). The other is the pin on the DC solenoid which activates the micro switch.

Yes you are correct the burner will not light unless the pin retracts on the differential valve (it is as you say a mechanical interlock). However, the pump has to be running first to create the pressure differential on different sides of the diaphragm in the valve. This allows the pin to retract when the DC Solenoid is energised. The DC Solenoid has to be energised and pump has to be operational before the pin retracts and opens the gas valve.

The Relay on the PCB has heavy weight contacts to switch the power drawn by the pump (contacts occasionally burn out). The state of the relay is controlled by the micro switch on the DC Solenoid.

If I remember correctly. When the solenoid is energised its contacts are open. When the relay is de-energised (-ve removed from E4 by activation of the DC Solenoid micro switch) the PCB relay contacts close and the pump starts (providing everything else is ready for operation).

So yes the pump has to start before gas will be allowed to the burners by the mechanical interlock.

I suspect Bunnyman may have an abbreviate explanation and exasperated by my verbose nature.

Hope this helps and doesn't confuse.

All the best

Bill

PS Just ordered Vaillant Microswitch VC VCW 126215.

 

[Agile]

But thats not the ONLY way the pump is activated.

In CH mode the pump normally operates continually and the burner step modulates and if it is then still too hot the gas solenoid is turned off but the pump STAYS on!

Tony