Vailant CH & HW goes luke warm

[steveiw]

Hi,
I would be really pleased if any one has an idea what my problem is, I have an old but incredibly hardworking and normally reliable Vailant VCW sine 18 T3W combi ( about 25 years old) It has developed an intermittent fault in the last few days and I have tried to find it but failed.
Normally on cold switch on, the boiler would fires up and in a few seconds the burners go from about 25% flame to max and stay that way until the CH system is up to temp.
Now the burner stays at 25% and takes for ever to warm the system. The HW is the same, switches over and back OK but doesn't call for more gas, the only way to get really hot water is to reduce the flow to a trickle.
It has reverted back to normal again today but has let me down twice this week, so I know its going to get me later. The boiler is old I know but it is so well built and apart from a couple of diaphragms and a micro switch has given top service. Any help will be appreciated.

 

[stevetheplumber]

Hi,
I would be really pleased if any one has an idea what my problem is, I have an old but incredibly hardworking and normally reliable Vailant VCW sine 18 T3W combi ( about 25 years old) It has developed an intermittent fault in the last few days and I have tried to find it but failed.
Normally on cold switch on, the boiler would fires up and in a few seconds the burners go from about 25% flame to max and stay that way until the CH system is up to temp.
Now the burner stays at 25% and takes for ever to warm the system. The HW is the same, switches over and back OK but doesn't call for more gas, the only way to get really hot water is to reduce the flow to a trickle.
It has reverted back to normal again today but has let me down twice this week, so I know its going to get me later. The boiler is old I know but it is so well built and apart from a couple of diaphragms and a micro switch has given top service. Any help will be appreciated.

aint seen one of these for a while now and im having trouble remembering whats what try earthing the ntc lead see if that brings in full flame its the sensor on pipe left of boiler single wire its low voltage just taken of and earth it to pipe
modulation is achieved by voltage to the solenoid coil at the base of the gas valve full gas was abot 15 volts if i recall
i agree with the well built comment probably the best combi ever built unfortunatly technology moves on and it might be a good time to get the £400 scrappage allownce against a new one id recomend a rehema advata for similar quality where are you ?

 

[steveiw]

Thanks for your help Steve.
It went again this morning, after working fine all yesterday. I tried earthing the NTC wire as you suggested but it shut down on earthing. It appears the resistance decreases as the water gets hot so 0 ohms shuts of the supply. I had a good look around the solenoid area this morning and I think I have found the problem. The micro switch mounted on the coil is suspect using my meter it appears to be clicking but not operating every time. I have replaced it and its fired up every time, so far!
This Boiler is one of the best comb's for sure totally over engineered but seem to go for ever, built to last with no profit in spares, or should I hold my breath?
I live on the not so sunny at the moment, Isle of Wight Steve. If it fails again I will as you say think of that scrap offer.

 

[Puddle]

Thanks for your help Steve.
It went again this morning, after working fine all yesterday. I tried earthing the NTC wire as you suggested but it shut down on earthing. It appears the resistance decreases as the water gets hot so 0 ohms shuts of the supply. I had a good look around the solenoid area this morning and I think I have found the problem. The micro switch mounted on the coil is suspect using my meter it appears to be clicking but not operating every time. I have replaced it and its fired up every time, so far!
This Boiler is one of the best comb's for sure totally over engineered but seem to go for ever, built to last with no profit in spares, or should I hold my breath?
I live on the not so sunny at the moment, Isle of Wight Steve. If it fails again I will as you say think of that scrap offer.

Make sure you greese the little brass pin that brings the micro switch on when the solinoid lifts,as this dries out and sticks and can damage micro switch making it spongy

 

[steveiw]

Yes ok Steve the pin may well need a little tlc and I will give it a smear. It does appear to be sorted now and as I said on my first post in 25 years of fairly hard constant use it has served me well. I have have worked on many that have not lasted 10 years without major part replacement. I fitted this one in the early days of combi's and quess I struck lucky with Vailant. I doubt if I will get anything built to the same spec again to replace it but will take your advise and look at advanta boilers when the time for change is inevitable.