Hello, I'd really appreciate some help with my central heating boiler.
I have a Vaillant ThermoCompact VC GB 112 EH, with a Danfoss FP715 timer unit.
Most of the time the boiler works fine. Now and then, it will stop working completely, for a couple of days or so, and then start working again.
I've called out two plumbers to have a look at the system, but they haven't been able to shed any light on the issue.
It seems to me that whenever the central heating or hot water is "advanced" for an hour (ie when it's activated outside of the usual timings, for example at the weekend if we're in the house during the day), then the next time the hot water or central heating come on as scheduled (usually the next morning), then the boiler cuts out after having heated the water only to luke warm.
The boiler then refuses to light.
Anyone got any ideas? I'm freezing! Although it will probably be working again in a couple of days.
We need to know a bit more about the rest of your system. Is it fully pumped? If so, does it have 2 zone valves (S-Plan see
formatting link
or one 3-port mid-position valve (Y-Plan)?
When the boiler fails to light, is it receiving a "go" signal from the rest of the system - in other words, is its Switched Live connection live? This is important, because it isolates the problem either to the boiler itself or to the external controls.
Are you sure that the cylinder and room thermostats are working ok?
It's probably an electrical or control equipment fault then. Plumbers deal mainly with pipework; you need a central heating engineer. People here will be more than willing to help, but as "Set Square" says, we need more info from you to work on.
Is it fully pumped? If so, does it have 2 zone valves (S-Plan see
I'm fairly sure I have a fully pumped, S-plan system (there are definitely two valves and hot water and central heating are independant of each other.
I'm not sure what you mean by "is its Switched Live connection live?" I attempted to start the boiler by setting the timer for the central heating/hot water to come on and by "advancing" hot water / central heating. In both cases, the boiler failed to light.
Well, having switched off the boiler at the mains for a while, and then switched it on again, hot water and central heating have both come on as per the timer.
I don't know if this will help, but when it wasn't working, the boiler sounded as though it was "stuck" - the fan was going and it seemed about to spark up, but didn't actually get that far, even though central heating and hot water were not due to come on at this time. Turning off the mains supply for an hour or so appears to have "reset" the boiler, so when the mains was switched back on, the boiler didn't make any attempt to start...until the timer instructed it to do so a few hours later.
Any help is very much appreciated. Thanks for your time so far.
There should be a multi-core cable connected to the boiler. One of the cores is the "switched live" (as opposed to a permanent live) connection - and is at mains voltage whenever the external control equipment (timer, thermostats, zone valves) is collectively calling for heat. You will need a boiler circuit diagram and a volt meter to be able to answer the question.
If this signal *is* present, but the boiler isn't lighting, there's a problem with the boiler itself.
If this signal is *not* present when the programmer and stats are all calling for heat, there's a problem external to the boiler - either a wiring fault or a failure of a thermostat or zone valve.
This is very odd! If the programmer and stats are OFF, the boiler should not be trying to light - unless, perhaps, there is a frost stat somewhere which brings the heating on regardless of the state of the programmer.
Maybe you have more than one fault!
If your system *is* an S-Plan, you will see from the reference I quoted earlier that the programmer and stats simply drive the zone valves. Each zone valve has an independent set of contacts which close when the valve is open. These contacts turn the boiler and pump on. If one of your zone valves is stuck open (maybe the spring return is a bit sluggish), it *could* turn the boiler on even when the programmer is off.
If the boiler's fan is running, the boiler must be being told to light. I don't know your particular boiler, but I assume that it has a fan-assisted flue. In order to light, a certain sequence of events has to occur. Typically, the fan runs and generates enough pressure to operate a pressure switch which switches on the pilot gas valve and the spark mechanism. A bit of electronics - using flame ionisation or some such similar wizardry - then turns on the main gas valve, and bingo! The whole thing can fail at any stage along the way. If you have an unstallation manual for the boiler, it should have some diagnostic charts to help you to pin-point the problem.
You can sometimes find out what is happening simply by switching things on and off, but this is often inconclusive - and you need to roll up your sleeves and get *inside* the system - or find others who know what they are doing to diagnose it for you.
Thanks, yes, I realise this...when I said plumber I meant heating engineer. My 2nd engineer is a good bloke and knows what he's doing, but it's difficult for him to diagnose the problem as each time he's been round the boiler's working again.
I'll check this out.
You're telling me! It's very confusing.
This sounds like a possibility...so you're saying that the programmer and stats drive the valves, and these drive the boiler? As everything else seems to work this could be the problem. Maybe one of the valves is closing prematurely once I've advanced the heating or hot water?
Yes - a few things happen...I (with the help of engineer no.1) first thought that the fan control circuit board was faulty, so this was replaced then I thought the air pressure switch was faulty, but since the boiler works consistently (for weeks at a time) until after it's "advanced", I don't think there's anything wrong with the boiler itself. The thing is, the boiler comes on as programmed after it's advanced, but cuts out before it's finished heating the water.
I've gone through the diagnostics, as have both my engineers, and it hasn't really helped.
I've tried this too, as have my engineers.
I think the thing to do is to wait till it's not working and call out my engineer again.
But if anyone has any other ideas please let me know.
Yes. You can see this if you study the S-Plan wiring diagram in
formatting link
switches inside the zone valves (which make when the valves have been driven open by the programmer/stat) have a permanent live feed from (1) in the junction box, and feed this to (10) - and thence to the boiler and pump - when the switch is closed.
I'd split the problem between the boiler and the controls. I'm fairly sure that on this model boiler joining terminals 3 and 4 together brings on the heating.
So find out how the controls are meant to work and then see if they are doing it right.
I'm fairly sure that this Vaillant will be like all others - boiler go is performed by joining a pair of terminals.
For S-plan this means those terminals should connect to the grey and orange wires of the zone valves.
Given that people have been down the diagnostics check list for the boiler that would imply that the external controls are giving the right signal to the boiler. (Invariably the check list is something like: Is the elctric on? Is the gas on? Is the boielr full of water? Is there 230V ac on pin X or some such?
Hello, my boiler has just failed again. This is the scenario:
hot water comes on at 5 - 6pm - this worked fine
Central heating comes on at 7 - 8pm - this failed.
The boiler is "switched live" - I've tested with a multi-meter. The thing is, the programmer shouldn't now be calling for heat as it's after 8pm, but the boiler is still "switched live". When I toggle the mains supply off and then on, I can hear the fan start and the air pressure switch clicks. Nothing else happens.
Anyone got any ideas? Is it the boiler AND the programmer that are faulty??
I've just had a look at the installation manual for a ThermoCompact boiler on the Vaillant site - but I don't know whether it applies to your exact model.
If it does, the electrical connections appear to consist of a strip with 9 connections, labelled 7, 8, 9, L,N, (Earth symbol), 3, 4, 5. Is yours like this?
If so, 7-9 are low voltage, and not used in UK, L, N, and E are self-explanatory - with L being *permanent* live, even when the boiler isn't firing. The important one for your investigation is 4 - which is the "switched live" to which I referred earlier - and which is turned on and off by the external control gear. When the fault occurs, does connection 4 have
230v on it, or not?
The other thing indicated by the manual is that you have a display panel - which indicates water temperature when the boiler is running - but displays a diagnostic message when something fails. Has your boiler got such a panel and, if so, what does it say?
Mine is quite an old version and the is wired slightly differently. I have terminals 1,2,3,4,5.
1 and 2 - have a permanent voltage across them
3 and 4, or 3 and 5 (can't remember which) are the "switched live", and when the prgrammer was calling for heat, there was a voltage across these terminals too.
The display panel consists of a water temp gauge and a system pressure gauge. It doesn't have a diagnostic message, unfortunately. If only it did.
The boiler has now failed completely which I'm actually quite pleased about as now my engineer has got a starting point.
I'll let you know what he says.
Thank you and everyone else for their efforts, it's very much appreciated.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.