URGENT Help needed please - Blown heating fuse...

[Richard Marx]

Hi all

Moved into a new place recently, and there were problems with the heating as some of you may have read.

I invested in a new programmable hallway wall thermostat and installed it today. However, during installation the main heating fuse just blew (in the white on/off wall switch). Turns out it had been fitted with a 13A fuse not

5A. Yet another problem the sellers left me, grr.

Anyway, I have replaced the fuse and the wall switch now lights up as before, however the heating still won't come on. Hot water is fine.

Before it blew, the new stat was working fine. I have also put back the old one, but that doesn't turn on the radiators either as it used to. I doubt both stats have gone.

What else in the electrics could have blown, if not the wall stats and not the main wall switch fuse?

Thanks for your urgent replies :)

[Fentoozler]

There may be a small (1 amp) fuse on the PCB inside the boiler?

Angus

[Richard Marx]

Thanks but the boiler itself is working just fine (for hot water) so I assume there's no point checking the fuse?

It's just the 'call for heating' circuit that seems to have a gremlin in it.

Any other ideas anyone?

[Andrew Gabriel]

Did you find out _why_ it blew? I might hazard a guess you had the heating relay contacts wired across the mains, in which case the insides of the relay won't look very pretty now.

[Richard Marx]

Funny you should say that ;)

When removing the old stat, it had 3 wires attached - red (pin 1), green or yellow (pin 2) and blue (pin 4) PLUS earth (not attached, loose in wall).

As the new stat also had 3 connectors (No, Com, Rc) I attached them as follows: Red - Com Green or Yellow - No ...at this point all worked fine... Blue - Rc ...then the fuse blew.

Excuse my ignorance but where does one find the "relay" you mention in a Y-Plan system? Have looked at

but can't see it. Also, aside from that, what is/was the Blue wire for, and can I just put some electrical tape round it and tuck it into the wall?

[Richard Marx]

PS: Do you mean the relay inside the 3-way valve?

[Dave Liquorice]

Of course it would the blue is most likely neutral which you have just connected, via the stat contacts, to live. This has might have fried or welded those contacts together.

As to the old stat now not working are you sure that you have wired it back exactly how it was, there does appear to be some vagueness about wire colours... Even then the dead short you put on may well have zapped a PCB trace in the boiler or elsewhere.

May I respectfully suggest you eat humble pie, call a domestic heating engineer to come and sort it out for you. Before you either kill yourself or create a dangerous installation.

[John Rumm]

I expect he means relay inside the heating controller (which would be in serise with the room stat to be able to inhibit operation of the call for heat when the heating ought ot be "off").

[John Rumm]

So far so good...

No = normally open I would guess - hence it closes when the stat switches - hence calling for heat... so far so good

RC = relay closed perhaps - ergo connecting COM (Live) to Blue (Neutral)

Pop!

The 13A fuse will not have helped you since the current passing though the whole circuit will have been substantial for the short time it would have taken to blow the fuse.

Chances are it has shagged the relay in the heating controller. As a stop gap you can just wire this out (i.e. leaving the CH on all the time for the moment and relying on the room stat to do its stuff).

If you have a multimeter, you can test which bits of kit (i.e. controller, stat etc) still work). If you have not got a multimeter a quick trip to Maplin sounds to be the order of the day!

[Richard Marx]

Thanks, where do I find the 'heating controller'? Is this the same thing as the 3-port valve?

[meow2222]

The relay mentioned will be inside the new prog stat.

The blue wire is presumably neutral, needed to power the compensation resistor in old mechanical thermostats. This should NOT be connected to the prog stat, else it'll go bang.

If you find the relay on the fried prog stat, if you open it it should be poss to file the contacts clean with a rats tail file. Then again if it popped a 13A fuse, you might find the contact assembly vapourised.

Ahhh, this tells me there's most likely been an error in rewiring the thing rather than a fried relay. You need to establish which wire does what, and correct how you've wired it. Probably anyway.

I trust you'll be investing in a 3A or 2A fuse!

NT

[Richard Marx]

Thanks, but I wired it back exactly as before. Green wired to 'NO', Red to 'COM'. Before bang-time it worked fine wired that way. After, nowt. Old stat also fails to work at all.

If it's not the wiring, not the old wall stat, and not the new wall stat, what does that leave?

[Richard Marx]

I appreciate your concern. I'm usually good at DIY and electrics (don't worry I always switch off).

I had called BritGas HomeCare a couple of hours back but they can't make it till tomorrow PM, so looking to resolve things myself.

I have tried the old stat and it is definitely wired the same as before - I took photos to be sure.

If the boiler PCB had shorted, would the boiler ignite for hot water?

Isn't the most logical culprit the 3-port valve controller? When I set the lever manually to mid-point, I get heat to my radiators just fine now.

[John G]

To be sure it leaves only YOU.

I think you should get your eyes checked out before you try to do any more wiring.

You talk about green OR yellow wire. Do you mean green AND yellow or just yellow.

Green is earth if wiring is very old as is green & yellow on more modern wiring.

I reckon you would be safer waiting that few extra hours and getting somone who knows what they are doing.

[Andy Dingley]

Ye gods no ! The last thing you want is a pack of Scouse fascists fooling with it - get in there and DIY it.

[Richard Marx]

I suffer from protanopia (very mild red/green colour 'blindness'). Don't laugh, I can tell the difference between red, blue, etc. but green & yellow appear similar. However as I stated already (your eyes are okay right?!): "When removing the old stat, it had 3 wires attached - red (pin 1), green or yellow (pin 2) and blue (pin 4) PLUS earth (not attached, loose in wall)."

Simply the problem is the blue and red connected by mistake and created a short somewhere in the system. If you can't give any indication as to where, if not the stat, that's fair enough but perhaps refrain from unhelpful comments like "get your eyes checked". :-S

I'm still alive, perfectly safe, power is always off. There's no danger here, I'm just trying to identify where the problem is. I have checked all the circuitry in the new stat and it's all looking happy. Must be a problem elsewhere. 3-port valve?

[John Rumm]

I expect he has a bit of bog standard three core and earth with Red, Blue, and Yellow cores + a bare earth wire.

No, the bare one would be the earth. Chances are the yellow is the switched live.

More milk tibbles? ;-)

[John Rumm]

No not the same as the valve.... that's called a three port valve! ;-)

Do you have some form of timer or programmer that let you choose when the heating is on, and how many times during the day?

If so, that is your controller. It may even be built into the boiler.

Often the call for heat from the stat also goes through the controller. Hence they both need to be saying "on" for anything to happen.

The final call for heat that goes to the boiler may also pass through a microswitch on the valve. It will depend on how this was wired as to if it is also at risk from damage from the short - without a full wiring diagram of how yours has been setup it i difficult to say for sure.

(I had a controller relay fail once when a rodent nibbled through a heating controller cable and cause a short!)

[Richard Marx]

Yep, I have a Danfoss wall timer (electric rotary type). I suspected something in there might have blown as it's the only thing left really that I can't check. I did order a new one funnily enough, a digital replacement of the same model to give me more control. If only it would arrive this morning!

The valve is also a standard 3-way Danfoss. Is this likely to have a relay too, or is the wall timer the likely culprit?

Thanks :)

[Richard Marx]

This is all correct. It is 3-core + earth. There's no way I can mistake the earth for anything else - even someone with mild colour blindness can see a stripy green & yellow bit of shielding pushed onto a bare wire tucked into the wall! The other wires were red, blue and yellow (thanks John!) as I already depicted.

My mistake, in hindsight - stupidly, was connecting the blue to the stat, just as the old stat had it. I hadn't realised that new stats don't require it to be connected, or that the old stat worked differently. If only the installation booklet told us such things, tch. Live and learn...

Meow.