Central Heating Electrical Fault.

Joined
11 Sep 2009
Messages
41
Reaction score
0
Location
Nottinghamshire
Country
United Kingdom
This is kind of an update to a problem I have had for a couple of years now but is still not fixed despite a lot of money and effort being applied.

My system is a fairly simple Y-plan system with HW tank in the airing cupboard.

To try and keep this simple the problem I have is the fuse blows in the fused spur supply to the heating system - which is mounted to the wall at the side of the programmer. The RCD does not trip but the 3A fuse does blow. This problem started a long time ago and came on slowly, getting more and more frequent in a linear fashion. This is when I started to replace components and has led to every component in the circuit being replaced.

The situation in which is blows is the same each and is basically when you have CH on or it has been on recently and you either manually turn on HW or the tank stat clicks on and turns on the HW. So in simple terms the CH is on and the HW tries to come on. The fuse blows. You replace the fuse and it all fires up and runs as per normal until the next time that same situation applies. If you reverse that and the HW is on and the CH comes on it is totally fine.

In short I have had replaced all components in the loop with brand new ones (Room stat, Tank stat, 3 way valve, Programmer, Pump & Brand new Worcester boiler). The boiler was replaced by a Gas Safe Engineer who re-checked all of the wiring and is happy it is wired correctly. After it blew fuses with the new boiler fitted we then tried another 3-way valve and a new programmer and it still blew the fuse.

I have since had an Electrician to look at the cabling in the walls as this seemed the only thing left to check and he Megger tested all the cables and found no cable or insulation faults. He suggested that the fault must be a live to neutral fault as this would blow the fuse and not trip the RCD, whereas a live to earth would trip the RCD. He also did a test on the RCD and this is working fine.

I'm at my wits end now as everything is new and the wiring has been checked both for being wired correctly and also not damaged in the walls.

This fault has been getting worse and worse for about 2.5 years and no matter what you do it seems to always blow the fuse in the same given situation. The cabling was Megger tested from the fused spur outwards to all components and gave no inclination of any fault.

Bear in mind that the system runs fine when CH or HW runs on it's own. Also when the fuse blows when you put a new one it it runs perfect again. One thing I have noted is that sometimes when the fuse blows it fuses the contacts in the tank stat.

The only thing I can think of now is to wire out every component in the circuit and run cables up the stairs to the various components just to completely disprove the wiring.

Does anyone have any ideas what to try next as I'm stuck?????
 
Sponsored Links
In short I have had replaced all components in the loop with brand new ones (Room stat, Tank stat, 3 way valve, Programmer, Pump & Brand new Worcester boiler).
Could you please give the make and model numbers of all components.

Does everything connect via a wiring centre (junction box)? If so a pic with the lid off would be helpful

Do you have a multimeter?
 
Here is the link to the original thread which contains photos.

//www.diynot.com/forums/viewtopic.php?t=191610&postdays=0&postorder=asc&start=0

Tank stat is Drayton HTS-3
Room Stat is Drayton Digistat+
Programmer is a Honeywell 9400C
Pump - I don't know I'm at work now but it is brand new.
Boiler is Worcester 15Ri
3 way valve is Drayton MA-1

So far 3 different tank stats have been tried, 3 different 3-way valves, 2 different pumps, 2 different boilers, 2 different programmers & 3 different room stats. Hence the reason why it all seems to point to the wiring. But the daft thing is the system works - and once it has blown and you replace the fuse you can turn each loop on and off to your hearts content and it all works fine in each & every combination - leave it a while and it will blow in the given scenario. Hope this makes sense it's hard to describe in simple terms.
 
faults like that can be difficult to find. But you dont say how often this happens or under what other situations.

Where does the heating system get its supply from? Is it shared with another applicance? Immersion heater?

The cyl stat is a big clue as we know the overload current flows through that loop.

I went three times to a situation like that where it seemed that the hidden wiring was failing but that was every 1-3 days and of course was not doing it repeatedly when I was there.

There are a few little tricks like adding low value fuses in the supply to key components, bypassing fixed wiring etc.

Unfortunately the one that I went to was for two Chinese doctors and whilst the husband was totally fine the wife was very neurotic and told me that I must find the fault on my next visit or......... ( or what was not defined ). She was not a nice person at all and I pity the husband or anyone having a baby in her area. I dont accept threats like that so I apologised to the husband and told him that it was too stressful for me to work for somebody like that. He totally understood. Its a pity because it would have been interesting to know what the fault actually was.

There can be all sorts of causes and a Megger only tests at 500v. A cable fault may only present under certain temperature/expansion/building movement conditions. A test at a higher voltage may often identify the problem. I have a variable supply at up to 15 kV which I can use for tracing cable and ignition faults like this if more usual methods fail to identify the problem.

Its not the cause in your case, but some boilers have a VDR which is a component designed to prevent voltage spikes and goes low resistance if the voltage exceeds 325 v. Its designed to create enough current to blow the fuse. Sometimes, particularly if they have been overloaded they can start to operate at a lower voltage than rated.

Tony
 
Sponsored Links
The way it works is like this. I have the HW set on the programmer to come on at 4:00pm and switch off at 9:30pm everyday. If for arguments sake it's cold i'll put the CH on for 1 hour at say 6:00pm. Then run a bath anytime between 6:00pm and the 9:30 switch off time as soon as the tank stat clicks to turn to call for HW the fuse pops. Replace it and it works fine with both loops running together. Then it will be fine until the next day when the same happens again. Through the summer when you don't use CH it runs perfect for months without any problem.

The supply comes from the consumer unit but I don't think it has it's own RCD - just a fused spur from the unit. There is no immersion heater.

The megger test was done at 500v but all tested to infinity - not even a trace of a dodgy reading. Like I said I reckon the only option now is to bypass all the house wiring.

It's so frustrating and been a pain for several expensive years and even now a Heating Engineer and Electrician do not have the answers. It had been suggested that a picture hook or something daft like that could have been a possibility but the megger test hopefully disproved that.

And yes I do have a meter.
 
I vaguely seem to remember reading about a similar fault on a forum, perhaps here.

Although I dont remember the actual details, I think it turned out to be a wrongly wired cylinder stat.

That would need to be VERY carefully checked both at the stat and more importantly at the other end of all three wires from it.

Tony
 
It did seem to point to the tank sat wiring as this is what turns on the HW and also has it's contacts fused occasionally but I'm told it is wired correctly and from the checking I have done I do believe this to be correct. The cable from the junction box to the stat has also be replaced with new too.

I think I'll try and find another Heating Engineer / Electrician for a second opinion on the wiring. Can't think of much else to do.
 
Many heating engineers are not very good at wiring and perhaps a third even have to call an electrician to wire up their instals.

You will need someone who would say that he is something of an expert on wiring/electronics. If he does not build amplifiers, have an amateur radio licence or possibly build/repair computers then he might not be the man.

Its easy to learn what you need to do if you have the time and many would say its probably better if you try yourself. D Hailsham on here has learnt by himself all about wiring for CH and he has the time to guide you.

Tony
 
Well to be honest I am a Mechanical Engineer by trade hence I am very interested and involved in all that has happened and have done whatever research and checking I can myself. I've downloaded wiring diagrams and followed what I can, seperately wired out the 3 cables that run from downstairs (2 boiler, 1 room stat) to the t.box in the airing cupboard and the fault remains.

I'm not an Electrician nor presume to be but following wires and diagrams is not rocket science but still nobody can tell me what is wrong. Hence now why I reckon i'll have to run the seperate cables through the house and completely isolate all house wiring and see where that gets me.

Every compoent has been replaced at least once so it must be wired wrong or the wiring at fault somehow. Also bear in mind that this problem has grown over months and months it didn't just happen one day so something somewhere has degraded over time.
 
I am just a DIYer, but are you sure there is no water leak dripping onto an electrical part? Especially round the pump or motorised valves, or inside the boiler casing?

And you have checked all cables and flexes for damage by mice, nails, or having something heavy stood on them, including a floorboard? I have seen PVC cables damaged where they were pressed tightly against a hot pipe under the floor.
 
There are definately no leaks as all components are visible in the airing cupboard. The boiler is brand new and there are no leaks anywhere.

Like I said I took a single cable and ran it up the stairs then one at a time wired out the 3 that go from downstairs to upstairs and the fault remains. Next I will try running all 3 cables up the stairs but if A cable was damaged then substituting it for a new one should have fixed the problem. Also bear in mind the Megger test should show up any break or damage to the cable insulation.
 
OK I have looked at your earlier topic and the pictures

The situation in which is blows is the same each and is basically when you have CH on or it has been on recently and you either manually turn on HW or the tank stat clicks on and turns on the HW. So in simple terms the CH is on and the HW tries to come on. The fuse blows. You replace the fuse and it all fires up and runs as per normal until the next time that same situation applies. If you reverse that and the HW is on and the CH comes on it is totally fine.

I have since had an Electrician to look at the cabling in the walls ... He suggested that the fault must be a live to neutral fault as this would blow the fuse.

One thing I have noted is that sometimes when the fuse blows it fuses the contacts in the tank stat.
You may find it helpful to read Mid-position valve working and Diagnostics.

You need to check the cylinder stat wiring carefully:

Common to Programmer HW ON
Terminal 1 (Call) to Boiler ON (same as valve Orange)
Terminal 2 (Sat) to Valve Grey and Programmer HW OFF

Make sure there are no strands of wire waving about which could short two terminals together.

Here is the wiring diagram for a Y Plan when the pump is connected direct to the boiler.

View media item 13167
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top