Thermostat dies, new one blows the fuse. HELP!

Thermostathelp · 26 Nov 2005

I was stripping some wallpaper from around the thermostat, and when I had finished, realised there was no heating.
Thermostat clicked on and off but wasn't firing up the boiler.
I figure there may be some bits of dust/paper/debris blocking it up, so remove the front cover, but all seems fine ( the 'click' is a tiny bit duller than it was ).
Voltage on Live is 240, voltage on Neutral is 0. These voltages remain the same even after the thermostat has been 'clicked' over?
The thermostat is by Danfoss ( I think it is a RMT-230 ).
I go out and buy a new one anyway - a Wicks one for a tenner.
I turn the power off to the boiler, take off the old thermostat, put the new one in it's place, wire it up as instructions say, and switch the power back on to the boiler.
As soon as I 'click' the thermostat over to fire up the boiler...the boiler control panel dies.
I go to the boiler cupboard, check the fuse - it's blown.

Does anyone have ANY idea what the problem could be?
Was it the thermostat in the first place?

ChrisR · 26 Nov 2005

No, it was the water you got into the electrics when you were stripping the wallpaper.

Thermostathelp · 26 Nov 2005

ChrisR said:
No, it was the water you got into the electrics when you were stripping the wallpaper.

It was the last piece of paper, and no water was used.

breezer · 26 Nov 2005

sounds like you have wired the stat wrong

how many wires ar there at the stat?

older stats had live neutral and switch wire, (and earth) it sounds like you have connected your new stat across live and neutral

Thermostathelp · 26 Nov 2005

breezer said:
sounds like you have wired the stat wrong

how many wires ar there at the stat?

older stats had live neutral and switch wire, (and earth) it sounds like you have connected your new stat across live and neutral

There are three wires - L, E & N.

This is how it says it should be wired, and I have done this.

Is the main problem likely to be a thermostat fault anyway?

slippyr4 · 26 Nov 2005

Thermostathelp said:
Voltage on Live is 240, voltage on Neutral is 0.

eh? voltage is a potential difference. to what did you attach the other probe to when you tested it??

If you have standard Y plan on your RMT-230, neutral goes on the leftmost pin, and the middle and rightmost pins are switched together when there is heating demand.

Turn off the heating at your isolator. Put your multimeter on lowest resistance. measure resistance between middle and right pins - should be infinite ohms when no demand. turn the stat up, and it should then be very very low resistance. If the stat is working.

It would be suprising if water would kill an RMT 230, it's a mechanical stat.

As soon as I 'click' the thermostat over to fire up the boiler...the boiler control panel dies.

that sounds like you got the neutral and the live on the middle and right pins. The neutral is only needed for the anticipator, which is on the left.

swapping the wire that is on the left at the moment with the one which is not 230V, will probably fix the problem.

ChrisR · 26 Nov 2005

[code:1]It would be suprising if water would kill an RMT 230, it's a mechanical stat. [/code:1]

Nobody suggested it would, but it could stop the boiler working and blow a fuse. If customer disturbed the stat and the boiler stopped, it probably is NOT a coincidence.

There are three wires - L, E & N.

No they aren't, even though the wires are probably coloured that way. Your Danfoss was never wired properly, which is very common.

Don't understand your diagram
What is "remote switch", and what have you got wires connected to? It may help if we knew what stat you had bought - probably a Landys and Gyr?

Thermostathelp · 26 Nov 2005

Right then here goes.

This is the new thermostat. It is a Wickes home brand one, but looks very similar to the one I took off, as you will see in a moment.

This is the diagram which I drew, it is on a small sticker on the inside of the new thermostats housing.

This is my old thermostat.

It is a Potterton HE boiler, and a Honeywell controller.

This is on the back of the thermostat packet.

This is where the fuse blew, in the airing cupboard.

I took the 240V reading from the AC Live and Earth, while the old thermostat has power to it.

Anything else, let me know.

Thermostathelp · 26 Nov 2005

slippyr4 said:
swapping the wire that is on the left at the moment with the one which is not 230V, will probably fix the problem.

From the top picture I have provided, what wire swapped with what one?

slippyr4 · 26 Nov 2005

your previous post was in the middle of me writing my previous post.

can't read the wiring diagram on the new stat from the picture. If you have only 2 wires, then you have no neutral, so the two wires would go in the middle, and the right, terminals on the danfoss one.

I'd suggest reconnecting the danfoss, properly, with a new fuse and seeing if that works. The danfoss unit is double insulated - there is no earth terminal and the earth should be sleeved and tucked out of the way.

Nobody suggested it would, but it could stop the boiler working and blow a fuse. If customer disturbed the stat and the boiler stopped, it probably is NOT a coincidence.

True, poor reading on my part. But now it transpires its a 2 wire job. So, the only scenario that could blow a fuse remotely from the stat is a live/earth fault. The other wire is meant to be shorted to the live on heating demand.

breezer · 26 Nov 2005

just because wires are coloured does not mean thay are what you think they are.

can you tell me what the other end of the flex is connected to, no not where you think it is, but where it is really connected to. in particular the wire that is green and yellow.

from the pics, it looks like it may be going into the box above your sfcu, as i can see what looks like a flex running up the righ of the pipe, open it up and please have a look and let me know

slippyr4 · 26 Nov 2005

Thermostathelp said:
slippyr4 said:

swapping the wire that is on the left at the moment with the one which is not 230V, will probably fix the problem.

Click to expand...

From the top picture I have provided, what wire swapped with what one?

I hadn't seen your original post; i assumed you had a 3 wire setup (which you need in order to take advantage of the anticipator built in to the danfoss stat). But you have the 2 wire one.

slippyr4 · 26 Nov 2005

breezer said:
but where it is really connected to. in particular the wire that is green and yellow.

ooh, breezer could be on to something there. hadn't considered naughty earths

Thermostathelp · 26 Nov 2005

Just want to make sure I understand what is going on here.

This is the flex which was going into the Danfoss thermostat, L, N & E - although are you saying that these wires may not actually be what they appear to be.
This flex is going into the ceiling and I assume it is going into the box in the airing cupboard, as yet I have been unable to determine exactly where they go.

Below is a clear photo of what is on the inside of the NEW thermostat.

Below is a clear photo of what is on the inside of the Danfoss thermostat.

All three wires were used on the Danfoss thermostat, and the 'Earth?' wire, was going to Pin 4 ( I think ).

The Danfoss thermostat was working, and had been for years, until I stripped off some wallpaper around it.

breezer · 26 Nov 2005

Thinking which pin it went to is not a lot of use, you must find out wher it WAS, not where you think it was.

as i said open the box above your sfcu and find out, i see 2 flexes, one is probably the pump or boiler the other your stat

open it up and look and post a nice clear pic too, i still say the green / yellow wire is your switch wire.

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Thermostat dies, new one blows the fuse. HELP!

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