Wiring from Programmable roomstat to remote temperature sensor

[Ian]

Hello Group Help! I am advised by manufacturer (Danfoss) that I should use 1.00mm2 two solid copper core cable to connect their remote sensor to a TP7000M prog roomstat and that the cabling must not be in close proximity to any mains cabling. The original cabling (1mm T&E, earth not connected) ran very close to mains cabling. This caused wildly fluctuating temperature sensing (as much as 5 degrees C on consecutive readings,which seem to be taken by the controller at one minute intervals) I have now re-routed the cabling to be , for the most part, 8-12 inches from any mains cable. I have used the 1mm T&E again. The situation is considerably improved but I am still getting fluctuating readings albeit more in the +- 1 degree C range. If I run a shortish (about 1 metre) length of cable to the sensor absolutely nowhere near any mains cabling the temperature readings are totally steady. Since a reading even 0.5 degree below 'actual' temperature results in a call for heat which then operates the zone valve and fires the Oil boiler, the next reading may be at a heat higher than target and shuts down everything, I can be left with the boiler running in bursts of <

1 minute. I would like to avoid this if possible. Any ideas on any appropriate cable to use - perhaps screened in some way - or would some form of twisted pair help in any way? Is there any 1mm2 two core twisted pair available? I'd like to resolve this before replacing floorboards/redecorating....... Many thanks for any input Ian Please reply to group - email address is not monitored Ian

[Aidan]

Screened twisted pair. Connect the screen to earth at one end only, this is usually at the controller. There's usually only mA of current involved with sensors, I don't know why 1 mm2 is needed.

[Dave Plowman (News)]

I'm not sure why they specify 1mm except that it's the smallest TW&E available. I can't see the resistance being a factor.

The very best cable for interference pick up rejection is star quad mic cable. Might be difficult to get cut lengths, though.

But do you know what the cable carries? If only low voltage DC, it might be possible to reduce the AC pickup by other means.

[Dave Liquorice]

Smallest cable that has mains rated insulation? I suspect that even the "low voltage" side of the device is not fully isolated from the mains thus any external cabling needs to be mains rated.

Donno if you can get mains rated screened twisted pair. Machine control maybe?

[Roger Mills]

ISTR that you can use telephone cable to connect the remote sensor on a Honeywell CM67. Presumably the battery powered logic in this stat is completely isolated from the mains side of the switching relay.

[John Rumm]

Failing that, you can buy braided tubular sleaving (designed for military and industrial screening applications). You could earth that at one end and run the T&E down the middle of it.

See RS part number 244-5203 for an example (there are a range of sizes)

[Ian]

I do not know what it carries but it is not mains. Their installation instructions are silent on the matter. The advice to use 1mm2 solid core - with a maximum length of 50m - was in response to my question of wildly fluctuating readings . I guess it is resistance related matter rather than anything else. I am only looking at a length of about 8m.

I am minded to try some of this stuff

its relatively cheap but would be interested if anyone sees any problem in so doing.

As I am sure is obvious to you guys, my knowledge of control system cabling and its inherent difficulties is non existent!

Many thanks to all the respondents so far Please reply to group - email address is not monitored Ian

[Dave Liquorice]

Foil screens aren't the best but worth a try I guess. I'd also get onto the makers direct and find out why they specify 1mm T&E in case it is down to insulation requirements.

[Ian]

Thanks Dave, I'd come to the conclusion to talk to Danfoss again myself last night whilst turning down the target temperature again! I am sure your average heating installer doesn't do anything different to what has been done so far...but then I guess the average user won't be too bothered either. I'm beginning to wish I'd stayed with the old anthracite boiler. You knew you were going to be too hot/cold with that. Please reply to group - email address is not monitored Ian

[Steve]

Just a thought Ian, turning the problem around, do you have any dimmer switches or any other thyristor/triac controlled devices connected to your mains?

Some of these devices, if inadequately decoupled for RF, can put very sharp spikes onto the mains. These spikes, being radio frequency (RF), can easily couple into nearby wiring either capacitively or inductively.

If the controller is high impedance and, in turn is poorly decoupled, the spikes could upset things.

In the old days of dial-up telephone connections to the internet, I was scratching my head as to why my connection was slow and unreliable on early winter mornings. My wife never had problems. Then it dawned on us:- I would invariably put the main light on via the dimmer, but my wife would always put on a standard lamp or two. In the evenings there was no problem because she always had her way - lighting wise! We switched the dimmer off and my problems disappeared. I had run the telephone cable too close to the mains wiring and it was coupling into the modem and causing havoc.

Maybe a red herring but just an idea worth checking.

HTH

Steve.