Using a contactor

I am hoping that someone can help with a problem I have been asked to solve regarding a thermostat control for underfloor heating. My learned old sidekick is on holiday for a week so I can't ask him.

The problem seems to be a thermostat controlling a large under tile wire heating system which must have the contacts fused on, as the floor just keeps on getting hotter.

It is on a 50A breaker so is drawing a high load and wired in 10mm2 T+E. From the little I know of such things I am guessing it should have had a contactor between the thermostat and the load, which would have protected the thermostat in a similar fashion to their use for car headlights.

The customer said their old electrician wired it for them but he's retired to spain and there is no paperwork.

The board is in the room and is just over a year old but the heating has only been used for the last few months, so the question is should I put in a contactor and a new controller, and if so, as I have never seen one, do they need an enclosure or do they come as a surface mount box?

Hope someone can help,

Nick.

I have never seen or heard of an underfloor heating system pulling that much. Are you sure ?

Try to find a make and part No and Google it. Stats dont normally switch that much current.

Have not tested it but the roon must be 50m2 and apparently has three different zones all wired together, and the systems I looked up earlier range from 100 to 200W/m2.

The customer is not particularly useful when it comes to answering questions about anything remotely technical and without any paperwork I can only aim for worst case.

Just found this for a similar system: "The maximum thermostat load is 16 amps (3680 W). Installations exceeding this load will need to be controlled with a suitable power contactor or alternatively by more than one thermostat"

So I guess I probably will need a contactor which changes my question to - what do these things look like and do they need their own enclosure or do they surface mount like a s/f/spur?

You are correct with the 16A max; and I`ve installed floors drawing (much) more than this.

For a domestic room, you`ll want a contactor that DIN rail mounts, somewhat like an MCB.......

It`ll need an enclosure - small CU will work nicely.

If you have the ends of all the zones available; it may be better to seperate them, and keep your contactor(s) smaller, and easier to wire; i.e. smaller cable

HTH

KME

Any queries, come back. I`ve done UFH quite a bit.

The problem seems to be a thermostat controlling a large under tile wire heating system which must have the contacts fused on, as the floor just keeps on getting hotter.

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Oh that poor old 'stat. What was the guy thinking???? I take max current values in things like stats and timeswitches with a huge pinch of salt unless I can actually see the contacts for myself. Oftentimes you find a poxy little micro-switch is in fact the switching element and it hasn't a hope in hell of passing and interupting 2A let alone 13 or 16A! How many times have you seen burnt out cylinder timers and I've attended a few welded up stats. Saying that I've also attended welded up contactors!! :_|

It is on a 50A breaker so is drawing a high load and wired in 10mm2 T+E....

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How the hell did he manage to terminate it in the first place?

From the little I know of such things I am guessing it should have had a contactor between the thermostat and the load, which would have protected the thermostat in a similar fashion to their use for car headlights.

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Most definitely, but don't confuse relays and contactors. Auto's use little encapsulated plug in relays, we're talking about a step change up to something like this as reccomended by KME fitted in a small CU....

http://www.tlc-direct.co.uk/Products/MK6420.html

Split the zones onto their own MCB's and from the output of the breaker take each zone feed to its own pole on the top of the contactor, then bring the u/f ccts to the bottom terminals of the ctr.

The customer said their old electrician wired it for them but he's retired to spain

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Good riddance!

...and there is no paperwork.

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:^O :^O...nice one centurion! :^O :^O

The board is in the room and is just over a year old but the heating has only been used for the last few months,

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How old is the underfloor installation exactly? If I was the punter I'd be doing a ****in' wardance about that!

...so the question is should I put in a contactor and a new controller, and if so, as I have never seen one, do they need an enclosure or do they come as a surface mount box?Hope someone can help,

Nick.

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Not so much a new 'controller'. It's only a room stat wired to the coil of a ctr. isn't it or are there other protection/overtemp circuits to consider?

Carter:

The electric UFH units also have a floor stat attached to them - primarily designed to stop the floor temp rising too high. (thermistor - 14K cold - 8K hot).

With regard to the contactor (s), the following points will need to be borne in mind:

1. keep the low current coil wiring between T-stat and contactor away from the heating coil wiring.

2. make sure you use BOTH poles of the T-stat switching for the contactor; i.e. don`t just use the phase. It tends to send EM pulses back to the stat when contactor (de) energises, and upsets the stat.

3. It is usually necessary to put a capacitor across the contactor coil. Again, cuts down the EMI, and negates callbacks.

You will possibly find, if you call the helpline, they`ll tell you no. 3 is not required. DON`T listen to `em. I`ve had the problem, and it took a while before I could stop the t-stat resetting, losing all programmed times, and the current time, every time the contactor switched OFF.

HTH

KME

Carter:The electric UFH units also have a floor stat attached to them - primarily designed to stop the floor temp rising too high. (thermistor - 14K cold - 8K hot).

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Precisely, that's why I asked. You'd think there'd be some form of feedback cct for just such an eventuality. Yer man seems to have an extremely basic installation on his hands and that's being generous. Sure it isn't just a run of frost protection tape (current self limiting) run under a screed?

1. keep the low current coil wiring between T-stat and contactor away from the heating coil wiring.

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true, the contactor's coil is highly inductive and it's not so much the steady state current that does for contacts it's that ability to create an arc that taters them.

2. make sure you use BOTH poles of the T-stat switching for the contactor; i.e. don`t just use the phase. It tends to send EM pulses back to the stat when contactor (de) energises, and upsets the stat.3. It is usually necessary to put a capacitor across the contactor coil. Again, cuts down the EMI, and negates callbacks.

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Sounds like a good plan.

It was the note about the `stat that set me off- almost all the under floor systems use the same range of stats - I`ve fitted a few hundred of `em in the last few years, including my own house.

Note to anyone installing this system. It is VERY easy to forget the floor probe, and a lot of the thermostats won`t work without it. I also put a standard resistance meter (NOT insulation) on the probe, and test the reading before burying it in the floor.

Thanks for all the advice, I am going to look at it Saturday morning and should be able to give the customer an idea of what's involved.

As a rule I try and avoid other peoples cock ups as they often end up an expensive can of worms, especially as their so called electrician has aparrently wired up a sauna and shower room for them. Can't wait to see if the sauna is on a 32A breaker and run in 1.5, thats why it's so hot in there.

Oh God there's more???