Relay/Contactor Advice

[japers]

Hello all

I have been sent here from Tilersforum so hopefully posting in the right place! I bought some Heatmiser 12v Network Thermostats and need to use one of these to actuate my underfloor heating (240V/2KW so that is roughly 8A).

I am therefore looking at 12V coil Relays or Contactors that would be able to switch this on. From reading through some of the threads on various forums, it seems a relay would be better if possible. Anyway I have found the following from Maplin which is 15A so lots of spare capacity in theory:

The bit that concerns me is the "Maximum switching power 450W resistive" - sounds like it won't handle the load or would it? (Maplins also do a 30A one but the resistive is 840W).

15A SPDT Miniature Relay : Power Relays : Maplin

Alternatively RS do a 16A one but there is no mention of resistive load:

Finder | Relays, Switches and Indicators | Relays | General Purpose Relay | Interface Relays |48.61.7.012.0050SPA

I'd be so grateful if someone could point me in the right direction as am a bit confused! Hopefully posting the above product links is ok...

 

[Steve1412]

Tyco Electronics | Relays, Switches and Indicators | Relays | General Purpose Relay | Power Relays |RM805012

Not really my area mate but will stick my two penneth worth in....I would go for something on a more industrial scale for switching that load with the amount of times it will probably be switched on and off it can have quite an effect on the life of the contacts. Something like the link would be my idea.
Just out of curiosity the thermostat in question appears to be part of full system...is this where you derive your 12 Vdc from???

Am sure one of the lads who do heating more than I will have a more definitive answer for you.

 

[japers]

Thanks for the advice and recommendation Steve!

The system I will be using is made by Heatmiser and it will be two thermostats - 1 for the combi boiler (easy to find a relay for) and 1 for the underfloor heating.

These two thermostats are linked together (power and communication) by CAT5E and they are powered from what seems a fairly standard 12V DC transformer. The website is quite complicated to understand! Each Thermostat needs 4 wires (in my application) - 2 for power and 2 for comms so for my situation CAT5e is best. Each thermostat also has a further 2 connectors which are a 12V switch to power a relay. Hope it makes sense...

 

[Steve1412]

Just had a nosey at one of heatmisers brochures and it is a little complicated just for a heating system......i love y plans!!!!!!!!! If the second contacts are 12 dc out for switching then that relay will suffice....the spec of some of the thermostats I have seen state they are volt free contacts for switching...this means you have to take a voltage to it to be able to switch it......if you know what i mean....if the stat is fed in 12volts you should be able to loop the + to the switching contacts, a switch from that to the relay and a - straight to the other side of the relay coil. As long as the 12vdc supply can handle the load of the relay coil.

 

[japers]

Thanks Steve, the main aim is to have a heating system which we can turn on or off remotely as we are very unpredictable in our living patterns. I had originally bought a Z-wave switch which would turn on/off the boiler via a laptop but getting a USB Z-wave controller that works is a mission! We have since done our kitchen which has meant a lot of rewiring including moving boiler from kitchen to loft and the underfloor heating. It therefore makes sense to get the 12V Heatmiser Thermostats which only cost a bit more than a standard thermostat and can be hooked into one of their remote controllers when we can afford one!

The Heatmiser bits are arriving in the next couple of days so I will then be able to see if the output of the 12V transformer is enough for the relays - fingers crossed otherwise I will have to fiddle some more!

 

[pmac10]

Hello all

I have been sent here from Tilersforum so hopefully posting in the right place! I bought some Heatmiser 12v Network Thermostats and need to use one of these to actuate my underfloor heating (240V/2KW so that is roughly 8A).

I am therefore looking at 12V coil Relays or Contactors that would be able to switch this on. From reading through some of the threads on various forums, it seems a relay would be better if possible. Anyway I have found the following from Maplin which is 15A so lots of spare capacity in theory:

The bit that concerns me is the "Maximum switching power 450W resistive" - sounds like it won't handle the load or would it? (Maplins also do a 30A one but the resistive is 840W).

15A SPDT Miniature Relay : Power Relays : Maplin

Alternatively RS do a 16A one but there is no mention of resistive load:

Finder | Relays, Switches and Indicators | Relays | General Purpose Relay | Interface Relays |48.61.7.012.0050SPA

I'd be so grateful if someone could point me in the right direction as am a bit confused! Hopefully posting the above product links is ok...

you have nothing to worry about, the 450W figure you looked at refers to DC loads, the 3600VA figure is the one you need to concern yourself with as you are switching AC. 3600VA=roughly 3.6kW.

 

[japers]

Thanks for the advice. Have gone and got the Maplin one, just because it is easier (local store) and cheap! I am now waiting for the flooring to test it... If it works (and I am sure it will) then I may upgrade to a better one a bit later

 

[ik_cw]

You can also check out: audon.co.uk/webcontrol/X300.html

They have the X300 that has internal relays and can be used in a thermostat mode to control other units, and you can use a resistor if you're worried about the load not being able to handle it properly.