Replacing an old SMC Central Heating Control Unit

There's not much difference in principle - ignoring any earths needed, you simply have a live and neutral in, the neutral being for the clock motor or programmer electronics, and two switched live feeds to control the heating and hot water. Some types of boiler had low voltage control, so in which case there would be a low voltage in as well. Nothing terribly complicated if you check *carefully* how the old one is wired.

Thanks for the reply Dave! Nice to know somones out there who is able to advise!

You make it sound very simple, and Im sure on face value when I look at it, it will be simple, but I do get a little bogged down when it comes to circuit diagrams and the type of system which I have, I basically dont know what system I have. My concern is that having read other diy forums, replies tend to say "if you have this system wire it this way, and if its this system wire it this way".

I will get the old control box opened up tonight and draw out how it is wired up, and see if this matches up at all with the new digital control box I hope to get!

On a side note, do you know if there are any regulations in place that state that a wualified person must do this work?

Thanks aga> > > Could anyone advise on whether this would be a sensible thing for me to

I haven't the same problem as you but I've just replaced a digital timer that required a degree in electronics, the instruction manual and at least two new swear words everytime you wanted to change the programme. I replaced it with a nice new analogue mechanical timer made by Horstman and am very happy indeed.

I discovered that the old timer had the wiring identification on its rear (hot water demand. hot water satisfied etc etc) and it was a very simple job to swap wires over like for like, especially as the new timer came with standard wiring diagrams for umpteen other controllers.

Be brave, give it a go!

Julian.

Thanks Julian!

I think I will be brave and go for it! Will amke sure I know exactly what Im doing before I rewire anything in though!

Im h> > I have an old mechanical central heating control unit made by SMC. It

No such animal in my book. They are all a nightmare to change!

I got the Horstmann one from the Screwfix book, cheap, reliable and soooo simple and quick to change!

Anyhow, good luck....

If the old one worked ok then all you have to do is wire the new one the same. Most basic programmers are simply two changeover relays operated by the timer mechanism. A changeover switch is the same as a lighting two way switch - one common terminal which is the input in this case, and two output terminals - one which is made with the relay off, and the other with it on. The 'off' one isn't required on some installations. The common terminals are often both wired to the incoming line so in practice you may only have two to be confused over. ;-)

Like all electrical circuits, it helps to 'break' them down into the simplest of blocks no matter how complicated they look at first. In this case you have three such blocks - the 'motor' which controls two changeover switches.

The main FAQ covers control systems. There is easily enough out there about the subject. If you come across any misunderstandings then ask here. If this is all too much for you then you will have to get in someone who's done it before.

Hi Ed I hope you can help me with my problem i haved moved the manifold from one room to other and put all pipes back as thay off but all rads are cold all up rads are normal hot the system is 8mm microbore the check valves are the ones with both pipes going to one valve only one valve on each rad please email direct if you would i will give my phone number and would be better to explane things thanks steve

replying to Thomarse, Janet wrote: Hi Thomarse Did you manage to find a solution for your SS heating control unit as I am in the same situation?