Replacing a timer switch?

I am an unqualified muppet so my advice is worthless...

Many Timer/Programmers Present 4 signal lines to the boiler and the Hortsmann Centaur Plus C27 appears to be no different... Apart form AC in, the terminal connections are:

1) H/W Off
2) C/H Off
3) H/W On
4) C/H On

as per page 2 of the table here...
http://www.heatweb.com/pdf/Invensys/Programmer%20Compatibility%20Chary.pdf

One challenge for you is making the correct connections in your boiler (you do not mention the make & model) - a specialist might be your best solution.

If your current timer is mechanical an electronic replacement is very probably not "Like for like" - you will need to decommission the current timer and install your electronic one external to the boiler. It will need AC power and a four-conductor control cable to the boiler.

Good luck...

Peccavi
January 2010