Ancient Potterton/Horstman timer has died

[Eastbath]

Hi
I have a very old electro mechanical boiler switch that says Potterton on the outside but the clock mechanism says Horstman and it is stamped a "Q Mk 3"

It has stopped rotating so the boiler HW and CH have to be switched on manually.
I'd like to replace it so I'd really like suggestions on what 7 day/separate HW/CH timer would be suitable and what the current wiring means.
I see the mains supply in.
I also see the mains earth and supply out to the boiler but there are two wires (presumably HW & CH controls) which are connected to screws 8 & 10.
I'd like to know where these should be connected to on a new timer?
Plus anything else you think I should know!
Many, many thanks in advance
Tim

 

[D_Hailsham]

I'd really like suggestions on what 7 day/separate HW/CH timer would be suitable.

Does your existing timer allow you to control the HW completely independent from the CH? Or is it the type where you can have HW by itself in the summer but must have CH and HW on at the same time in the winter?

 

[Eastbath]

Yes you can have HW only but to have CH you have to have both on.
Thanks
Tim

 

[D_Hailsham]

Yes you can have HW only but to have CH you have to have both on.

Then you have what is called a "gravity" HW system. It's a poor description,but it means that the boiler water is pumped through the radiators, but not pumped through the heating coil in the HW cylinder. Instead it relies on natural circulation (hot water rises, cold water falls).

Suitable programmers are: Honeywell ST9400C or the Drayton LP722. Both would need setting for use with a "gravity" system, but this is easy and explained in the installation instructions.

As for the terminals, I can't identify the timer from the info you give, so don't know which terminal is CH and HW. However there's an easy way to find out.

Turn power off
Turn room thermostat down
Remove the timer for the backplate
On the backplate, join Mains Live terminal (probably red wire but could be brown) to either terminal 7 or 8 with a short length of wire.
Turn power on

If the boiler starts, that terminal is the HW connection.

If the boiler does not start the terminal will be the CH connection You can confirm this by turning the room thermostat up. The pump should start.

Gravity type systems are very restrictive, particularly in the winter when you have to have HW on whenever you need CH - even if you don't need one. There are two ways of overcoming this: Convert to fully pumped or convert to a C Plan.

 

[Eastbath]

Great info.
Many thanks
It is likely we will be replacing our ch this year as its well overdue, just unfortunate timing on the control giving up right now!
Thanks again
Tim