Replacing CH controls - Advice please

22 Nov 2003
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United Kingdom
I have an Ideal Mexico boiler with a Switchmaster 400 timer, which is faulty and a Satchwell TLX2356 room stat. I am planning to replace the controls with the following:-

Honeywell CM67 prog room stat with opt start
Honeywell ST799 programmer.

I plan to have the heating on constant and have the room stat look after the timing. The HW will be on twice a day, set by the programmer.

Does any one see any problems with this setup or can offer any wiring advice, also would there be a better better set of components?
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Your existing system will have to be capable of heating only, if it is a standard gravtiy HW and pumped CH then you will need motorised valve/s and appropriate wiring to do timed HW and Progstat heating.
The current system is an 'S' plan (1 Pump, two 2-way motorised valves)

The only changes I am making is to replace the controls, just wanted to make sure they were the right choice and to see if anyone has had problems with these.
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Make sure you have a bypass fitted.

My controls are exactly as yours, but the programmer came first by a long way. You don't need a 2 channel programmer - no point having more than a single one for the HW. 799's are darned expensive too, unless you have to have a single pattress sized one.

You can get systems which let you program the HW temp/time, or you could fit an extra stat higher or lower on you cylinder and use a timer to decide which one is in use. You can have timers on all your trv's... etc
I have seen some of the programmers that have the HW control, unfortunatley non of these seem to have the optimum start functionality of the Honeywell, unless anyone knows of one?
You may well be right there. Check the webites for the Smartfit and the 2/3 equivalents. Which I daresay you've already done!

I do wonder how effective o/s is. I've had to turn it off for half the customers of mine who have it. It's confusing to have the boiler sitting there Off when YOU aren't warm enough!
Just purchased my ST799(single patress type) and the CM67 and am about to start the intsallation. I have taken photo's of all of the connections in the old programmer and room stat, beats making a diagram!.

I thought I would start with the cm67 first.

The existing room stat has a twin and earth wich comes from the fcu, plus another pair (red and light green) which runs to the so called wiring centre, which is a single gang patress with lots of connector block bits (not very helpfull, so i think i will replace with a proper honeywell wiring centre).

The live from the twin and earth is to T3 on the room stat. The neautral is to T2 and also wired to the light green via connector block. The earth just goes to the backplate for the stat. The red from the other pair is to T1 on the room stat.

The cm67 requires 2 wires only. I assume these will be the live from T3 and the other red from T1. Can someone please confirm. I assume that the neutral is only for the shunt heater in the stat.

Curiously enough the green in the other pair is not connected to anything in the 'wiring center??

The motorised valve is a honeywell f8-9616, of which there are two on for ch and one for dhw.

Thanks in advance.

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