Replacing a Switchmaster CT1 with a Drayton HTS3

[JohnPonty]

My cylinder thermostat a Switchmaster CT1 has stopped working and I am looking for a replacement. I am thinking of buying a Drayton HTS3 Cylinder Stat.

My existing setup is the heating and hot water are controlled by 2 Tower 2 port zone valves VAL222MV. The Switchmaster connects to a white plastic box that has a strip of choc blocks. The cable has 4 colours as follows
1 Yellow/Green, 2 Blue, 3 Red, 4 Black

At the cylinder the connections inside the Switchmaster are as follows from top to bottom
Yellow/Green attached to the metal casing underneath the blue control screw
Blue cable attached to second connection
Red cable attached to middle connection
and Black cable attached to bottom connection.


I realise there is a difference connecting to a new thermostat because the Switchmaster has 4 cables and modern thermostats usually have 3.

I have downloaded the Drayton instruction manual and can see the connections/cable colours for replacing other cylinder thermostats but the Switchmaster is not listed.

Can you tell me which colour cables go to which connection on the Drayton HTS3 Cylinder Stat.

I am going to leave the connections to the choc blocks as they are and connect the cables to the connections you advise. I realise the Drayton does not need earthing and I will attach pvc tape around it.

Can you please advise me?
Thanks

If the picture is not clear. The black cable is connected to number 1 stamped on metal case, red cable is connected to number 3 and blue is attached to number 2

 

[stem]

I believe that's a 'Smiths Switchmaster Cylinder Thermostat 1' abbreviated to 'Smiths SCT1'

In which case the 'conversion table' is:

The green/yellow 4th wire connected to the metal casing will be an earth. The HTS3 is double insulated and does not require an earth connection.

With spring return 2-port valves such as Tower VAL222MV, the satisfied connection isn't required. However, your system may have had different motorised valves fitted in the past.

 

[JohnPonty]

Stem,
Thanks for quick reply. Found this picture on Ebay of the parts that were in my house when I first moved in. The three way valve was not installed in my house. The were 2 Switchmaster motorized valves instead. One failed and I replaced them with the Tower motorised valves. I understand just putting PVC tape on the earth which is not required. So you think I should be able to connect the Drayton with 2 cables, but which goes where and what colour? The Drayton installation lists the Smiths thermostat.
Thanks

 

[stem]

The Drayton installation lists the Smiths thermostat.

Correct

Look carefully at the photo of the instructions you posted....

I believe that's a 'Smiths Switchmaster Cylinder Thermostat 1' abbreviated to 'Smiths SCT1'

'Common': Switchmaster 1 = HTS C
'Call': Switchmaster 2 = HTS 1
'Satisfied': Switchmaster 3 = HTS 2
Isolate / insulate the earth

 

[JohnPonty]

Thanks will have a look

 

[stem]

All a cylinder thermostat is; is a switch, and you are simply exchanging one switch for an another with identical functions. I would connect all three wires exactly as they are.

 

[JohnPonty]

I looked on Ebay for a Switchmaster and saw one with a full pack of controls. Apparently the full name is Smiths Switchmaster Cylinder Thermostat 1 abbreviated to Smiths SCT1 which is listed in the Drayton installation manual for the HTS3. I bought a Drayton HTS3 from Screwfix and installed it. I also contacted Drayton for advice on connections. As I mentioned earlier when Switchmaster was installed there was no standard installation for connections as there is now. I tried various suggestions about what to connect where but still no joy.

I then suspected that the Tower valve was faulty. To get hot water I manually locked the valve open with the lever at the end of the valve. This works but only when my other Tower valve is open for heating When moving the lever it sounded noisy whereas the other Tower valve was not. I suspected the synchronous motor was faulty. There are a couple of videos on Youtube on how replace it. Note that the synchronous motor has 2 leads of the same colour and you can use either to connect to the existing connections you have with your model of motorized valve. Buying advice for synchronous motors I learnt while researching where to buy a motor. Amazon sell them for less than £15 but although they are stamped “Original” on the outside they are cheap copies and will not last long. The Drayton motor has “Made in USA” stamped on them and they cost £23 odd at Screwfix or you can buy direct from Drayton if you wish.

Overall a great saving instead of buying a new motorized valve, draining system (if you do not have isolation valves), expense of a plumber etc.