Changing cylinder thermostat help!?!?

[Steve_uk]

I am a novice with regards to central heating systems but have managed to change like for like parts on my system in the past..... However, I'm a little stuck with my latest task. I'm looking to replace my cyclinder thermostat which is a Drayton HTS3 for an electronic dual cylinder thermostat. (Model DDS1). I was hoping it would be a like for like with regards to the wiring......
My current Drayton has just 3 connections being live, neutral and common.
When I wire into the new DDS1 thermostat with the same 3 wires being live, neutral and common I have a problem. Basically it seems to constantly call for heating from the boiler and will not switch off.... I don't know how to make it only call for heat when requested from the timer.
I'm not sure if this thermostat is compatible or if it is simply my lack of knowledge now showing and I'm missing something obvious.
(The new thermostat has extra ports being NC and NO which the Drayton does not)
To help further, my current heating setup consists of a 3-port valve, and a tado smart room thermostat and hot water timer downstairs.
Any help would be massively appreciated. Thanks.

 

[Newboy]

My current Drayton has just 3 connections being live, neutral and common.

More accurately common, call & satisfied.

I'm not familiar with your new stat but hopefully others with more knowledge than me are ............................

 

[echoes]

As above, you do not have a neutral at your old stat; you have 3 live wires, and the stat connects com to either no or nc (normally open / normally closed), depending on whether the cylinder is cold or hot.

Your new stat requires these connections, and also requires a permanent live and neutral. So you will either need to run an extra cable, or replace the existing cable with (at least) 6 core (l,n,com,no,nc,earth) flex.

 

[stem]

The Drayton HTS3 wiring connections are as below:



But I am a bit puzzled, because the wiring diagram I found for the DDS1 is as below:



But doesn't match your description:

The new thermostat has extra ports being NC and NO which the Drayton does not.

Based on your description:
NC corresponds to Drayton 'Satisfied'
NO corresponds to Drayton 'Call for Heat'
Common or Com, the third wire will be a switched live supplied from your time controls.

Then if the new thermostat requires a permanent Live and Neutral, as per echoes post above, this will need to be added by connecting the L & N to the same fused spur supplying the boiler and the rest of the heating system.

 

[oilhead]

Above comments are generally correct, except the NC connection will be 'call for heat' and NO is 'Satisfied@

 

[stem]

Above comments are generally correct, except the NC connection will be 'call for heat'

Possibly. But it's not always the case. Seems to depend on the manufacturer. In the example below NO is 'call for heat'. There isn't an NC connection.



OP. In any event it's not a major issue, if you find that the thermostat works in reverse, ie turn it up and it switches 'off' turn it down and it switches 'on' simply swap the NC and NO connections over.

Calling the terminals 'call for heat' and 'Satisfied' is much easier IMHO

 

[Steve_uk]

Thanks for all the help and advice. I now have a better understanding so will taken a look at the existing wiring. I think I might just need to add a permanent live and neutral to my existing wires..... just need to figure where from!?
It seems with my lack of inderstanding that I had incorrectly thought that two of the three existing wires were a permanent live and neutral which is how I wires them into the new thermostat. I think this now explains why it was constantly calling for heat!!
Thanks again for all the help.

 

[ReganAndCarter]

The diagram below clearly shows how things work on your 'Y Plan' heating system when hot water only is on. When the hot water only comes on on the programmer, it sends current to the common terminal of the cyl stat. This is then relayed to either one of the other two terminals - ie. if the water is cool then current is passed to terminal 1 (on diagram) and when the water is heated/satisfied, the current then goes to terminal 2.

http://octaveblue.co.uk/c_heating/hw_only.htm


http://octaveblue.co.uk/c_heating/