Wiring Room Thermostat to Honeywell ST 7000 Programmer

Discussion in 'Electricians' Talk' started by Axel, Oct 31, 2011.

  1. Axel

    Axel New Member

    Hi,

    I want to connect a room stat to a Honewell ST 7000 programmer which currently has no room stat and is wired as follows.

    Brown - L - labelled L
    Blue   - 4 - labelled CH On
    Black - 3 - labelled HW On
    Zero   - 2 - labelled HW Off
    Green - not connected and cut off at entry to programmer.

    The question is what terminal do I connect the Com from the room stat to?  There is no earth from the stat.  The live needs to go to L and the neutral to Blue (4).  The Com from the stat must go to 3 or 2 both labelled as hot water.  Connecting to 3, the black cable which will be the live to the boiler, means that the stat is doing nothing so I think it must go to 2 but the labelling is confusing the issue.  I believe there are normally 2 live feeds to the boiler on the programmer one for HW and one for CH so the single feed is confusing and obviousy I do not want the room stat to control the hot water.  This is an old system due to be changed next year but we need to get through the winter with some control on the heating.

    Can anyone clarify how to wire this please?

    Axel     
  2. wally

    wally Screwfix Select

    Usually the room stat goes in series with c/h on, its just like another switch.   But without knowing what system you have cant be sure.
  3. sinewave

    sinewave Screwfix Select

    Terminal 3 should go to a wiring centre somewhere and if a typical S plan should be connected  to the orange wires from the C/H & H/W 2 port valves.

    You need to install the Stat in series between the outgoing Brown live feed from the C/H 2 port valve to the Stat common then the return trigger feed from the Stat to the communal Orange wires which will also be grouped with the Boiler pump live.
  4. removed 7

    removed 7 New Member

    It seems likely that you have a 'Y-Plan' system here.  The new thermostat can be wired directly to the  ST7000 programmer if you don't have a conventional wiring centre, or it can be wired straight to the circualation pump, via a joint box.

    If you can only wire to the programmer, then it's a simple matter of removing the wire to 'CH ON'  and connecting the brown feed to the 'stat there, then the switched live return from the 'stat will connect to that disconnected wire via a small connector, 

    Depending on the type of room stat, you might also need a Neutral connection, which will entail the use of 3 Core & Earth cable.


    Lucia
  5. wally

    wally Screwfix Select

    Be interesting to know what system he has, you could be correct Lucy asumed you based it on H/W off wire on the clock but sine might also be correct because some of the very old satchwell valves needed to be driven closed but I doubt any of them exist nowadays.
  6. removed 7

    removed 7 New Member

    Who knows, Wally. in the absence of fuller information? I don't need to compete with Sine on heating systems because he does't know pig's @rse from pork in this area.   But he should be aware, that whatever the system a thermostat always goes before a valve and certainly not after it for obvious reasons!



    Lucia
  7. wally

    wally Screwfix Select

    I never really read sines post, only refering to the Y & S plans, but now you mention it, well spotted.
  8. Axel

    Axel New Member

    Hi,

    Not sure what system I have.  Y or S plans does not mean anything to me.  The programmer is remote from the boiler and there is no wiring centre so I need to wire the stat directly the programmer.  Logically I know that the room stat needs to interupt the power supply supply to the boiler to be of any use.  With this in mind the the post from Lucy makes sense, although I am still concerned that the HW feed is not affected and the HW will still work from the programmer.  I will be switching the programmer to "always on" for CH and the CH will be controlled by the room stat temperature   The stat has three connections L, N & COM.  Does this provide enough information to clarify the connections between the programmer and the stat?  It is in fact a wireless programmable room stat so the transmitter is mounted directly adjacent to the programmer and I only need to get the connections right.

    Thanks in advance.

    Axel 
  9. wally

    wally Screwfix Select

    It does not need to interupt the supply to the boiler, it would interupt the supply to the motorised valve.    We need to know what your system is in regard to these valves.     Have you got 2 motorised valves each with a pipe in & a pipe out (2 port)  or have you got a valve with 3 pipes (3 port) usually in the airing cub
  10. Axel

    Axel New Member

    Sorry I meant the supply to the CH.  There is a three port valve fitted.

    Axel
  11. sinewave

    sinewave Screwfix Select

    Yes Looche, I'll admit I worded that incorrectly, tempting as it may have been by other posters to quickly 'edit' said post to reflect the intentional wording of the return trigger feed to be connected to the brown of the 2 port.


    I'm quite happy to admit to my erroneous Data typo's! 
  12. removed 7

    removed 7 New Member

    Most gracious of you Sine.

    I took the opening post to be evident of a Y Plan, 3-port valve system, since both HW ON and HW OFF terminals were used on the ST700o (as HW OFF is not required on S-Plan).

    So I refer the O/P to my instructions for wiring the 'stat with 3 core&E  directly to the CH on terminal, with the assurance that this will not interfere with the DHW settings.


    Lucia.
  13. Axel

    Axel New Member

    OK, all connected up as advised and working fine.

    As it is a 3 port valve we can have HW without CH but not CH without HW (I believe that two motorised valves are required for that).  At this time of year the programmer is set to CH & HW.  One question (3 parts).

    If the programmer is set to manual, ie "always on" so that the CH is controlled by the temperature on the programmable room stat what happens with the hot water?  There is no cylinder stat so how can the room stat switch the boiler or pump or valve off when it is still programmed for hot water.  It does appear to, as the boiler can be switched on and off with the room stat. If it is set to HW only what switches the boiler off when there is a tank of HW but no cylinder stat?

    Axel    
  14. madhatter1uk

    madhatter1uk Member

    There has to be a stat on the feed into the boiler that switches the boiler directly before the timer, otherwise the boiler would overheat and explode, There are no other signal wires going to that programmer so it cant over ride the progamer to do it via a central heating heating pump and valve plus a master rad. All that stat is doing is overriding the timer output. You shouldnt need to have it on manual to use the stat either, the stat will work on timed or manual.
    But a boiler has to have an outlet in case the water boils up. On some syatems you either have a pump and master radiator and or an expansion tank.
  15. Lectrician

    Lectrician Screwfix Select

    Lets have a photo of the valve.  If you have a standard Y plan system with a three port valve, it is designed for HW only, CH only or both.  If it does not work this way it is wired incorrectly, or you have the timeclock commisioned incorrectly (there is usually a switch on the rear that says "fully pumped" or "gravity".  you want "fully pumped".

    An expansion vessel handles expanded hot water when ever a system heat up, it is not there to handle an overheating system.  Thats what pressure relief valves or header tanks are for.

    A boiler will have an integral stat to control the system water temperature which will operate independently off the cyl and room stat.  This is almost like a backup incase of failure of other controls, or wiring by numpties.  If this stat fails, thats when there are issues which the PRV will take care off, or water will expand and boil up into the header tank (not good).

    "master rads" as you call them are used as heat dumps on older systems, not on modern systems.  A system using a three port mid position valve needs no heat dump or bypass arrangement as there will ALWAYS be a path open for the boiler to dump heat - a three port valve cannot be fully closed, it remains open one way or the other.
  16. sinewave

    sinewave Screwfix Select

    Overheat?....................Maybe!


    EXPLODE?..................
  17. madhatter1uk

    madhatter1uk Member

    Yes thats what I said, just in a different way
    Master radiators are still used on coal boilers.
    A three port valve will have nowhere to dump heat to if all the radiotors are off and the hot water is boiling you have to have a stat in the boiler .
    An expansion TANK, is a header tank, its just another name . perhaps not the correct name but its the same thing.
  18. madhatter1uk

    madhatter1uk Member

    ye Explode, caboooom. pehaps not that dramatically though but if its stuck on and has no radiator output and its boiling up its possible. But it doesn't happen because we have stats, and master radioators and expansion pipes. I wonder what would happpen. we need an old house to test it and find out I like the idea of an exploding boiler. Used to happen on steam trains, still does sometimes.
  19. Axel

    Axel New Member

    Hi,

    Thanks for the replies but I think we have gone off at a tangent here!  I said that there was no cylinder stat so with the absence of a room stat, or the room stat satisfied, do we depend on the boiler stat to switch off the system when the hot water demand is satisfied?

    Axel 
  20. removed 7

    removed 7 New Member

    Axel, you've only recently mentioned that you don't have a cylinder 'stat. This I find very odd as you said earlier that you have a 3-port valve. And these two usually go together - typically on a Y-Plan system.  So God knows what kind of system you really have there, particularly as you now say it's "connected up as advised, and working fine"........

    Even though no harm can come of adding a room 'stat to control the circulating pump for the heating, as advised - I think you ought to seek the advice of a reliable heating engineer, because I'm no longer convinced that you know your system or what you're doing with it.

    Pay no attention to the previous scaremongering posts about exploding boilers or cylinders etc: Overheating will either be dealt with by the boiler 'stat or the expansion pipe.  But do seek local, expert advice, dear.........


    Lucia.

Share This Page