Difficult to be sure without seeing how it is wired, but do you have 2 elements in the tank (one high for just heating half a tank, one low for heating a whole tank) or one element and a change over switch?

I'm also assuming you have 2 sets of cables from your meter, some that are live all the time and some that are live only at off-peak times.

Please check this is true before reading the below, as it may not be the case.


The 6400 is normally used with hot water and heating,  and is in effect a pair of time clocks, that operate two groups 

of  contacts that can keep different times.  manual can be downloaded here 

As such this is not normally used with E7. Equally you could have the two clocks wired with one on each supply, or there may be a change over relay somewhere so that whenever the E7 supply is on, the system uses it, but if there is demand at other times, then it gets put on the day rate supply.

Basically to be able to help we will need you to do a bit of detective work.

I think most E7 systems charge everything at the off-peak rate overnight - so an off-peak supply can sometimes be emulated by using a single 24-hour circuit and a time switch that only switches on during the overnight period. That saves running an extra cable and so on, but has the disadvantage that if your clock and the supplier's get out of sync you can inadvertently end up using electricity at the peak rate instead - or indeed get into a right tangle if the user doesn't understand exactly what's going on and tries to re-program their clock.

   - Andy.

You say “an immersion heater” so presumably just one immersion heater, therefore the two switches are linked together after the switches, therefore both lights come on.


Set the clock and leave the peak switch off so it runs through the clock.


Andy B.

Check the time the suppliers meter is set to and set the immersion clock to match if the time is out.


Andy B.

If the clock is in fact an ST6400c then it only has 3 amp switching contacts.

Unless the immersion heater is controlled through a relay, which I really doubt.

Hi guys,


thank you all very much for your response, although half of it doesn't mean much to me as I'm, well, without any knowledge on this topic.

I'm not even sure now whether it's an immersion heater (as I have emailed honeywell as well and they said they don't use their controllers for immersion heaters, what means im most likely wrong)


I have figured out it's gonna be easier for you if I add some pictures to the topic so you don't waste your  time on me too much.

Please see attached. I now know, they are separately connected to the top and bottom of the tank, which I haven't checked before (apologies). I thought I could read it off the tank what sort of a heater this is but can't figure it out..(the agency nor landlady are not any good which is annoying and weird at the same time). As you can see, the onpeak is wired into the bottom, off peak is wired into the top. As I've mentioned before, the off peak, even tho it gets lit as per timers settings, does nothing if the onpeak is not switched on. (I have tested it tonight) I hope this gives you more insight - i'm definitely learning from your answers. 

I should have mentioned that I have switched electricity supplier recently. (this was finished less than a week ago)

Also, I'm not entirely sure how to set the controller against the timer in the utility room as well, it just rolls . The offpeak hours seem to be running until about 7:15 in the morning what strikes me as a very odd time. 

Thank you

Kind regards

Tom