Fitting rt500bc on Worcester 28i Junior

[alekc]

Hi, I recently moved in a new home where we have a Worcester 28i Junior with an mechanical timer (judging by it's code it's GRASSLIN ELECTRO MECHANICAL 24HR TIMER) which is driving me crazy with ticking. So I thought to replace it with rt500bc.

Wiring diagram on the back of the old timer and on the rt500bc is the same, so I assumed that it should be an easy swap. Once wired up, receiver is not pairing (no light at all).

I contacted salus support asking them if there is something wrong in my wiring (maybe they need 4th link as well?) or the receiver unit is faulty, and got a response that rt500bc is not compatible with Worcester 28i, and I should by myself a ST620WBC.

I am a bit perplexed by this (given the fact that the old unit is powered by a simple mechanical relay with a timer), so would like to hear your opinion as well.

This is back of my current (old timer):

Back of rt500bc

I connected wires in following way:
red to 1, black to 3, and blu to 4

Any thoughts?

 

[stem]

First of all, the Salus needs a mains electrical supply to L & N to power it, but you do not have a connection to the L terminal.

Secondly, the voltages stated on the back of the existing timer are not mains voltage, so I doubt you will find the 230 vac required.

Having said that, the Salus switching contact is electrically isolated from the mains supply, so it can be used to switch the voltages stated, but it can't be powered by them.

 

[alekc]

Damn, didn't think about voltages, my mistake.

So from what you are saying my options would be
1) return it and go with ST620WBC (if I decide to stay with SALUS)
2) Connect L &N terminals directly to receiver's line 1 & 2 (getting them directly from the main pcb, in picture on the left), and keep 3&4 as they are for using switch on/off logic (whole procedure not extremely complicated but not without danger either ).

Am I correct?

 

[stem]

Sounds about right to me.

If the black and blue wires are the switching contact, which they appear to be (easy to check with a multimeter) then you could wire them to the Salus contacts using terminals 3 & 4, then you would need to make a separate mains L & N connection to the Salus to power it. The red wire currently going to the timeswitch motor would not be required.

I have never seen a Salus ST620WBC but I believe it would be a straight plug and play swap for your boiler, so I guess it operates at the same voltage as the existing Grasslin. Being specialised, I imagine that it will cost a bit more though.

The Salus units are getting popular now, so if you keep it I would recommend changing the operating frequency, I have come across two instances where heating was being switched by the thermostat next door because they were operating on the same frequency. Doesn't apply if you live on an isolated farm of course.

 

[alekc]

Yeah, I saw couple of threads about this issue (fq conflict) when doing my research about this unit, hopefully it will not be the case.

Thanks for hints, I'll confirm that 3&4 are indeed switching contact, and decide if proceed with installation or replacement (yes, st620wbc is about 15 quid more, and I kinda prefer rt500bc estetic)

Cheers

 

[stem]

The terminals LS and NS on your boiler PCB terminal strip can provide you with a live and neutral respectively and are provided specifically to power remote thermostats / programmers. It will be easier than trying to get the wires in the same terminals as the main supply.

 

[alekc]

Should I add new wires to existing ones or it's safe to remove the old ones?

 

[stem]

Sorry I don't understand. Which wires are you referring to?

 

[alekc]

Maybe I got you wrong. By "pcb terminal strip" I assumed you were speaking about white plastic strip shown in my pic above, which is on the left.

If that's the case, Ls and Ns terminals (which should be 3 and 4th screw from the right based on worcester manual https://www.worcester-bosch.co.uk/support/document/download/8716106766 page 19 fig 31) are already used by grey and black(?) wire.

That's why I asked if I should add new wires (which will go to receiver) to existing one or if I should remove old wires.

 

[stem]

The terminals shown in the manual you have posted the link may not correspond to your photograph, so it might not be for an older / newer your version of your boiler. The note on the cover says it's for models newer than FD485.

I can't see too clearly in the your photo, but aren't terminals Ns and Ls the empty ones next to the blue wire? the letters Ls stand for Live supply and Ns is for Neutral supply. Bosch Worcester provide them specifically for external controls.

I don't know what the grey and black wires are for, possibly a frost thermostat?

 

[alekc]

Yes, you are right. It's not clear from the angle (I can't confirm it till I get home), but looks like they are indeed Ls and Ns, and they are free. If that's indeed the case, I can reuse them without issues.

Thanks again for advises (and patience )

 

[alekc]

Ok, for anyone who will find this topic with google: Once 2 wires (LS and NS) are connected properly, receiver starts to work. Unfortunately it's not possible to use rt500bc with worcester 28i junior because receiver behaviour is inverted compared to original timer. When salus tries to switch on the boiler, contact status on pin 3 and 4 is the same as "switched off" on old manual timer.

In theory I suppose I could overcome this hurdle by trying some combination with an relay, but I don't think it's worth the hassle. So I will be returning this unit for now.

 

[stem]

Thanks for the post. That was very helpful. It does appear that some versions of this boiler don't correspond with the majority of Worcester boilers. The manufacturer provides the same terminals with the same labeling, yet they perform different functions.