Denfoss randall SET2 replacement issues

27 Dec 2014
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United Kingdom
Hi guys.

I have a randall set 2 timer switch which has gone faulty. Im not a plumber but the plumber found out that this was the fault on our heating system. So he said just need to change this timer. Im currently doing my NVQ in electrical installations so I said I can do that. For a temporary solution he did some connections to bypass the timer and just control the heating/hot water by the the boiler manually. So I got the new timer switch today which is a danfoss FP975. This should be the replacement for the set 2 timer that I had. Today I opened the old timer and well I'm a bit confused. I can see he did some temp connections but not sure where. I wrote everything down so I can put it back if it all goes wrong. The connections are as follows. N-neutral , L- live, 1(DHW ON)- hot water cable , 2(DHW COM)- link to L , 3(DHW OFF)- N\C , 4(HTG ON)- Heating cable , 5(HTG COM)- N\C, 6(HTG OFF)- N\C. So there was this link with earth cable which I think is the temp connection. This was on the top of the other connections. 1 was link to 2 and 4 in a terminal block. So I read that the live should be connected to 2 and 5. So I did that. Also removed the temporary connection. When I switch it on I had hot water when I pressed the hot water but the heating did not worked when I pressed the heating button. Just looking at the set2 wiring diagram that suggest to connect 1 to 5. If anyone could help would be greatly appreciated! Thanks [/b]
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If you post a pic of the timer backplate and also say what system you have, eg S / Y plan or other it would help
Hi. Thanks for the reply. The heating and hot water controlled by a HSA 3 valve. The boiler is Potterton Prima F 30 F boiler. I start to understand more of the whole thing. If the htw on connected to com the hot water comes on. Same with the heating. So my hot water cable connected to htw on and my heating cable connected to heating on. The commons are linked together. When I switch the heating on it sounds like it try to start. So i think the controller gives the power to it but then it stops. However when i press the hotwater. The boiler kicks in and starts like it should. Could it be the hsa 3 playing up? Also when I put the hsa3 valve into Manual mode and press the heating, the boiler kicks in and the heating starts. Here are some pics.
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Okay here is how it works now.

1, when I press the hot water button the hot water comes on.

2, when I press the heating button the system starts a little then stops and there is no heating.

3, when I have both of them on they both working (I can feel the hot water and heating pipes getting really hot)

I thought it might help to tell you this. 😃

Are you sure there has never been a connection to terminal 3 of the programmer?

To get the three port valve into the heating only position you need a HW_OFF signal.

From the look of it, at the moment this voltage will only be available when the Danfoss programmer has HW_ON selected and the cylinder thermostat is satisfied.
It won't be available when you have the HW system OFF, so the valve will only move half way and the boiler won't fire up.

Can you supply a picture of the wiring centre?
Hi. Thanks for replying. What you are saying is kinder what i realised after posting last. Basically my old set2 programmer had 2 options. Hw or CH+HV. So I never actually had an only CH options. What threw me is that this new controller has a separate button for each. And the 1st thing the instructions says you should try out both buttons. Im not 100% but maybe this is the way it should be with the current wiring. There was no more cable other then I stated before. What do you think? Thanks. Here is a pic of my old controller.
I was hoping there would be another wiring centre or junction box where the room and cylinder thermostats, boiler/pump and three port valve wiring all connect together. That might shed more light on the type of wiring plan in use.
O that. It's not a nice and tidy junction box. Just cables connected and twisted together. I did open it to look but didn't want to disturb anything so I closed it back😃. Did you read my previous message? What do you think? Thanks
Sorry, it looks like you'll be stuck with the old setup of Hot Water only or Heating + Hot Water. It would need more information on the present system to decide if it was possible to rewire and get a Heating-only option to work.
I know what you mean. But thats okay. As long as I fixed/replace what it was before Im happy. I can see that with more research I could probably improve the system. Thanks for your help

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