Replacing a central heating / hot water timer controller

Replacing a central heating / hot water timer controller

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Dr Mike Oxgreen

Original Poster:

4,101 posts

164 months

Wednesday 22nd February 2017
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I need to work out why my Dad's central heating isn't working. For some reason, he had taken the electronic timer control unit off the wall - but I don't know why, and alas he died about six weeks ago so I can't ask him. I fear he may have been living without central heating through the winter and never mentioned that there was a problem.

The timer unit is a Honeywell ST 7100, pictured below. Also pictured below is the back-plate behind it (note the lovely wallpaper).

I'm going to have a go at simply bunging some new batteries into the controller and screw it back to the plate and see what happens, but I doubt that's going to fix it. I will also try taking the clear plastic cover off the back-plate and poke a multi-meter around to see if we're getting 240V in appropriate places.

Assuming I can't fix it, I'd like to try buying a suitable replacement timer/controller. I can't see an identical unit on ebay - perhaps it's too old and obsolete. Can someone tell me whether this back-plate is a standardised fitting, and whether newer control units would simply plug straight into it? Could I simply buy this, for example? (It says "fits industry standard backplate" on the box, which sounds good...)

Edited to add: Just realised that one I've linked to is only single channel, which won't do. How about this one instead? (This one also says "fits industry standard backplate", which sounds good...)

The old timer/controller:


The back-plate on the wall:




Edited by Dr Mike Oxgreen on Wednesday 22 February 15:58

smn159

12,446 posts

216 months

Wednesday 22nd February 2017
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Don't know whether the one linked to will fit straight on the existing backplate, but assuming it comes with its own backplate they're not difficult to swap over

moles

1,794 posts

243 months

Wednesday 22nd February 2017
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That ain't an industry standard backplate!. Will need to be reworked not hard just jiggling around with what's already there.

Cerbhd

338 posts

90 months

Wednesday 22nd February 2017
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As already stated a new controller wouldn't fit that back plate, very easy to swap for new though, just live, neutral, earth, hot water on and central heating on.
I would suggest fitting existing controller and putting central heating on, check you have a supply getting to the room stat, just because your late father disconnected it, it may not be the faulty part.
Sorry for your loss by the way

Dr Mike Oxgreen

Original Poster:

4,101 posts

164 months

Wednesday 22nd February 2017
quotequote all
Ah, okay. So do these devices usually come with the backplate, or,do you have to buy that separately?

If it's just a case of disconnecting all the wires and connecting them to the equivalent terminals of a new backplate then I agree it doesn't sound hard.

Alucidnation

16,810 posts

169 months

Wednesday 22nd February 2017
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Many of the new programmers have conversion wiring instructions to follow, depending on what you are replacing.

Cerbhd

338 posts

90 months

Wednesday 22nd February 2017
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Yep, the new one will come with a backplate and is an easy job to swap but why do you think it is the programmer that is the problem?

Rickyy

6,618 posts

218 months

Wednesday 22nd February 2017
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You can bypass the programmer simply by putting the CH and HW on wires in with the common wires (after safely isolating the power, obviously). If everything works, its new programmer time.

They are simple to wire up...



Drayton LP series are my preferred model of programmers.

Dr Mike Oxgreen

Original Poster:

4,101 posts

164 months

Thursday 23rd February 2017
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Cerbhd said:
Yep, the new one will come with a backplate and is an easy job to swap but why do you think it is the programmer that is the problem?
Well, simply circumstantial evidence: my Dad had removed it from the wall so presumably he thought there was a problem with it, but you're right that his logic may have been faulty.

I like Rickyy's idea of wiring the "CH on" and/or "HW on" directly to the common live, flip the power on and see what happens.

I'll be at the house this weekend, so plan of action is:
  1. Replace batteries in existing controller and re-attach it to the backplate. Power on - does it work?
  2. If not, power off, remove controller from the wall, remove clear plastic cover from backplate, power on, and check for 240V in appropriate places.
  3. If power is good, power off, wire "CH on" direct to a common live. Power on - does it work?
  4. If not, then it's the boiler or possibly a motorised valve failing to move into the correct position. Call a pro.
  5. If it does work in step 3, then order a new controller.
  6. Er... anything else I can try?
Many thanks for all your help so far!



Edited to add: Should have mentioned that the room stat in the hallway is a programmable one. So if the heating works when connected directly to common live then I could leave it like that for now as a temporary measure. The CH would then be under the control of the programmable stat (which is what you want) and I could set it to a continuous 12°C or something like that while the house is empty.

Edited by Dr Mike Oxgreen on Thursday 23 February 06:29

Cerbhd

338 posts

90 months

Thursday 23rd February 2017
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Sounds like a good plan, If it is a sticking motorised valve then you can just change the syncro motor for a couple of quid, no need to drain down.
Good luck!

mikeiow

5,286 posts

129 months

Thursday 23rd February 2017
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Blimey, exactly the same as ours.
We did have to change the battery this year. Fixed the problem.

Keep meaning to investigate an RF one.....it is awkward to set (in dark cupboard). Not overkeen on the idea of a hive/nest etc, but would like the ability to easily change the settings!

Good luck with it, & sorry for your loss.

Dr Mike Oxgreen

Original Poster:

4,101 posts

164 months

Thursday 23rd February 2017
quotequote all
mikeiow said:
Blimey, exactly the same as ours.
You mean the wallpaper? hehe

mikeiow said:
We did have to change the battery this year. Fixed the problem.
Would be nice if it's as simple as that!

Slight potential complication. The house clearance guys were there yesterday and have "totally cleared the kitchen". I'm fearing that they may have binned the controller! hehe

Dr Mike Oxgreen

Original Poster:

4,101 posts

164 months

Thursday 23rd February 2017
quotequote all
D'oh!

I went to the house this afternoon, and sure enough the clearance guys have done an amazing job on the kitchen. Unfortunately I can't find the heating control unit anywhere, so I reckon they've gathered it up with all the other crap and got rid of it.

But, I was able to verify that 240V is getting to the places it should, and I rewired the "CH on" wire into one of the common lives and switched back on. The boiler fired up, and the rads started getting hot. After a while the boiler cycled off (its circuit temperature control knob was only set to number 3, but it came on again when I turned it up). I figured out how to put the stat into continuous mode, and set it to 12C, and after a while it too cycled off.

So the system is working perfectly, except for the now missing control unit. I guess I'll be buying a new one anyway!

Can't really blame the clearance guys - they can't be expected to know everything that should stay, especially when this house really is chock full of rubbish (Dad was a full-on hoarder like you've seen on the telly).

Thanks for your help, guys!

Cerbhd

338 posts

90 months

Thursday 23rd February 2017
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Glad it's sorted, sort of!

Dr Mike Oxgreen

Original Poster:

4,101 posts

164 months

Saturday 25th February 2017
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Hmmm...

I've just received the new controller.

And it doesn't come with the backplate!

No matter what I search for, I can't find any backplates to buy. Just popped round to Screwfix, and the helpful young chap spent quite a while searching, but also drew a blank.

Please can somebody point me to where I can buy the backplate? The box says "fits industry standard backplate".

Dr Mike Oxgreen

Original Poster:

4,101 posts

164 months

Saturday 25th February 2017
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^^^ Disregard!!!!

I was being an utter numpty. I had failed to identify that part of the controller is in fact the backplate.

It does come with the backplate. I was being a plonker!