Talk to me about CH timers/stats!

[NiceCupOfTea]

Hi all, we have a fairly standard central heating/hot water system. Recent boiler, but ancient CH timer and thermostat.

The stat is one of those 1970s/80s Honeywell electromechanical ones and I want to replace it with a digital one with a timer on it - that way I can leave the CH on all the time at the timer, and use the thermostat to trigger it/time it.

Looking at a Honeywell CM907.

Any obvious gotchas with swapping them over?

Also want to replace the timer. I have read a lot about a standard backplate and timers just attach to them - however, ours is an ancient Randall 4033 - I see they are still sold under the Danfoss name, but ours looks much older. When my parents got a new boiler in the early 80s they got one that looked identical to ours. It works fine but want more control over the timing for the hot water (rather than the 2 per 24 hours we have at the moment).

Is there a 7 day digital timer that will just attach to the same backplate?

[Ferg]

No.
What are your wiring skills like?

[PF62]

I have a Honeywell 927 (the wireless version), which I installed a few months ago to replace an older model that had been in use for 10 years, but recently failed.

Instalation is not difficult, but you do need to think about what you are doing, be able to read a wiring diagram (and work out which of the multiple examples in the fitting instructions is the same as your system) and remember that you are dealing with mains voltage. The wiring diagrams are available to download from the Honeywell website.

Apart from the benefit I bought the device for (of the house being more comfortable, which it easily achieved), I found an unexpected benefit of significant fuel savings. These were achieved by -

- Not needing to 'turn up the dial' when the house felt cold, because it never does.
- If you do need to 'turn up the dial' because it is a time when you are not due to be in, when it gets to the next timed seting, it just goes back to the normal setting.
- A 'party' mode which can be used to extend the heating time, but can be used to reduce the temperature for a set time (for example if you are going out in the evening, but still want a warm house when you get back).
- A 'holiday' mode, lower temperatures for x number of days, before going back to normal.
- A 'tomorrow is Sunday' mode, so it uses Sunday's settings for tomorrow (or for x number of days) and doesn't start the heating at 6am or whatever like a normal weekday.

What is good with all these functions, is that you don't need to consult the manual to work out how to use them. The device is 'idiot proof' in use.

There is also a big saving is from the 'optimisation' system it runs. For example you set it that you want the heating to be 20c at 6:30am, but it doesn't turn the heating on at 6:30. It works out what time it needs to turn on, so it is 20c at 6:30am, and it changes this automatically throughout the winter, depending on the ambient temperature. Obviously this helps from not having the heating on earlier than needed, but also means that the house is always at the 'right' temperature, and again the tweaking of the dial is resisted.

Do consider the wireless version (which can be picked up for not much more money than the wired ones off ebay), and allow you to put the sensor control panel where *you* want it, or even move it around the house with you. Previously my thermostat was in the hall, which had a very different heating/cooling pattern than my living room, so I was always too hot or too cold. Now the wireless sensor is in the living room, and the problem is removed.

On a scale of 1-10, these devices rate 11.

[Simpo Two]

If it helps, this is the point where even DIY Impressario Simpo quit and called a man in. I did the actual wiring but only after somebody cleverer than me told me where each one had to go.

[NiceCupOfTea]

Thanks guys - I'm OK with wiring up to a point but if it's too complicated I'll get my dad to have a look or call somebody in.

I understood with the stat that it's just a couple of wires so "should" be easy enough. If the Randall timer I've got is too prehistoric to have the standard backplate, surely it's just a case of swapping the wires over (assuming of course that it has all the same wires...)

[Ferg]

Wiring heating is very straight forward.
All the 'earths' and 'neutrals' are commoned and it's merely a case of organising the 'lives'.

What actually needs wiring?
There may be...
Boiler
Pump
Programmer/Timeswitch
Room Thermostat
Cylinder Thermostat
3-port valve
2 or more 2-port valve
Frost thermostat.

[NiceCupOfTea]

Ferg -

Sorry, I don't know

I was hoping once I got the cover off the Randall box it would be relatively obvious which wire would go where in the new timer! I realise this may be a pipedream though If not immediately obvious I would "get a man in".

With regards the 'stat I am hoping it should be pretty simple - 2 wires iirc.

Would I be better off with the wired/wireless one? I was going to go wired as I thought that I would just end up leaving it in the wrong room or losing it. We don't have thermostatically controlled radiators - would wireless be better in this case?

[philmots]

Sorry to but into this thread but i'm after doing a similar thing, everything we have in the house is on a combi boiler. I want to be able to leave the manual timer that is currently on the boiler to on all the time and control all the features of the heating system from where i currently have a dial type thermostat in the hallway... I'm assuming this is possible?

What would be the typical wiring set up behind my dial thermostat? I guessed at like 2 wires as it's only currently controling temperature?

Can someone recommend a product!?

[Ferg]

philmots said:

Sorry to but into this thread but i'm after doing a similar thing, everything we have in the house is on a combi boiler. I want to be able to leave the manual timer that is currently on the boiler to on all the time and control all the features of the heating system from where i currently have a dial type thermostat in the hallway... I'm assuming this is possible?

What would be the typical wiring set up behind my dial thermostat? I guessed at like 2 wires as it's only currently controling temperature?

Can someone recommend a product!?

A 240v thermostat will have 3 wires and probably an earth. Essentially you will no longer require the Neutral and you should isolate it at the boiler end. The other wires are simply a switch and will be replaced by virtually ANY programmable room thermostat.

[Ferg]

NiceCupOfTea said:

I was hoping once I got the cover off the Randall box it would be relatively obvious which wire would go where in the new timer! I realise this may be a pipedream though If not immediately obvious I would "get a man in".

With regards the 'stat I am hoping it should be pretty simple - 2 wires iirc.

Would I be better off with the wired/wireless one? I was going to go wired as I thought that I would just end up leaving it in the wrong room or losing it. We don't have thermostatically controlled radiators - would wireless be better in this case?

The Randall timeclock is used as a junction box. This is why there is so much complication. You need to find out what you have in the way of Motorized Valves.

Y-Plan = 1 x Three Port Valve
S-Plan = 2 (or more) x Two Port Valves.

Then, get a 10-way junction box and wire everything back to this including any thermostat or timer. I'll point you at a wiring diagram and BELIEVE ME it isn't difficult.

[philmots]

Ferg said:

philmots said:

Sorry to but into this thread but i'm after doing a similar thing, everything we have in the house is on a combi boiler. I want to be able to leave the manual timer that is currently on the boiler to on all the time and control all the features of the heating system from where i currently have a dial type thermostat in the hallway... I'm assuming this is possible?

What would be the typical wiring set up behind my dial thermostat? I guessed at like 2 wires as it's only currently controling temperature?

Can someone recommend a product!?

A 240v thermostat will have 3 wires and probably an earth. Essentially you will no longer require the Neutral and you should isolate it at the boiler end. The other wires are simply a switch and will be replaced by virtually ANY programmable room thermostat.

Thanks for that. I assume the thermostat will come with a wiring diagram to explain this? Why would i no longer need the neutral? could i not leave it in anyway? (i'm trying to keep it as easy a possible (i'll be doing it myself!))

Thanks.

[Ricky_M]

I'm a heating engineer and just fitted a CM927 in my house, boiler isn't on half as much as it used to be and the house sits at the perfect temperature.

Wiring is simple enough.

Your old clock will probably have about 7 or 8 wires when you take it off. But basically the only connections for a twin channel clock should be a Live and a Neutral and a CH output (ch on) and a DHW output (hw on), maybe a DHW off if you have a three port valve.

You'll need a single channel programmer for the DHW, that will be simple to fit.

If you are going for the CM907 (hardwired) they are simple to wire up, just two wires, common and demand for heat. I'd spend a bit more and go for the CM927 (wireless) as I take mine into whatever room I'm in and never gets too warm.


If you need any advice on wiring just ask!

[NiceCupOfTea]

Thanks for that - are there any CH timers that have an extra channel that can control an immersion heater? Always forgetting to turn the damn thing off...

[Ferg]

philmots said:

Ferg said:

philmots said:

Sorry to but into this thread but i'm after doing a similar thing, everything we have in the house is on a combi boiler. I want to be able to leave the manual timer that is currently on the boiler to on all the time and control all the features of the heating system from where i currently have a dial type thermostat in the hallway... I'm assuming this is possible?

What would be the typical wiring set up behind my dial thermostat? I guessed at like 2 wires as it's only currently controling temperature?

Can someone recommend a product!?

A 240v thermostat will have 3 wires and probably an earth. Essentially you will no longer require the Neutral and you should isolate it at the boiler end. The other wires are simply a switch and will be replaced by virtually ANY programmable room thermostat.

Thanks for that. I assume the thermostat will come with a wiring diagram to explain this? Why would i no longer need the neutral? could i not leave it in anyway? (i'm trying to keep it as easy a possible (i'll be doing it myself!))

Thanks.

In a traditional thermostat the neutral is used in an 'anticipator' circuit (a heater, really) to avoid quick cycling. The modern thermostats use thermistors and have circuitry built in to avoid this. There is no neutral terminal in a moderm programmable room thermostat. You could, I suppose, terminate at the thermostat end, but you may not have the physical room for a safe termination.

[Ferg]

NiceCupOfTea said:

Thanks for that - are there any CH timers that have an extra channel that can control an immersion heater? Always forgetting to turn the damn thing off...

Not that I know of.
A heating programmer is fused at 3amps. Immersion heaters require 13amps.

[NiceCupOfTea]

Ferg said:

NiceCupOfTea said:

Thanks for that - are there any CH timers that have an extra channel that can control an immersion heater? Always forgetting to turn the damn thing off...

Not that I know of.
A heating programmer is fused at 3amps. Immersion heaters require 13amps.

Thanks - what I need is one that turns itself off!

[Gareth79]

I will second (or fourth!) the Honeywell 927 - I read other forums and the pros recommend it for not going wrong, and working very well. I often use the "party mode" to turn the heating down if I'm out all day - as long as you calculate the number of hours correctly it's warm for when you get back. "Holiday mode" can be used to set it permanently low for days at a time.

I was replacing an older Honeywell digital timer (which didn't do weekends) and much older dial type thermostat (which didn't seem to work at all). As mentioned the 907/927 doesn't control hot water, so I left the old one in place to just control that side of things. To control an immersion heater then just perhaps get an old-fasioned clock-type timer?

[NiceCupOfTea]

Excellent!

So, a Honeywell CM907/927 to deal with heating/thermostat duties
One of these to deal with the Immersion heater.

But I still need something to switch the hot water. Any ideas? Something simple and one/two channel is fine (the heating "channel" would just stay switched on and be controlled by the CM927 but a 2 channel might be easier to wire in as it would just replace the 2 channel mechanical timer) but with a digital timer and an advance/override function.

Any recommendations?

[andy43]

Something like this would do for the hot water.

[philmots]

Ferg said:

philmots said:

Ferg said:

philmots said:

Sorry to but into this thread but i'm after doing a similar thing, everything we have in the house is on a combi boiler. I want to be able to leave the manual timer that is currently on the boiler to on all the time and control all the features of the heating system from where i currently have a dial type thermostat in the hallway... I'm assuming this is possible?

What would be the typical wiring set up behind my dial thermostat? I guessed at like 2 wires as it's only currently controling temperature?

Can someone recommend a product!?

A 240v thermostat will have 3 wires and probably an earth. Essentially you will no longer require the Neutral and you should isolate it at the boiler end. The other wires are simply a switch and will be replaced by virtually ANY programmable room thermostat.

Thanks for that. I assume the thermostat will come with a wiring diagram to explain this? Why would i no longer need the neutral? could i not leave it in anyway? (i'm trying to keep it as easy a possible (i'll be doing it myself!))

Thanks.

In a traditional thermostat the neutral is used in an 'anticipator' circuit (a heater, really) to avoid quick cycling. The modern thermostats use thermistors and have circuitry built in to avoid this. There is no neutral terminal in a moderm programmable room thermostat. You could, I suppose, terminate at the thermostat end, but you may not have the physical room for a safe termination.

Thanks Ferg. I might graft in a bit of external help just to keep it all safe!