honeywell cm927

[mark206000]

Hi all,

I'm looking to replace the old timer switch on my 15 year old oil boiler. The timing part of the existing switch is broken and I just use the rocker switch on it to turn the heating off and on.

Mine is a basic boiler that heats up water and pumps it round the house and into the hot water cylinder. No separate part for heating hot water by itself.

I asked a plumber about the honeywell cm927 and he said 'this will only work with a combi oil boiler and will be ineffective on a conventional'.

I couldn't see why this was the case as surely it only needs to switch a relay, basically what i'm doing at the minute with my finger on the rocker switch on the old timer.

Sorry for the long message but I just want to be sure I'm getting the right thing before I buy it now, he's put doubt in my mind !

I see in the online manual wiring diagram, section 4
- a. Burner (direct control)

http://www.honeywelluk.com/Documents...l/pdf/1069.pdf

I assume this is how I need to wire it and the plumber is talking nonsense ?

Sorry again for the long message

Thanks,
Mark

 

[Whitespirit66]

The plumber is right. You need a 2 channel programmer, one channel for ch, the other for hot water. The 927 is only single, as well as being a thermostat. You say you need a timer (programmer), not a stat.

If you want a good quality one, this one will do you -

http://www.heatingcontrolsonline.co.uk/honeywell-st9400c-7day-programmer-p-374.html

If you already have the wires in place, you don't need wireless.

 

[Agile]

ST9400C is the two channel programmer designed for timing CH and HW.

You will also need a room stat to control the temperature!

The CM927 is a programmable thermostat. It is a single channel controller which can set different temperatures at different times.

So you could use the CM927 for the heating and a simple one channel timeclock to set times for the HW.

So in this case the plumber was right although he could have been more helpful to suggest what you could use.

There are many different controls and most installers just use a few models they are familiar with.

Tony

 

[mark206000]

Thanks for the reply !

I have no other way of heating hot water other than turning the central heating on.

I've only got one switch which turns the oil boiler on or off. Nothing else to do the hot water separately. If the central heating is not on, no hot water.

I want wireless because the timer is out in the back of the garage at the minute and want a thermostat to have more control of the temperature.

Thanks,
Mark

 

[Whitespirit66]

After I posted, it did run through my mind that maybe your system maybe lacking on the controls front.

The trouble is, adding a single channel programmer with a room stat will mean that the hot water won't heat if the rads have satisfied the stat.

It would be a good idea to add a motorised valve to seperate out the hw and ch.

This will entail pipework changes. Depends how much you want to do. In it's current state, your system will be inefficient, and time consuming to operate. Small changes shoould be quickly repaid. Overheating your hw is also not a good idea, especially if you are in a hard water area.

 

[Agile]

Yes, you need to add a motorised valve(s) so that the system is fully controlled. We used to charge about £240 for that.

Only then can you get fancy controls!

Tony

 

[mark206000]

Thanks for all the info. It's all getting a bit out of control tho !

I have an old broken timer, it's got one 3 way switch, timer - off - on. That's it, nothing else.

I get no hot water any other way apart from when the central heating is on. It's an old basic oil boiler.

I don't know about motorised valves or anything else. I'm happy how everything works when I put the switch to on.

All I want to do is replace the old broken unit with it's one switch that turns the boiler on or off, with another one.

The existing one is a grasslin thing, analogue clock on it and one switch, can't see model number, had to remove bottom cover to see the grasslin name. The house is 16 years old, so I'd say the timer is the same.

Thanks !
Mark

 

[cherryred]

If you just to replace like for like you just want a time switch.
Here is one cheap and cheerful which will give the same on/off times every day.
You could of course go for a 7 day which means you could set different times for each day.
Have a look on Ebay. There's plenty to choose from.
You could even get an electronic one which will change time at daylight saving.



http://www.ebay.co.uk/itm/24-Hour-M...al_Electrical_Fittings_MJ&hash=item3cbdd18cd5

 

[cherryred]

Another possibility to give some form of control and would not break the bank or involve any change to the pipework is to fit a cylinder thermostat and a room thermostat. These could be wired in parallel so that if any one calls for heat it will bring on the boiler. When both are satisfied the boiler would be switched off. Its not ideal but it would be an improvement on what you have. I assume the boiler cycles on its own thermostat and you adjust that to control the heating?
If your cylinder is gravity fed you may be able to control the pump from a room thermostat for Heating control and use the cyclinder 'stat to control Hot Water.
If you could offer more details eg is the cylinder heated by gravity?

 

[mark206000]

Just for info, I've removed the old timer switch and installed the cm927 in its place and all is working as it should be.

Just followed the manual for the wiring for a conventional boiler and no problems at all.

Cheers !
Mark