Help please re.Honeywell CM907 7 Day Programmable Thermostat

KeenDIYer · 17 Sep 2008

I've just moved in to a new house which has no room stat for the central heating. The programmer controls both hot water and central heating which you can select either hot water operation or central heating (which I assume also heats water at same time).

I've just bought the above room stat but now pondering how to wire as I can potentially see conflict with the programmer.

I'm thinking that I should use the switched live from the existing programmer to feed the room stat and then return to the boiler. Will I then need to set the programmer to constant 'on' in order for the room stat to switch on/off as set?

Also, how will this affect hot water; will this then always be heating?

Help/advice gratefully received.

Thanks

D_Hailsham · 17 Sep 2008

KeenDIYer said:
The programmer controls both hot water and central heating which you can select either hot water operation or central heating (which I assume also heats water at same time).

Make and model of old programmer?

Do you have a pump, motorized valve(s) and a hot water thermostat on the side of the cylinder (not the immersion stat)?

I'm thinking that I should use the switched live from the existing programmer to feed the room stat and then return to the boiler. Will I then need to set the programmer to constant 'on' in order for the room stat to switch on/off as set?

That's one way of doing it.

Far better to disconnect the switched live from the old programmer and connect it to a permanent live, then there is no chance of someone accidentally turning the old stat to twice/once a day and stopping your new sat working

how will this affect hot water; will this then always be heating?

Depends on what controls you have (See my earlier questions)

KeenDIYer · 17 Sep 2008

Looking at the Screwfix site, it looks similar to a Drayton LP112 though badged British Gas and only has 2 on/off settings.

It's an old floor standing boiler with a single pipe central heating system. Yes it is old and I intend to replace full system but not until next year if I can help it. I just want to make it as energy efficient as I can with the stat for now.

It does have a pump but unsure about motorized valves; how would I check?

There is no thermostat on side of cylinder but I intend to fit one as part of this exercise.

Sorry for my limited plumbing knowledge.

Thanks

D_Hailsham · 17 Sep 2008

KeenDIYer said:
Looking at the Screwfix site, it looks similar to a Drayton LP112 though badged British Gas and only has 2 on/off settings.

My guess is you have a pumped CH, gravity circulation HW, system. If you remove the old programmer from the wall and look on the back there is a link. If it is on G then my guess is correct. In this case you cannot have CH ON by itself - it has to be CH and HW. If the link is on P, then you have independent control of CH and HW

It's an old floor standing boiler with a single pipe central heating system.
It does have a pump but unsure about motorized valves; how would I check?

MVs look like this, but I don't expect you will find any.
View media item 800 View media item 801

There is no thermostat on side of cylinder but I intend to fit one as part of this exercise.

Very unlikely if you have a single pipe, no MVs and the programmer set to G.

Do you intend to fit a a cylinder stat before you convert to two-pipe?

It might help if you could post pics of your installation. The overall view is better than close-ups of each part, then we can see how it is plumbed together

KeenDIYer · 17 Sep 2008

I think you're right in that CH cannot come on without HW as there is a single on/off time setting. HW can run alone but CH runs to time set for HW.

Is it still worth looking a the back of the programmer; and does this make the use of my Stat impractical?

I was going to fit a cylinder stat before changing to a 2 pipe installation, is this not wise?

I'll try and get the pic's loaded.

Thanks

KeenDIYer · 17 Sep 2008

I've attached photo's as asked in album titled boiler.

Does this help?

Thanks

D_Hailsham · 17 Sep 2008

I suspect that the two large pipes disappearing into the ceiling go to the HW cylinder. What does the pipe out of the pump connect to, apart from the rads?

A pic of the HW cylinder and the surrounding pipework will confirm what I think.

A HW circuit is essentially a single pipe system, so fitting a motorized valve and cylinder stat should not be a problem. These will give you the idea:

View media item 827 View media item 73

KeenDIYer · 18 Sep 2008

Thanks for the diagrams, very useful. One question though remains about my existing programmer. If the CH isn't independantly governed by the programmer (ie. HW automatically fed when CH operational) then is it possible to use the room stat I bought or do I need to change the programmer first if this is possible?

I'll add some pics of the immersion tank later.

D_Hailsham · 18 Sep 2008

KeenDIYer said:
If the CH isn't independantly governed by the programmer (ie. HW automatically fed when CH operational) then is it possible to use the room stat I bought or do I need to change the programmer first if this is possible?

Can't see any real problems. In the summer (HW only) the CH stat will be off and the boiler will be controlled by the HW side of your old stat. In the winter, when CH is on, the HW times will be determined by the CH times.

The reason you must have HW on when the CH is on is due to the fact that the HW is a gravity circulation system so there will be circulation through the HW cylinder whenever the boiler is on. The CH ON control starts the boiler and the pump running.

I'll add some pics of the immersion tank later.

That will be helpful.

KeenDIYer · 18 Sep 2008

I've attached the immersion pics.

Unsure where the other pipe from the pump runs to. It appears to come up by the immersion and continue up into the loft???

D_Hailsham · 19 Sep 2008

KeenDIYer said:
I've attached the immersion pics.

Thanks.

You definitely have gravity circulation hot water, which can be converted to a C plan. The valve could be put into either of the horizontal pipes going into the side of the cylinder.

If you do what I suggested earlier and connect your new programmer to its own 230v supply (i.e. not from the CH ON of the old programmer), your old timer will just control the HW. It is wired up to control the valve to the HW tank, so you will only get the HW being heated when the valve is open; and this will only happen when the cylinder stat says that the tank needs heating up. You will then have independent control over HW and CH times/temperatures.

When you convert to a two pipe system, both old and new programmers can be retained, but you may need to alter the wiring to the valve.

Unsure where the other pipe from the pump runs to. It appears to come up by the immersion and continue up into the loft???

Which way is the pump pumping, towards or away from the boiler? (there should be an arrow on the body of the pump.) If it is towards the boiler, this pipe is probably the cold feed from the F/E tank. If away, it will be feeding a radiator ~~somewhere.~~

KeenDIYer · 20 Sep 2008

Pump arrow points upwards.

Assuming this is correct, I'll now go ahead and do the work.

Thanks so much for your help.

D_Hailsham · 21 Sep 2008

KeenDIYer said:
Pump arrow points upwards.

Assuming this is correct, I'll now go ahead and do the work.

Go up in the loft and check where the pipe from the pump goes, before doing any work. I'm only guessing that it feeds a radiator.

The motorized valve you need is a Honeywell V4043H1106 28mm Zone Valve.

As for the wiring, the diagram, repeated below, needs modifying to account for your CM907.

1. The time controller shown is your old programmer. You should remove the wire CH ON terminal as it is not needed.

2. The room thermostat shown will be replaced by your CM907.

3. The CM907 L and N terminals go to the L and N terminals on the terminal strip.

4. Put a wire link between the L and A terminals of the CM907.

5. Connect the CM907 B terminal to terminal 5 on the terminal strip.

View media item 73

KeenDIYer · 21 Sep 2008

Thanks so much, that's next weekend planned then!

All the best

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Help please re.Honeywell CM907 7 Day Programmable Thermostat

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