Wiring Difficulties Replacing Very Early Drayton Digistat

[PEDANTICVINDICTIVEMAN]

Alas poor Yorik, I knew him well, electrocuted himself, as red and black he couldn't tell, is this a wire I see before me.........
War and Peace sold volumes, but was long a boring
Chuckle chuckle chuckle

 

[Whitespirit66]

Alas poor Yorik, I knew him well, electrocuted himself, as red and black he couldn't tell, is this a wire I see before me.........
War and Peace sold volumes, but was long a boring
Chuckle chuckle chuckle

I imagine the OP's butler will be collecting his cards soon if he doesn't get the red and black wires the right way around.

He could always sell a couple of mummy's Gainsboroughs, and put the proceeds towards a few of hours of a 'trades person's' time.

Where do they get them from?

 

[Capvermell]

I imagine the OP's butler will be collecting his cards soon if he doesn't get the red and black wires the right way around.

He could always sell a couple of mummy's Gainsboroughs, and put the proceeds towards a few of hours of a 'trades person's' time.

Where do they get them from?

Mummy normally uses a now mainly retired tradesperson (but still doing jobs for old clients) with electrical skills but who seems to have a wife who never ever talks to him and who consequently likes to bore for Britain to all his clients. He likes to fix you with his beady eye and talk at you for an hour about all the minute details of his life before he gets down to work. But he is competent and also very cheap (being one of the old school who is not in touch with the fact that a young tradesperson now feels entitled to earn as much as a primary school head teacher) and I am sure can do the job for £30 if needed. Problem is he may not be available for a week or two.

Anyhow Mother has now got over initial furious shock of room stat not working and being taken off the wall and has been persuaded that the replacement item will be a great deal less hassle to run once in place. Plus in the interim I have successfully fixed her dodgy Watchman oil level sensor unit on her oil tank by taking the top off the unit and cleaning out all the gunge and corrosion plus lifting out the oil float tube so any old polluted oil in the tube that was blocking the sensor beam is now replaced by new fresh oil.

It seems to me that quite a lot of you on here are very two faced indeed about offering help and if you perceive someone as one of the better off classes you resent any possibility of a lost opportunity to stitch them up for the £200 you would charge for job that you would only charge £50 for to your impoverished mate on the council estate.

As it happens I haven't been employed for some while due to redundancy in mid 40s hence trying to make myself useful by taking on some of these jobs normally done by a tradesperson.

At the end of the day I am sure that the more honourably motivated amongst you will still help with advice and those who won't are clearly the guys who are bitter that another opportunity to rook an educated but technically incompetent person has possibly been lost.

 

[Happyplumber]

who gives a **** if you have university education,degrees coming out of your ass your still **** and deserve no help whatsoever

Show a tradesman some respect and they may help you,but i would not urinate on the likes of you if your caught fire !

 

[PEDANTICVINDICTIVEMAN]

Alas poor Yorik, I knew him well, electrocuted himself, as red and black he couldn't tell, is this a wire I see before me.........
War and Peace sold volumes, but was long a boring
Chuckle chuckle chuckle

I imagine the OP's butler will be collecting his cards soon if he doesn't get the red and black wires the right way around.

He could always sell a couple of mummy's Gainsboroughs, and put the proceeds towards a few of hours of a 'trades person's' time.

Where do they get them from?

These Gainsborough showers his mummys got on the wall, are they 8.5Kw or 9.5Kw, dont think she will get much for them anyway as they were never up to much when they were new

 

[Capvermell]

who gives a **** if you have university education

Well you and that massive chip on your shoulder quite clearly do. Hence why you then also object if people use words with more than two syllables, construct sentences with more than eight words or use paragraphs with more than two sentences.

Show a tradesman some respect and they may help you,but i would not urinate on the likes of you if your caught fire !

Can I presume though that you would therefore cheerfully urinate on me in any other circumstances and also be found leerily calling me baldy across the street as you roll out of the pub much the worse for wear and/or be found pursuing me for the next 50 miles waving your fist and screaming out of the window in an insane road rage wanting to punch my lights out if I had flashed your white van to try to encourage you to move over and give up what you assumed was your god given right to try and obstruct the outside lane to prove how hard and cussed you are.

Coming back to the topic I have identified that in the mains switch box for the pump which the room stat controls the circuit for that the incoming black wire from the room stat is terminated on a piece of choco bloc on which an incoming blue wire is also terminated. There is a red wire running from the upper side of the pump circuit mains switch and a brown wire from its lower connection point the other side of the switch. I'm not sure how much exactly this all helps though when the incoming live wire at the room stat is coloured red.

Or would the cheapest analogue roomstat I can get from a DIY shed also now have terminals 1, 2 and 3 and if so could I use this to test whether attaching red to 1 and blue to 2 at the room stat end blows up the stat or not and if it doesn't if it switches on the circulation pump. Or do these analogue stats all still also need a neutral wire attaching them to switch the circuit?

 

[D_Hailsham]

The HW is always heated when the heating circuit is in use, which rather suggests there is no valve to isolate the hot water circuit.

OK. So there is no separate control over HW temperature.

The existing central heating control is a Drayton Tempus 7.

There are two jumpers on the back. The top one should be set to the B position. In this position the CH and HW times are always the same and the boiler is turned on and off by the HW ON connection. All the CH side does is turn the pump on and off.

The CM907 surely does the same job but just adds time related control of the set temperature several times a day and/or on different days of the week.

If the CM907 is set for a lower temp overnight and in the middle of the day and on the HW circuit on the Drayton Tempus 7 left on constant it surely comes to much the same thing as operating the Tempus 7 on Twice and leaving the CH and HW switched off overnight?

You would have to ensure that the times when you want the heating on coincide with those when you want the hot water on.

As for wiring, Red is live and goes to A on the CM907, Blue is switched live and goes to B on the CM907, Black is Neutral and should be made safe. C on the CM907 is not used.

The reason I suggested that the CM07 might not be suitable is that it is designed to control the boiler directly, particularly the PI side which determines when the boiler runs and for how long, so as to prevent overshoot. There is also the Optimum Start feature to consider.

 

[Capvermell]

There are two jumpers on the back. The top one should be set to the B position. In this position the CH and HW times are always the same and the boiler is turned on and off by the HW ON connection. All the CH side does is turn the pump on and off.

Do you mean that if this jumper is altered the Constant, Twice and Once settings will then always be the same for both the CH and HW circuit sides and one cannot set CH for say Off and the HW for Twice? In practice any operation of the heating circuit does actually turn the HW circuit on though. You can however set the HW to say Twice and the CH to Off and the heating does not come on when the HW fires up in that configuration.

If the HW cylinder has no circuit valve that shuts off when it reaches temp there must I presume be a chance of seriously overheating the water in very cold weather. Obviously this wastes energy but could it ever get so hot the tank explodes? I know that in very cold winter weather the hot water in the tank does become already become very very hot (feels like 65C to 70C) if no water has been drawn in recent times.

As for wiring, Red is live and goes to A on the CM907, Blue is switched live and goes to B on the CM907, Black is Neutral and should be made safe. C on the CM907 is not used.

So you are saying that Blue for Switched Live and Black for Neutral is actually the normal colour scheme in a central heating room stat circuit? Obviously this is confusing for the novice compared to use of brown for live and blue for neutral in the plug wiring of a normal appliance. I'm certainly feeling happier about risking attaching Red to A and Blue to B on the CM907 now and I know that it was when Red was attached to Live and Black to Dem on the Digistat but with Blue unattached that it blew up. When Blue was attached to Sat and and Black to Neutral or Black to Sat and Blue to Neutral the Digistat was not blown up at that point. However that makes sense as in effect both those terminals did nothing on the Drayton Digistat as it was configured and only Live and Dem were active?

 

[DeltaT]

I'd like to thank Capvermell, he has cured my insomnia!!

 

[Capvermell]

I'd like to thank Capvermell, he has cured my insomnia!!

I'm happy to hear that you found my post useful then.

 

[D_Hailsham]

Do you mean that if this jumper is altered the Constant, Twice and Once settings will then always be the same for both the CH and HW circuit sides and one cannot set CH for say Off and the HW for Twice?

No. If the jumper is in the A position you can have the heating on by itself. But you said this was not possible, so the jumper must be in the B position. I was asking you to confirm this.

However, you seem to be saying that the jumper is in the A position, which is illogical as you say you can't have heating on its own. Unless you are just theorising and haven't checked the actual position of the jumper.

In practice any operation of the heating circuit does actually turn the HW circuit on though. You can however set the HW to say Twice and the CH to Off and the heating does not come on when the HW fires up in that configuration.

That is what I would expect! You can have HW on by itself or HW and CH together, but you cannot have CH on by itself.

If the HW cylinder has no circuit valve that shuts off when it reaches temp there must I presume be a chance of seriously overheating the water in very cold weather. Obviously this wastes energy but could it ever get so hot the tank explodes? I know that in very cold winter weather the hot water in the tank does become already become very very hot (feels like 65C to 70C) if no water has been drawn in recent times.

As the boiler temperature will not exceed about 80C (controlled by the boiler stat), the water in the cylinder cannot exceed the same temperature, so there is no possibility of it exploding. In any case there is a safety vent coming out of the top of the cylinder which discharges into the cold water tank. Many people with this type of system will turn then boiler thermostat down to about 65C in the summer. This allows the water to heat up to about 60C.

If you want better control, you have two choices:

1. Convert the system to fully pumped This will require a change in the piping, a motorized valve and a cylinder thermostat.

2. Add a motorized valve and cylinder thermostat to existing system. This will not be so efficient as option 1 since the water will still circulate by gravity from boiler to cylinder.

Here are diagrams of the two options; 1 on left, 2 on right.

View media item 69 View media item 827

So you are saying that Blue for Switched Live and Black for Neutral is actually the normal colour scheme in a central heating room stat circuit? Obviously this is confusing for the novice compared to use of brown for live and blue for neutral in the plug wiring of a normal appliance.

No! There is no standard convention for wiring heating systems. Each installer has his own method, some better than others. I deduced from the info you gave that red must be live and black neutral so, by deduction, blue must be the switched live.

when Red was attached to Live and Black to Dem on the Digistat but with Blue unattached that it blew up. When Blue was attached to Sat and and Black to Neutral or Black to Sat and Blue to Neutral the Digistat was not blown up at that point. However that makes sense as in effect both those terminals did nothing on the Drayton Digistat as it was configured and only Live and Dem were active?

The switch inside the Digistat is a two-way one. It either connects Live to Dem or Live to Sat. If the room temperature is below stat setting it connects Live to Dem and if it is above, it connects Live to Sat. When you connected Red to Live and Black to Sat (blowing the Digistat) the room temperature must have been above the stat setting.

In an earlier post you said

When I had red connected to terminal 1 and Black to terminal 3 but also the Blue wire to Terminal 4 (Neutral) this did not result in the Digistat being fried, even though the heating pump did not run.

Presumably this was before you fried the Digistat, so the only logical explanation is that the room temperature was below the stat temperature when you did this test. In any case, if black was switched live it would not fry the stat.

So you are saying these enhanced features won't operate here because they relate to information coming back from the boiler by being attached to the boiler's fire up or stop firing command circuit?

Unfortunately they are not that sophisticated. In any case an older boiler, like your mother's does not supply the necessary feedback. I was just wondering whether you would get the same level of control over the room temperature if all the CM907 did was to turn the pump on and off? It may work, all you can do is try it.

As for the Optimum Start feature, you would have to ensure that the HW always came on before the CM907 started the pump. This could be an hour or more before the time set on the CM907, depending on the outside temperature and how long it takes to heat up the house. If you set the Tempus 7 so the CH side is On then the HW side will also be permanently On. Similarly if the CH is set to All Day the HW will also be on All Day. As you said, turning on the CH always turns the HW on. It has to, otherwise the boiler would not light.

 

[Capvermell]

However, you seem to be saying that the jumper is in the A position, which is illogical as you say you can't have heating on its own. Unless you are just theorising and haven't checked the actual position of the jumper.

I never actually mentioned the jumper and I have never actually so far seen the back of the Tempus 7 or the current jumper position. It was actually you who introduced the question of the jumper setting as I didn't previously know that it existed. All I said was that as configured the Tempus 7 programmer will always turn on the HW circuit light when the CH is timed to come on even if the HW switch is at a lower frequency of operation than the CH (eg CH constant and HW Twice).

As the boiler temperature will not exceed about 80C (controlled by the boiler stat), the water in the cylinder cannot exceed the same temperature, so there is no possibility of it exploding. In any case there is a safety vent coming out of the top of the cylinder which discharges into the cold water tank. Many people with this type of system will turn then boiler thermostat down to about 65C in the summer. This allows the water to heat up to about 60C.

Correct there are some vent pipes in the loft that go in to both the central heating header tank and the cold water tank. Also thanks for the other useful and reassuring information that it should not be possible for the HW tank to explode due to this arrangement.

If you want better control, you have two choices:

1. Convert the system to fully pumped This will require a change in the piping, a motorized valve and a cylinder thermostat.

2. Add a motorized valve and cylinder thermostat to existing system. This will not be so efficient as option 1 since the water will still circulate by gravity from boiler to cylinder.

Here are diagrams of the two options; 1 on left, 2 on right.

View media item 69 View media item 827

Thanks for the helpful information although this would definitely go beyond my skills and require the use of a professional plumber. Coming from a computer background I always feel comfortable working with electrical circuits than with water under pressure.

No! There is no standard convention for wiring heating systems. Each installer has his own method, some better than others. I deduced from the info you gave that red must be live and black neutral so, by deduction, blue must be the switched live.

Unfortunately all the connections to the old terminal points were obscured by masking tape and as the wires fell off with the masking tape attached I didn't manage to fully keep track of which terminal the blue and black wire had been attached to. I am sure that the Red wire was attached to Terminal 1/Live on the old Drayton Digistat. All I can be sure of after that is that there were two wires attached to two of the other three terminals that were black and blue. The only other thing I know is that the Digistat was switching the pump on and off latterly but it could originally have been switching the motorised valve on the downstairs circuit open and shut for the early part of its life and this was possibly then changed to controlling the heating pump at the boiler end by a boiler engineer when the downstairs valve failed and was removed from the circuit. Or did that all happen before the Digistat was installed as I just can't remember now. All I can be sure of is that in recent years the Digistat switched the central heating pump on when the living room temp fell below the set temp and off when it reached the set temp. This cause the boiler to fire soon after as the temp in the boiler dropped as water circulated. Obviously the boiler also fires if a lot of hot water is used even if the CH circuit is off and only the HW circuit is operating.

The bottom line is that I cannot be absolutely sure what the blue and black wires were for. You seem to imply that on the very early Drayton Digistat (when there was only one Digistat and they did not have a number of models) that the stat needed both Dem and Sat wires to be connected and that therefore there was no Earth as only 3 wires were in use? But then how will the CM907 switch the pump on and off when it only has Live (A) and B terminals that I can use or are you saying I need to use B and C as long as neither of them is a neutral.

To complicate matters there are also taped off unused yellow and green wires behind the central heating thermostat wall plate and unused isolated red and black wires in addition to the red and black wires that are used.. As I said before some of these wires arrive at the stat point in a thick yellow plastic sleeving and some arrive on their own without any sleeve encasing them visible (it could of course still be there in the plaster).

In an earlier post you said "When I had red connected to terminal 1 and Black to terminal 3 but also the Blue wire to Terminal 4 (Neutral) this did not result in the Digistat being fried, even though the heating pump did not run"

Presumably this was before you fried the Digistat, so the only logical explanation is that the room temperature was below the stat temperature when you did this test. In any case, if black was switched live it would not fry the stat.

No this is not strictly correct as I blew the 3amp fuse in the wall switch plate controlling the fused spur that supplies current to the pump and the Drayton Tempus 7 CH programmer three times in the course of the proceedings but did not blow up the Drayton Digistat on any of these occasions. Only attaching the red and black wires to terminals on the Digistat without the blue wire connected at the same time succeeded in blowing it up with an unpleasant burning smell and some scorching marks on the back of the Digistat circuitry board.

I was just wondering whether you would get the same level of control over the room temperature if all the CM907 did was to turn the pump on and off? It may work, all you can do is try it.

I don't see why not as the current Digistat simply switches on or off the CH pump and the CH pump coming on always makes the boiler fire up within less than a minute due to the drop in water temp inside the boiler.

As for the Optimum Start feature, you would have to ensure that the HW always came on before the CM907 started the pump. This could be an hour or more before the time set on the CM907, depending on the outside temperature and how long it takes to heat up the house.

It doesn't sound like the end of the world if Optimum Start sometimes doesn't work quite as intended in terms of when the system starts up but if necessary I can always disable Optimum Start if it proves to be a pain. I am very happy with an older model of the Honeywell programmable roomstat at my own home that has 6 temp changes a day and a 7 days programmer and holiday setting etc but no optimum start. Or I suppose at some point I could install the CM907 at my house (where Optimum Start would work properly as the HW circuit is separate there) and transfer the programmer from there to my mother's house. However the very large digits etc on the CM907 were an attraction with an ageing mother in the picture.

Unfortunately I don't think I am really any further forward in terms of being sure I won't blow up the CM907 when I attach the wires to it though. So perhaps time to consider calling in an electrician with CH system experience?

 

[D_Hailsham]

I never actually mentioned the jumper and I have never actually so far seen the back of the Tempus 7 or the current jumper position. It was actually you who introduced the question of the jumper setting as I didn't previously know that it existed. All I said was that as configured the Tempus 7 programmer will always turn on the HW circuit light when the CH is timed to come on even if the HW switch is at a lower frequency of operation than the CH (eg CH constant and HW Twice).

Which confirms that the jumper is in the B position.

I am sure that the Red wire was attached to Terminal 1/Live on the old Drayton Digistat. All I can be sure of after that is that there were two wires attached to two of the other three terminals that were black and blue.

They must have been connected to the Call (Dem) and Neutral. The Sat terminal is only used for cooling systems.

All I can be sure of is that in recent years the Digistat switched the central heating pump on when the living room temp fell below the set temp and off when it reached the set temp. This cause the boiler to fire soon after as the temp in the boiler dropped as water circulated. Obviously the boiler also fires if a lot of hot water is used even if the CH circuit is off and only the HW circuit is operating.

At the risk of repeating myself for the umpteenth time. This is what I would expect with a pumped CH/gravity HW system, which is what you have.

You seem to imply that on the very early Drayton Digistat the stat needed both Dem and Sat wires to be connected and that therefore there was no Earth as only 3 wires were in use?

I have never implied that. The connections would have been Live, Dem (Switched Live) and Neutral. SAT IS NOT USED

But then how will the CM907 switch the pump on and off when it only has Live (A) and B terminals that I can use or are you saying I need to use B and C as long as neither of them is a neutral.

You connect red to A and Blue to B Black is not used and should NOT be connected to C.

To complicate matters etc etc

Provided the system works as it did immediately before you started trying to change the Digistat, I would forget all about the extra wires. They are not relevant to the problem.

Only attaching the red and black wires to terminals on the Digistat without the blue wire connected at the same time succeeded in blowing it up.

With three wires and three terminals there are a limited number of permutations for connecting them and only one of these will blow the Digistat. That is (a) when Live and Neutral are connected to Live and Dem and the wall stat is set above the room temperature; or (b) when Live and Neutral are connected to Live and Sat and the wall thermostat is set below the room temperature. Connecting Live and Switched will not blow the digistat, it will run the pump. Connecting Switched Live and Neutral will do nothing.

Red is Live - CM907 Terminal A
Blue is Switched Live -CM907 Terminal B
Black is Neutral. NOT CONNECTED

Unfortunately I don't think I am really any further forward in terms of being sure I won't blow up the CM907 when I attach the wires to it though.

Only if you don't trust the information you have been given.

 

[simond]

You made your bed, D_H.

He'll want you to bake him a cake next.

Everybody else on this thread can spot a verbose messer when we see one.

 

[PEDANTICVINDICTIVEMAN]

I should have got a bigger bag of popcorn