Fitting room thermostat - confused.

[AndyJ]

Hi everyone ... wonder if anyone can advise this DIY novice.

I've bought a Drayton Digistat 3 to replace our broken (and now binned) Honeywell room thermostat (which looks like, but was not identical to this type. ie the round dial on the front).

In the location of the thermostat there's three wires, red, yellow and blue. From what I can make out of the instructions and from my basic understanding on home electrics (which is not alot, so please do not discount the obvious here), I have the red wire connected to #1, and yellow wire connected to #3 - the blue wire has as been terminated, and there's nothing going to #2 on the Digistat. According to the manual, #1 is common, #2 is call for heating / heating satisfied/call for cooling, and #3 is call for heating (again???).

As you can probably guess, it doesnt work. So I popped up stairs to my boiler - and this is where I'm confused.

Since we moved in we have a timer (made by Wickes if it makes any difference), which is connected to the system. I've taken the timer off, and found that there's 6 connection points, N, L and 1 through 4.

The blue wire I mentioned i the paragraph above, is not connected to anything. Seeing as I terminated it downstairs I guess that bit was right.

The red wire (common) is, at the timer upstairs, connected to point # 4. I can't seem to find what the purpose of, what is common at the DigiStat, going to connector # 4 on the central heating timer is. Ideas ?

The yellow wire goes straight to my Honeywell "3 position diverter valve".

Sanity check .. does all that sound right?

So, ultimately, anyone have any ideas where I'm going wrong here? Please help, the house is nightmare to control, the dog is molting, and the wife is complaining

The DigiStat simply refuses to make the call for heat, when the current temp is less than what I've told the thermostat it should call for heat at.

 

[Paul Barker]

If all you have done is change the stat and everything was ok before then maybe is is of a type that requires the neutral to work, for instance acl. Likely that your blue is neutral and should be connected inside the programmer, and in the stat which you say it isn't.

Or just get a stat that doesnt require a neutral like the honeywell you had...

If that's not your problem secondly check for live at position 3 during operation when it should be live if it is your problem is further along the chain. Keep looking.

Remember boiler only comes on after mid position valve tells it to, not accordint to stat, so if stat functioning look at valve.

When you do get to the bottom of it don't forget to put your £700 test equipment on it all and fill out your forms, or pay a 100 quid to the council to test and inspect your work and issue your certificate.

 

[AndyJ]

I think my problem may be a bit more basic than I thought.

I'm failign to understand how the thermostat operates with the programmable timer.

Given that the programmer has two controls - one for hot water (off, once, daily constant etc) and the same for central heating .. where should both controls be given that the room thermo can (should) ultimately decide when the heating should be on off or indifferent??

Should the heating be on all the time (according to the programmer (constant) ??

Incidently, anyone know the purpose of what looks like two microswitches inside the Honeywell V4073 valve ?

 

[MANDATE]

The timer which you have set decides when CH is put to the on position, although you can over ride it without upsetting the settings. you can make 'off' become 'on' or 'on' become 'off'
Once 'on' it's then up to the room stat to control it by supplying power to the 3 port valve.
CH is demanded at 2 of the 3 ports. Mid positon is one where one of the microswitches comes into effect and holds the valve there and then there's the CH only positon and thats where the second micro switch come into effect by supplying power to the boiler, because the other source through the cylinder stat has been cut off

 

[AndyJ]

Thanks for that Mandate .. I'm getting a little clearer.

Came home today, unsupprisingly to a freezing cold house - clearly the room stat and/or boiler did not do its job.

Room stat was clearly signalling that it was calling for heat (desired temp was 22, current temp was 18). Popped upstairs and the 3port valve lever was to in the middle - which I believe is DWS, right? Left is CH? Yet the rads were cold. So I moved it to the right - and the radiators heated up. If I understood rightly, at the middle position - the valve should be feeding both the DWS and the CH??

One thing I've noticed is that moving the level manually to the right (to CH) makes the heating come on (expected) - but I can't seem to understand under what circumstances the lever srings back. Occasionally, when I've moved it to the right (-> CH), the spring wants it back in the DWS/CH position or in the DWS only position. Under what criteria does the spring pull back from CH only? When the temp is at a certain degree? (I think I read 70c somwhere for something - not sure if this is applicable).

Right now I have the lever manually pushed to the right which has the rads nice and hot - although I could not put the level onto the lip - to keep it permenently on open - sometimes I can - not sure why I can't this time and why I can't othertimes - again .. ideas ?

Thanks for the assistance though - really appriciated.

 

[Paul Barker]

We;ll of moving manually gets the boiler fired up the rstat is working properly. You need a new head for your mid pos valve or a syncronous motor might work. Search under screwfix or somewhere like that.

ACL Drayton ones have plastic head with plastic push piece which releases it easy as pie. Honeywell has tin cover that needs removing to get at synchronous motor.

 

[MANDATE]

Andy! As I understand it the lever is only there to hold the valve in the mid position while the system is being filled.
I've dismantled a valve, which basically comprises a quadrant with gear teeth on the arc which engage with the gear teeth on the motor.There were about six times more teeth on the quadrant than the motor, so in all the motor turns only six full turns to move quadrant from HW through mid way to CH.
When HW only is selected power going to the valve motor is cut off and the quadrant returns back to the start position by the spring, this also turns the motor back.
There are only two other choices, either HW and CH shared or CH only.
If you choose HW and CH shared, then the room stat powers up the white wire on the valve and turns the motor but at the mid positon the power driving the motor is changed by bringing in another circuit via the micro switch. It's this change that holds the motor and quadrant at the mid point and so it stays like that until one of the two systems becomes satisfied.
Depending on which one either the full power is restored and the motor/quadrant moves on to the final position (CH) or the power is removed and the spring drives the quadrant/motor back (HW).
The most likely problems appear to be either (a) a stiff spindle on the valve. (b) a faulty motor, finding it hard to turn the quadrant against stiff spindle and spring. or (c) micro switch that fails to supply power to the boiler due to faulty micro switch or the fact that it was not activated becuase quadrant did not move the full amount.
If you have only CH selected and you have moved the lever over so the boiler functions then you know that the micro switch itself is not at fault.
So that would suggest to me the quadrant is not moving the full amount.
I removed the actuator head, holding it upside down I could see the 'D' shaped hole that locates on the spindle. Then I tried HW only, follwed by HW/CH followed by CH only and the flat on the hole turned 90 degrees.
What had me fooled for a little while was that sometimes I had CH and sometimes not, but soon realised it was only there while HW was being called for. I ran some hot water off and sure enough the CH came on.
I also realised that with an insulated cylinder and no one using HW it going to stay satisfied for quite a period, whereas with the CH it is going lose heat quickly, so the valve does not stay in the mid positon for long periods.
Hope this helps.