Replacing Four Wired Thermostat

Discussion in 'Electricians' Talk' started by Nick Gerrard, Nov 13, 2017.

  1. Nick Gerrard

    Nick Gerrard New Member

    Hello Folks,

    I would like to replace my old thermostat (Eberle Lifestyle 15015.3521) which seems to have a low temperature accuracy with a digital one (perhaps the Honeywell DT90, any suggestions?).

    I was wondering if someone could help me with the wiring. The old thermostat has 4 wires (see photos), whilst the ones on the market now seem to have two wires generally. I know that one of the wires is mains (using voltmeter), and the third is earth, but the other two seem to be both netural. Do you know which ones I would need to connect if I was to fit a 2 wire thermostat?
    Front wiring.jpg Wiring diagram.jpg
  2. Nick Gerrard

    Nick Gerrard New Member

    (Not sure if this is necessary but here are photos of the unit where I set the times for the central heating + hot water).

    P.S. Would anyone know if my setup would be easily replaceable by one of those new thermostats/control units, like the Nest?

    Attached Files:

  3. nigel willson

    nigel willson Well-Known Member

    Looks to me like normal 3 wires! + earth . The two blacks are connected together, so effectively 1 cable!
  4. Nick Gerrard

    Nick Gerrard New Member

    Many thanks for the reply, so the idea would be to ignore the blue and yellow cable and just connect both blacks and the brown in the new thermostat? Or would I connect the blue to the same place as the blacks (from the wiring diagram in the second photo it seems like the blue and blacks are on the same line)?

    Any idea why there would be two blacks in the first place?
  5. Hi Nick.

    On your old 'stat, the terminals are - from top to bottom:

    1) Term 1 - Brown wire. This is the LIVE
    2) Term 2 - 2 x black wires. This is SWITCHED LIVE (the one that turns the boiler on when the stat tells it to)
    3) Term 3 - Yellow wire. This is the EARTH which is unlikely to be required on the new stat. What's more, it shouldn't be yellow, but yellow/green.
    4) Term 4 - Blue wire. This is the NEUTRAL. This will not be required on most digital room 'stats or Programmable room stats provided they are battery-operated (which most are).

    Why are there TWO blacks? Don't know, but I guess that one switches the boiler on and the other switches the pump on. Bear in mind that these black wires become LIVE whenever your stat 'calls for heat'.

    To fit a DT90, you would connect the BROWN (old Term 1) to 'A', and the 2x BLACK (old Term 2) to 'B'. (These blacks wires should really have a collar of BROWN tape wrapped around them to indicate they become 'live'.

    The earth (yellow) will be terminated in a suitable connection on the back box, and the neutral (blue) should be slipped in to a single screw terminal to keep it safely isolated (or simply snip off the bare wire end, turn the wire end over on itself and wrap in blue tape).

    To fit a Nest you'll need a 2-channel type if you also want to control your hot water. But, yes, it can be fitted. But I'm not sure we should be going that advanced considering what I presume is your limited leccy knowledge.

    What I would do in your situation, I think, is to live with the hot water heatings you currently have (ie keep the SIEBE there for DHW) but replace the old room stat with a programmable room stat like the Honey CM907 (or any other model that takes your fancy). Then you would simply set your SIEBE to be 'ON' 24hrs for CH, and let the prog stat do everything else.
    Peter Hing likes this.
  6. Nick Gerrard

    Nick Gerrard New Member

    Many thanks for such a thorough reply. I've had a look at the Honeywell CM907 and I think my current controller (front photo attached) can actually do something similar (just without as many independent temperature/time setting). As I have a single temperature which I want (when I want the heating on) I guess a simple digital thermostat would be enough. You're right regarding the nest, my plan is to switch to one when/if the boiler needs replacing, and get it fitted professionally.

    2017-11-13 20.31.43.jpg
    Deleted member 33931 likes this.
  7. MGW

    MGW Well-Known Member

    Old central heating had three or four devices which controlled it.
    1) Room thermostat.
    2) Frost thermostat.
    3) Tank thermostat.
    4) The programmer.
    Modern central heating does not turn the domestic hot water on/off, the storage tank is insulated well enough to not need to switch it off, and for the room heating the programmer, frost thermostat and room thermostat are all built into one programmable thermostat. To upgrade the old programmer is either bridged out or simply left on continuous.

    The whole idea of controlling one room and crossing ones fingers that other rooms will be good enough has also gone, today we use thermostatic radiator valves (TRV), so each room is independent. The whole problem which you have highlighted is called hysteresis, that is the gap between the thermostat turning off and back on again, the neutral supply to your old thermostat heated up a resistor to try to reduce the hysteresis.

    Although a digital thermostat may have just 0.5 degrees between on and off, it is more down to where it is in relation to the radiator. It may seem odd, but being close to the radiator but allowing for the heat from the radiator often works better than on the wall opposite to radiator and if the thermostat is at 90 degrees to radiator it has a very large hysteresis. Ways to reduce the hysteresis vary, fan assisted radiators are good (Myson) and the electronic head for the TRV also works well, the latter because of two things, one it has two temperature sensors one measures water temperature so corrects the air temperature reading, and the other is it does not switch on/off but slowly opens and closes the valve.

    There are wall thermostats which will also control the flame height of the boiler rather than simple on/off, most well known is OpenTherm but the boiler needs to be of a type which will work with them.

    There are also what seems to be simple wall thermostats which can reduce the hysteresis by starting to switch the boiler on/off before the set temperature is reached and using a mark/space ratio i.e. switching boiler on/off with different lengths of on/off so the over shoot is less, the Honeywell Y6630D Wireless Room Thermostat does this and it is reflected in the price. The Flomasta 22199SX is a lot cheaper and programmable and it is easy to think why pay more, and in my open plan house a similar one no longer sold in UK has been doing a great job for years, and in some cases the anti-hysteresis software can mean it does not work as well when combined with the anti-hysteresis software also built into the TRV electronic heads.

    Thermostats like the Wiser and Nest are designed to link into the buss of the boiler so give a very good constant temperature if the boiler is designed to use them. It would seem EvoHome is the best system for normal radiators it makes the like of Hive and Nest look as if designed in the dark ages. However the price tag is rather high.

    So step one work out what will work with your boiler, next is decide what level of control do you want, balanced with price, I made a mistake with mothers system, either go for cheap, maybe terrier i30 stand alone heads on the TRV and a cheap thermostat, or go whole hog and get something like EvoHome, those systems between the two extremes are a waste of money in most cases.

    I went for a system where I could alter room temperature with the phone, however if using the global positioning data of the phone to switch the heating up, then it needs to activate around 100 to 150 miles from the house, I rarely go that far from the house so rather pointless, and the idea of getting your phone out 2 hours before to start for home and switching on heating, well far better to use a simple timer.

    In my own house I have fan assisted radiators, this has spoilt me, they are so fast heating up a room I can suffer the half hour it takes to warm house. However they do make some noise, and also house cools down quicker too. Plus they will not work with EvoHome.
  8. If you are happy with one basic temp setting and a few timings, then you are absolutely right - no benefit in fitting a Prog Stat.
  9. Nick Gerrard

    Nick Gerrard New Member

    Many thanks for the advice guys. I decided to go with the DT90 which I was picking up today... but THEN black friday sales started and the Nest is at its lowest price ever. Eugh... now everything is up in the air again.

    Having had a look at the Nest wiring diagrams it seems that by method of elimination I have an S-plan system
    1. I definetly don't have a combi boiler
    2. I definetly don't have district heating
    3. I don't think I have an opentherm boiler- couldn't find anything re opentherm in the manual for the Vokera Mynute
    4. I'm pretty sure I have two motorized valves rather than one - which would rule our Y-plan (although I'll check this tonight)
    The S-plan diagram says the programmer needs 6 connections. This is where things become a little complicated. Although I have 6 wires leading in to my current programmer, 2 go into live, and 2 into neutral, so I am assuming that one of each of these would need to be diverted into heating common, and hot water common. Is there anyway to find out which is which?

    I'm assuming that "hot water on" would go to "hot water relay normally open" and "heating on" would go to "heating relay normally open"?
  10. Nick Gerrard

    Nick Gerrard New Member

    Decided to go for a professional install with the nest. Don't want to fry the cat accidently!
    Deleted member 33931 likes this.
  11. stateit

    stateit Well-Known Member

    With items such as the Nest, you will be going through the 'Eugh' feeling every year as a new model is brought out, and in the lead up time to that event every time the model you have is discounted so as to clear old stock...
    Deleted member 33931 likes this.

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