Honeywell CM707 / Nest Thermostat e

Discussion in 'Electricians' Talk' started by Ian Birch, Sep 27, 2021.

  1. Ian Birch

    Ian Birch New Member

    Hello all, hoping for some help. I have a Honeywell CM707 thermostat connected to a Logic Combi ESP1 35 boiler. This is in a new build house. I want to replace the thermostat with a Nest Thermostat e which I bought, but I'm struggling with the wiring.

    As per the image, my current thermostat has a brown wire connected to A (Live according to the sticker?) and a black wire connected to B (Neutral according to the sticker?). The Grey and Yellow/Green wires aren't connected to anything.

    I'm unclear what should be connected into what on the Nest Thermostat e (Y1, G, R, W1, C, *OB). Image below.

    Any help would be appreciated!
  2. bright_Spark

    bright_Spark Screwfix Select

    You need to alter the wiring at the time clock too, you cannot just connect the wiring from the old stat as it exists as the nest runs on 12v d c and the wiring you have at the stat is 240v so you will simply blow the nest. The other parts upplied with the nest is used to connect your old stat wiring to bring it down to 12vdc. All of the wiring diagrams are with the nest. You need to fit the other parts along with altering the wiring from your old time clock. You should have a permanant supply, a switched h/w (call) and switched heating wire (call for heat). This is all at the time clock.
  3. MGW

    MGW Screwfix Select

    You say Nest E but show a Nest gen 1 not sold in UK. The plate you show is for the USA system.

    It likely will need re-wiring, as the Honeywell CM707 thermostat is on/off, Nest e can be used on/off, but that would be a bit counter productive when boiler is OpenTherm and thermostat is also OpenTherm.

    But you also say "New Build" it is required to have zones in new builds, all that is required is TRV heads, but often one finds zone valves, but the Nest system does not seem to be designed to work with zones, EPH do an OpenTherm thermostat which can be set as Master/Slave but I know of no others, often the zone valves need removing to fit the likes of EvoHome or Wiser. The Nest thermostats are really designed for hot air systems, there is no provision to even link to a TRV.
  4. Ian Birch

    Ian Birch New Member

    Thanks @bright_Spark & @MGW . It appears I picked up a US version 'Nest Thermostat e'. Those details were omitted from the eBay listing :rolleyes: Back to the drawing board.
  5. MGW

    MGW Screwfix Select

    I found in mothers house the thermostat fitted by central heating installer was inappropriate for the installation. Thermostats work in a number of different ways, you have the simple on/off, and the modulating thermostat, and some boilers don't give you the option of using a modulating thermostat, but the on/off thermostat has two completely different uses. This does seem strange at first, but it can control the room temperature, or it can stop cycling.

    With an oil boiler in the main the output is fixed, so there is an on/off thermostat designed to reduce the hysteresis and it does this by as it approaches the target temperature it starts switching the boiler off and on, call a mark/space ratio, this gradually reduces boiler output so it does not over shoot. However with a gas modulating boiler this is counter productive, as every time the boiler is switched off by the thermostat any heat in boiler is lost out of the flue, and on switching on again it does so at full output, so the boiler never gets time to modulate and the mark/space switching does seem to work, but causes the boiler to work inefficiently.

    So with a gas boiler the TRV head is king, this is the device used to control room temperature, and because it gradually opens and closes it works in harmony with the boiler, allowing it to modulate as it is designed to do.

    However there is a limit to modulation, typically a gas boiler is 6 to 26 kW output, once it is down to 6 kW it starts to cycle on/off, however when the boiler does the cycling rather than thermostat, then it switches back on at minimum output, so far better, but it can never switch off completely.

    So we want a wall thermostat that on warm days will switch off the boiler, it seems odd, but a large difference between on and off temperature is actually an advantage, you don't want the boiler switching back on again if the sun is out and we have a warm day.

    We also have a third on/off thermostat, one which works out how much the temperature will over shoot, and turns off before the target is reached, but does not use a mark/space ratio to adjust boiler output. So we have three types of on/off thermostat.
    1) Simple on/off
    2) On/off with mark/space ratio
    3) On/off with predicted over shoot adjustment.
    There are also one other, Hive uses a link to the TRV heads and a 30 minute call for heat system, so it is really a hub telling the boiler when required by gathering information from the TRV heads.

    Some other thermostats also connect to the TRV heads, EvoHome and Wiser being two good examples, however these also have the option to fit an OpenTherm module and turn boiler up/down rather than on/off which is even better, Nest is odd one out in that does not need a module fitting all built in, but also has no link to TRV heads.

    There is a problem where zone valves are used, Wiser thermostats will only allow use of the Opentherm module when single channel, so with new builds often there are zone valves, so they will not work with the OpenTherm module, EPH seems to be the odd one out, it does not link to TRV heads, but it can work as master/slave, so even with zone valves it can still control boiler with OpenTherm.

    Some boilers like Bosch have not adopted OpenTherm, they have their own modulating thermostats, but these only work in one room, so unless open plan home not really very good control, it would seem the Hive system was designed to get around this problem when boiler manufacturers only allow on/off electrical control.

    The net result is there is no best thermostat, it all depends on your boiler and your home which will work best.

    Also of course life style. The geofencing is pointless for me as retired and very rare leave the house for long enough to be worth turning off. I found with the Energenie TRV head the anti hysteresis software was OTT, and it takes a few hours to get to set temperature, I would cheat and set two degrees higher then wanted for first hour to counter this. Wiser however has a memory built into the TRV head, so it knows at what point to start turning off, so can reheat a room faster.

    But then we also have cost, the maximum any smart heating is likely to save is £100 per year, so spend £1000 on smart controls and it will take 10 years to pay back if we forget about interest payments, longer if with include that. However it does also increase comfort, but the big question is if a simple TRV head like the eQ-3 or Terrier i30 will give you enough control without massive expense, I fitted 5 eQ-3 and 4 Energenie mihome TRV heads, the former at £15 each blue tooth version the latter £35 each wifi version and frankly the eQ-3 seem to do a better job, neither links to my Nest Gen 3 wall thermostat, and since using an oil boiler not using the OpenTherm option. In all rather disappointed with Nest. But it did do what I wanted it allowed DHW and CH to be both controlled with two wires.

    I must admit the Hive wall thermostat with linked TRV heads will likely work with most boilers, however unless at least some linked TRV heads fitted, with gas modulating boilers not an efficient system, it needs some TRV heads to stop it cycling. Where it fails is when zone valves have been fitted, then either Wiser or EPH seem king, depending if boiler OpenTherm enabled or not.

    But once you look at price of thermostat and maybe 10 TRV heads at £50 each then question is if worth it? I find my house does not cool that fast, even if out of house 10 hours on a very cold day it has only dropped 4 degrees C at most, so is it really worth turning off?

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