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Myson mep 2 to a nest 3rd gen learning thermostat

Discussion in 'Electrics UK' started by Tommonselina, 12 Jun 2021.

  1. Tommonselina

    Tommonselina

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    I am currently trying to install a nest 3rd gen thermostat on to an existing myson mep2 programme system.
    Can some one please help a guy out and let me know which wires go where and also can I put the wires from my room thermostat straight into the nest?
    Your help is much appreciated.
     

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    Last edited: 12 Jun 2021
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  3. CBW

    CBW

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    If you want to use existing thermostat position and wiring, then you’d be best putting the nest heatlink near the wiring center.
     
  4. Tommonselina

    Tommonselina

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    Thanks for the reply but I'd still need to know about wiring etc.
     
  5. CBW

    CBW

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    Nest pro or someone more familiar with wiring on here. I can do it, but not 100%.
     
  6. ericmark

    ericmark

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    The programmer upload_2021-6-12_22-7-42.png clearly 3 and 4 are switch wires, but there is some thing missing, you have red and blue at programmer and red, blue, yellow at thermostat, but seems likely behind the programmer there are other connections.

    So as CBW says start at the boiler, there are three ways to wire a boiler to a thermostat, extra low voltage on/off, low voltage on/off and varying voltage up/down, the latter is the best.

    Some boilers are limited to just one method, but many allow one to select which is used.

    So first thing is there are two very different boilers, on/off type and up/down type. Old boilers and oil boilers are normally on/off, but gas boiler normally work up/down, however working up/down using the return water temperature needs the water to circulate, so they also have some thing so in summer months it can switch fully off.

    So there are two very different uses for the thermostat, with modern gas using up/down it controls room temperature, using on/off it stops boiler cycling in the summer.

    The same applies to thermostats, anti-cycle normal place is in the hall, and temperature control in main room. Nest Gen 3 is rather a poor thermostat, most in that price bracket can either be set master slave or with connect to TRV heads, Nest Gen 3 is really designed for an open plan house where a single thermostat can control whole house, which is not that common, it was before google got involved linked to MiHome Energenie who did the TRV heads, but google removed support so now rather poor. I know as I use Nest Gen 3.

    So the thermostat should be installed in the fastest cooling room with no outside door, or alternative heating, my problem is there is no such room. Not the heating of the room that is important, it is the cooling, you can adjust the speed of heating using the lock shield valve on radiator, but no way to adjust cooling, so thermostat has to be in fastest room to cool.

    With things like Hive it does not matter so much, as if a room cools fast then simply fit a TRV head to tell Hive to switch on, same with EvoHome, Tado, Wiser, all but Nest. EPL is for when split into zones, think the Drayton Wiser is the same, it can be configured as a master/slave, the Wiser limited to three zones one being DHW, but can also connect to TRV heads, EPH up to 10 zones but does not connect the TRV heads.

    Tado I don't know, both Drayton Wiser and Honeywell EvoHome have extra bit to work OpenTherm. The same with boilers, some are OpenTherm, some need extra module and some can only use there own system (Bosch) and some can only use return water temperature. So there is no one thermostat does all. And how you do the control starts with boiler, as that is the unit your not changing in a hurry.

    It may work with wrong controls, but not work in an efficient way. And the whole idea of Hive, Nest, EvoHome, EPH, Wiser is to improve the efficiency so no point fitting one that does not match the boiler.
     
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  8. Tommonselina

    Tommonselina

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    Thanks for this, I think I either need to get a professional in or just send it back. Much appreciated.
     
  9. AndyPRK

    AndyPRK

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    maybe drayton is a bit easier, and cheaper.

     
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  10. ericmark

    ericmark

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    You can start with model and make of boiler. Then at least you will know what will work.

    The problem as I see it, is if it's worth it?

    It is all very grand saying XYZ is the best system, but if we look at the Drayton Wiser as an example. Basic Thermostat around £130, can't find a price for the Wiser Heat HubR so it can work with OpenTherm, the TRV head around £40 each, so for a whole system with say 6 rooms, looking at £400. And still needs fitting.
     
  11. AndyPRK

    AndyPRK

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    personally I wouldn't bother with smart TRVs

    keep it simple
     
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  12. ericmark

    ericmark

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    I did not see @AndyPRK post when I posted, I have 9 programmable TRV heads, 4 are Energenie MiHome, and 5 are eQ-3, the eQ-3 are bluetooth not wifi, and will only connect to one phone, but can be controlled from the device its self, cost me £15 each, the Energenie are wifi and need a phone or PC near impossible to control direct, originally they were designed to work with Nest, but Nest withdrew support.

    I used the Energenie in the last house with gas central heating and a modulating boiler and they worked spot on, once set, the display shows 4 TRVs-1.jpg the target and current, and if current is over target close the lock shield a tad, and if under open it a tad, and in the old house once set they were spot on, however new owners did not want them, so put the old wax type back, and guess what, now set, they also worked spot on. So yes @AndyPRK is right in a way, not required if heating all rooms at the same time.

    I will admit in this house with oil boiler which does not modulate not quite as good.

    But the programmable TRV head not only controls temperature, it also controls when, so if out at work and returning at 6 to first cook meal, then eat it, then sit and watch TV then go to bed, you can set the TRV heads to same sequence, kitchen warmed first, then dinning room, then living room, then bedrooms. This means as long as radiators are big enough you can heat rooms faster as only heating that room, and not heat rooms not in use at that time.

    Sounds great, but in practice I don't shut doors, so all this clever technology is wasted.

    The video @AndyPRK links to says what I have also found with Nest, it is too clever for its own good, and changes the scheduled heat patten without you doing anything, claimed to be a learning thermostat, but needs to go back to school, I had to turn off the learning features. The geofencing with Nest does seem to work, but also the TRV heads have geofencing so not really required.

    As to how to modulate the boiler, not so sure, whole idea of OpenTherm is the thermostat turns the boiler up/down, and the result in up/down rather than on/off is a smaller hysteresis, i.e. it does not over shoot, we found in old house sun on bay windows could cause the temperature to raise way over settings typically hit 32°C before the radiator had cooled, keeping the radiators cooler, using the programmable TRV and modulating boiler also means they stop heating faster, so still could hit 25°C when sun came out, but that was far better, it was due to bay windows, they were a sun trap, but reaction speed is important.

    I has a stupid Bosch boiler so OpenTherm not an option, however by fitting both a wall thermostat and TRV in the hall it could be set to work nearly as good. Idea is open front door and TRV opens reheating the hall, but before it switches off the wall thermostat, it starts to close again, so the wall thermostat only gets warm enough when the weather is warm to turn off, it stops the boiler cycling in warm weather, that's it only job, room temperatures are set by the TRV heads. It does mean the hall TRV and wall thermostat need setting to work with each other, but did it in last house so can say yes it works.

    However that means having an expensive wall thermostat rather pointless, if the wall thermostat is programmable also the TRV head needs to be programmable so they match, but don't need to be linked, linked is easier to set up, but not needed.

    So it is the TRV head that does all the work, so look at cheap first, the Terrier i30 and the eQ-3 both not wifi, the eQ-3 has two versions one with and one without bluetooth, the bluetooth makes setting easier but is not required.

    So this 61dmtMm13BL.jpg has two buttons plus a dial which you can also press so I have right hand button set to alternate between eco and comfort setting, so I walk in room and press button and it goes from 17°C to 21°C and at next scheduled change back to 17°C, simple, no phone, not wifi, can use phone but easier to press button. It also has open window detect, so in kitchen when we open door to unload car it auto turns of heating for 15 minutes while we unload. Since leaving EU price has gone up, but I paid £15 for bluetooth and seen non bluetooth at £10 each.

    Big question is why do you need a wall thermostat? All it does is switch off heating on a warm day, and I have fingers. TRV set room temperature, so what does the wall thermostat do? A Sonoff wifi switch can switch heating on/off as you approach or leave home.
     
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