Worcester Bosh Heatslave 20/25 Room Thermostat
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What make / model is it?
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The store will maintain temp but the divertor will not open to allow water to be pumped around the rads u necessarily.
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One of the issues with my boiler is the diverter valve, it doesn't seem to be a very good design and gets stuck open, so when the heating is off and you turn the hot water tap on, it sends the hot water around the heating system. I have replaced the valve at least 3 times over the years.
When you get it, if the wiring terminals look like this:
Then the wiring is:
Remove the wire link between boiler terminals 2 & 3
Boiler terminal 2 connects to L (thermostat terminal 4)
Boiler terminal 3 connects to 'Open' (thermostat terminal 2)
Boiler terminal 6 connects to 'N' (thermostat terminal 3)
As you have purchased a programmable thermostat, that includes both time and temperature control, the existing heating time control at the boiler will need to be set to have the central heating permanently 'on'. But don't panic, that will allow the new thermostat to take over control.
Then the wiring is:
Remove the wire link between boiler terminals 2 & 3
Boiler terminal 2 connects to L (thermostat terminal 4)
Boiler terminal 3 connects to 'Open' (thermostat terminal 2)
Boiler terminal 6 connects to 'N' (thermostat terminal 3)
As you have purchased a programmable thermostat, that includes both time and temperature control, the existing heating time control at the boiler will need to be set to have the central heating permanently 'on'. But don't panic, that will allow the new thermostat to take over control.
Make sure that you check it carefully though because there are several different versions of this thermostat (3 I think) and whilst the terminals are numbered 1 to 4 the wiring is different for them, so if when you get it, yours looks different in any way and you require assistance post back.
Here's a better photo.
Here's a better photo.
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Every thing else you say I agree with. However I followed the advice you are giving above for 2 years in my mothers house, I was pulling my hair out trying to control the house, I tried all sorts, starting with the lock shield valve on the hall radiator where the thermostat was, then tried a wireless thermostat which I could move around, to some extent that worked, however rather surprised at where it worked best, it was on a tea trolley just 3 foot from radiator at best, once it worked I realised it was down to the time taken for heat to reach thermostat from the radiator, and being close reduced the hysteresis, however the boiler was a modulating boiler, so it did not get the radiator that hot.
Then the thermostat started going faulty, it lost it's RF range and had to be placed closer and closer to the receiver. I did not want to buy another one the same, so next step was trying programmable TRV to an extent they did work, but it still had a problem, in the main the sun was problem, but also wind, the temperature of the rooms depended on cloud cover and wind direction.
So desperation I tried fitting a TRV in the hall where the wall thermostat was, OK took some careful setting, and as you point out the wall thermostat should be placed in a entrance level room, with no alternative heating, normally kept cool, with no outside door, and of course the hall must be one of the worst rooms, it has an outside door, and a stair case which can have a chimney effect sucking the heat away.
However setting carefully when front door is opened the TRV opens and allows it to quickly recover, however before it gets warm enough to switch the wall thermostat off, the TRV starts to close, so it extends the time it takes to switch it off. It worked.
OK as said it took some careful balancing, but worked.
However has not worked so well with this house, I blame the location of the stairs, at this time the TRV in hall shows target 20°C current 18°C and the wall thermostat shows target 20.5°C current 20.5°C so a 2.5°C difference between the two thermostats. In the morning the TRV moves from 17°C to 20°C but if I do the same with the wall thermostat then it over shoots, so it goes 17°C to 19°C then each hour adds an extra 0.5°C until finally at 20.5°C this ensures the boiler fires up at around one as hour so other rooms are not starved of heat.
In the main it works, however when I go out the occupancy sensor and geofencing turns the wall thermostat to eco to save money, that bit works, but on return it tends to over shoot.
I'm all for keeping things simple 
If the TRV is set lower than the room thermostat, the room will never reach the temperature set on the room thermostat, and it will stay permanently 'on' so the boiler will cycle indefinitely meaning there's no point in having the room thermostat at all because it won't be doing anything.
On the other hand if the TRV is set higher than the room thermostat, the TRV will never turn the radiator off, because the room thermostat will turn off the heating before the temperature on the TRV is reached, so there's no point in it being there.
In effect you would be trying to control the temperature in a single room with two separate devices, one of which will interfere with other. Like 'his' and 'her' TV remotes or tying to use two different satnav's to control a self driving car.
I have a small (small for the size of the room) radiator in my hall where the room thermostat is located, it doesn't have a TRV. The room thermostat switches on the heating and the rest of the house that is fitted with TRV's gets up to temperature, the hall takes a little while longer before the room thermostat switches all of the heating off. It works perfectly, the temperature in each of the rooms is maintained exactly as we want it, and we never have to adjust anything.
If the TRV is set lower than the room thermostat, the room will never reach the temperature set on the room thermostat, and it will stay permanently 'on' so the boiler will cycle indefinitely meaning there's no point in having the room thermostat at all because it won't be doing anything.On the other hand if the TRV is set higher than the room thermostat, the TRV will never turn the radiator off, because the room thermostat will turn off the heating before the temperature on the TRV is reached, so there's no point in it being there.
In effect you would be trying to control the temperature in a single room with two separate devices, one of which will interfere with other. Like 'his' and 'her' TV remotes or tying to use two different satnav's to control a self driving car.
I have a small (small for the size of the room) radiator in my hall where the room thermostat is located, it doesn't have a TRV. The room thermostat switches on the heating and the rest of the house that is fitted with TRV's gets up to temperature, the hall takes a little while longer before the room thermostat switches all of the heating off. It works perfectly, the temperature in each of the rooms is maintained exactly as we want it, and we never have to adjust anything.
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A TRV even an electronic TRV is analogue, it does not switch on/off but up/down, so set a TRV at 20°C and it adjusts flow as to the range depends a lot on the lock shield valve setting, but you have around a degree with electronic and more like 3°C with mechanical.
There is clearly a distance between the TRV and the wall thermostat, and so there is a time delay between the TRV reaching temperature and the wall thermostat reaching temperature.
With a modulating boiler adjusting the two devices to work together works well, I know it works as I have done it. Both used mechanical TRV and set at 3.25 approx with wall thermostat at 19°C and used electronic so can set both an eco and comfort temperature.
However the non modulating oil boiler is more of a problem, the radiator gets hot and the TRV closes, then the boiler switches off and the TRV opens wide so the TRV is constantly adjusting, one can hear the motor running as it adjusts, however one can look on the PC and easy see the target and current temperatures so you can quickly work out what the average TRV temperature is compared with the average wall thermostat temperature, in this house there is around 2°C difference between the two.
My problem is 4 doors and a stair case, as you very rightly said.
this can't really happen with my house, there are 6 radiators all able to send heat to centre of the house where the wall thermostat is located, it really needs moving to the living room, but on my to do list. Leaving a door open or closed, will affect the wall thermostat, so even with the algorithms of the Nest learning thermostat it can't learn as always changing.
However in most homes there is some compromise, be it morning sun shine, opening doors, or other factors there is no perfect location, I have stated entrance level with not outside doors, the only room which would fit that is the toilet/shower room, and that is not suitable for thermostat anyway, every room has doors to outside. And living room is south facing so morning sun can cause the temperature to sore.
In real terms my boiler may cycle, but the thermostat stays on for quite a long time, moving from 17°C eco to 20°C comfort in the morning takes some 2½ hours, if I let it do it in one hit, it would over shoot, so I go from 17°C to 19°C then raise it 0.5°C an hour to 20.5°C so it takes 4 hours to heat hall, it however does not take 4 hours to heat the other rooms, so dinning room, kitchen and living room have all reached temperature set within an hour of change, the stepped change in hall stops hall over shooting as it takes so long for the heat from radiator to reach the wall thermostat.
So main point is time, and set a radiator TRV at 20°C and a room thermostat at 20°C and since the room thermostat is mounted higher than the TRV and it is some distance away the room will heat to 19°C rapidly, then slow down getting the last 1°C as the TRV closes, but will still in the fullness of time reach the 20°C at the wall thermostat and turn off the boiler, however because the last 1°C was slow to be achieved, one every other room in house will have had the time required to heat up, and two the room with wall thermostat will not over shoot to any great degree so boiler will restart sooner so reducing the hysteresis.
My only real problem is geofencing, and occupancy detection, when the wall thermostat detects I am not at home, it auto goes to eco mode, and on return it goes to comfort mode, so if I have not been out long that's OK. However leave the house for 10 hours and when it switches back on it over shoots, I can get around this by turning down the wall thermostat on my return, then turn it back up an hour latter, however I forget, so end up chocking open the doors to rooms being used so hall can cool into those rooms.
There is clearly a distance between the TRV and the wall thermostat, and so there is a time delay between the TRV reaching temperature and the wall thermostat reaching temperature.
With a modulating boiler adjusting the two devices to work together works well, I know it works as I have done it. Both used mechanical TRV and set at 3.25 approx with wall thermostat at 19°C and used electronic so can set both an eco and comfort temperature.
However the non modulating oil boiler is more of a problem, the radiator gets hot and the TRV closes, then the boiler switches off and the TRV opens wide so the TRV is constantly adjusting, one can hear the motor running as it adjusts, however one can look on the PC and easy see the target and current temperatures so you can quickly work out what the average TRV temperature is compared with the average wall thermostat temperature, in this house there is around 2°C difference between the two.
My problem is 4 doors and a stair case, as you very rightly said.
Mine is very poor in its location with two doors between the wall thermostat and hall radiator, and two doors and a stair case on the other side.
this can't really happen with my house, there are 6 radiators all able to send heat to centre of the house where the wall thermostat is located, it really needs moving to the living room, but on my to do list. Leaving a door open or closed, will affect the wall thermostat, so even with the algorithms of the Nest learning thermostat it can't learn as always changing.However in most homes there is some compromise, be it morning sun shine, opening doors, or other factors there is no perfect location, I have stated entrance level with not outside doors, the only room which would fit that is the toilet/shower room, and that is not suitable for thermostat anyway, every room has doors to outside. And living room is south facing so morning sun can cause the temperature to sore.
In real terms my boiler may cycle, but the thermostat stays on for quite a long time, moving from 17°C eco to 20°C comfort in the morning takes some 2½ hours, if I let it do it in one hit, it would over shoot, so I go from 17°C to 19°C then raise it 0.5°C an hour to 20.5°C so it takes 4 hours to heat hall, it however does not take 4 hours to heat the other rooms, so dinning room, kitchen and living room have all reached temperature set within an hour of change, the stepped change in hall stops hall over shooting as it takes so long for the heat from radiator to reach the wall thermostat.
So main point is time, and set a radiator TRV at 20°C and a room thermostat at 20°C and since the room thermostat is mounted higher than the TRV and it is some distance away the room will heat to 19°C rapidly, then slow down getting the last 1°C as the TRV closes, but will still in the fullness of time reach the 20°C at the wall thermostat and turn off the boiler, however because the last 1°C was slow to be achieved, one every other room in house will have had the time required to heat up, and two the room with wall thermostat will not over shoot to any great degree so boiler will restart sooner so reducing the hysteresis.
My only real problem is geofencing, and occupancy detection, when the wall thermostat detects I am not at home, it auto goes to eco mode, and on return it goes to comfort mode, so if I have not been out long that's OK. However leave the house for 10 hours and when it switches back on it over shoots, I can get around this by turning down the wall thermostat on my return, then turn it back up an hour latter, however I forget, so end up chocking open the doors to rooms being used so hall can cool into those rooms.
The rear of the one I have ordered is the same as the one you pictured.
The instructions say to connect L and N to 220 voltage power, 5 and 6 to your gas boiler, 1 and 2 are thermal actuators, please do not connect them to your gas boiler.
My boiler is oil, and according to the FAQ's when I ordered, this thermostat will work with any boiler.
OK, thanks for the additional info. In that case:
Remove the wire link between boiler terminals 2 & 3
Boiler terminal 2 connects to L (thermostat terminal 4)
Boiler terminal 6 connects to 'N' (thermostat terminal 3)
Add a wire link between thermostat terminals 'L' and '6'
Boiler terminal 3 switched live connects to thermostat terminal '5'
Remove the wire link between boiler terminals 2 & 3
Boiler terminal 2 connects to L (thermostat terminal 4)
Boiler terminal 6 connects to 'N' (thermostat terminal 3)
Add a wire link between thermostat terminals 'L' and '6'
Boiler terminal 3 switched live connects to thermostat terminal '5'
I doesn't really matter terminals 5 & 6 are connected to either side of an on /off switch. When the thermostat is 'on' the two terminals are electrically joined together. Convention says that the terminal with the 'switch contact' attached, as per the below is used as the live, but it will work either way around.
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