Replacing Old Myson Room Thermostat with Honeywell DT135

[fuglyfish]

Hi there, I would like to replace an old Myson room thermostat with a newer Honeywell DT135

Old Myson Wiring....

so there are three wires
1. Brown ... Live?
2. Black with brown tape
3. Light grey with blue tape
3. Unused earth

The new thermostat only needs 2 wires from what i'm reading, i obviously need live (brown) , what do i do re the other wires please ?

picture taken from another site of the new thermostat inside as mine has not arrived yet

if someone could kindly give me a steer it would be appreciated ..thanks

 

[ericmark]

The old thermostat likely has a compensation resistor to reduce the difference between on and off temperatures, but they would work without it, so likely the blue is neutral.

With electronic thermostats they often have batteries and don't need the neutral.

However thermostats can be low voltage or extra low voltage, and analogue or digital, analogue is best, some thing like OpenTherm but digital (simple on/off) can be still used, in the main room temperature is controlled by the TRV (thermostatic radiator valve) and the wall thermostat is only there to turn off system in warm weather.

In the main there is some legend some where to show what is what, I see nothing on either picture, I can guess on that the wires are, but better to be sure.

 

[stem]

It's a bit tricky to be sure when you don't give us the Myson model, or show where the wires go. I suspect that it could be a MRT1. If it is, it will be wired as below and this diagram should be printed inside the lid.

If, and only if, it is a MRT1 then the wires you are interested in for your new thermostat will be those now in terminal 1 [Live] and terminal 3 [Switched Live] which you will find probably correspond to the brown wire and the black wire with brown tape. If so, the wires presently in 1 and 3 will go to the top two terminals of the DT135 marked as below, it's only a switch so it doesn't matter which way around they go.

The Honeywell DT135 is battery powered so doesn't need a neutral so that should be isolated / insulated and tucked safely out of the way. It is also double insulated so doesn't need an earth connection so that too should also be isolated / insulated and tucked safely out of the way.

If your thermostat is not a MRT1 or the wires don't correspond to the diagram above please don't proceed but post back further details and someone will be able to confirm.

 

[Mottie]

in the main room temperature is controlled by the TRV (thermostatic radiator valve) and the wall thermostat is only there to turn off system in warm weather.

I thought it was a big no no to have a TRV fitted to a rad in the room where the wall thermostat was fitted?

 

[EFLImpudence]

I thought it was a big no no to have a TRV fitted to a rad in the room where the wall thermostat was fitted?

I would say the statement:

, in the main room temperature is controlled by the TRV (thermostatic radiator valve) and the wall thermostat is only there to turn off system in warm weather.

is not true.



The odd person might do it like that if they wanted but hardly "in the main".

 

[fuglyfish]

It's a bit tricky to be sure when you don't give us the Myson model, or show where the wires go. I suspect that it could be a MRT1. If it is, it will be wired as below and this diagram should be printed inside the lid.

View attachment 314708

If, and only if, it is a MRT1 then the wires you are interested in for your new thermostat will be those now in terminal 1 [Live] and terminal 3 [Switched Live] which you will find probably correspond to the brown wire and the black wire with brown tape. If so, the wires presently in 1 and 3 will go to the top two terminals of the DT135 marked as below, it's only a switch so it doesn't matter which way around they go.

View attachment 314712

The Honeywell DT135 is battery powered so doesn't need a neutral so that should be isolated / insulated and tucked safely out of the way. It is also double insulated so doesn't need an earth connection so that too should also be isolated / insulated and tucked safely out of the way.

If your thermostat is not a MRT1 or the wires don't correspond to the diagram above please don't proceed but post back further details and someone will be able to confirm.

Hi there

you are correct i've had a peek and it is the MRT1

ill focus on the cables that you have mentioned and really appreciate all the input thanks

 

[ericmark]

I thought it was a big no no to have a TRV fitted to a rad in the room where the wall thermostat was fitted?

No not really, in an ideal set up having the wall thermostat in a room on the lower floor (clearly as heat raises) with no alternative heating (this includes sun through the window) keep cool (as don't want it to trigger in summer) and no outside doors (as they will clearly cool the room when opened) it may all work without a TRV in the same room as wall thermostat.

But in the real world such a room does not exist, so the room normally used is the hall, and this has a door in it, so if the lock shield is set to keep the room needing slightly more heating than rest of house, once door is opened the hall becomes not just cool but rather cold, so we fit a TRV in the hall as well, then when the door is opened the TRV also opens and reheats the hall reasonably fast, but before it turns off the wall thermostat the TRV closes, not completely off, but with vastly reduced flow, so the last degree to turn off the wall thermostat takes a lot longer, this allows modulating gas boilers to do as they are designed and modulate, and gain the latent heat in the flue gases, if a gas boiler is externally turned off, on turning it back on, it will run flat out, and so insulate a temperature hysteresis, but if it turns down then off due to the temperature of the return water, on turning on again it does so at minimum output, reducing the temperature hysteresis to a very low level.

What I am saying on/off control needs to be as infrequent as possible, if you use a modulating wall thermostat then what I have said is not required as the thermostat will not generate a hysteresis, so using the likes of OpenTherm gets around the problem and allows the gas boiler to gain the latent heat, also with an oil boiler in the main they don't modulate anyway so again what I have said it not really true, but I would think most people today do use a modulating gas boiler without ebus control so in the main yes we do use a TRV in same room as wall thermostat.

There are always exceptions, my last house was open plan, there was no hall as such, or doors between down stairs rooms, and the Myson fan assisted radiator ensured there was air movement between the rooms, so not TRV's on lower floor, and just one thermostat positioned between the two main rooms. TRV's stopped bedrooms from over heating, there was no need for them down stairs, couldn't fit them if I wanted on fan assisted radiators, but homes like that are rare.