Thermostat replacement

[Frank Noton]

Boiler is a Worcester Boch Highflow 440 with a TR2 room thermostat. This is an old style bimetal stat and I want to replace it with a digital one, e.g. Honeywell DT90E1012 two wire.

Is this going to be possible? I'm assuming that the current three wires are Live, Live Signal and Neutral and that I would use the Live and Live Signal whilst making the Neutral 'safe'.

Anyone done this sort of thing and can advise - 1) if it can be done; 2) how I identify which is Live and Live Signal.

Many thanks for any help on this matter.

 

[RayPooley]

Don't see what not. It has a switched line. All that needs is something that can apply power to it when heat is required and remove power from it when it is not. It just needs a switched supply. So any thermostat should do. Even a simple clockwork timer could provide that function. Unless I misunderstood your requirement.

 

[Frank Noton]

Don't see what not. It has a switched line. All that needs is something that can apply power to it when heat is required and remove power from it when it is not. It just needs a switched supply. So any thermostat should do. Even a simple clockwork timer could provide that function. Unless I misunderstood your requirement.
View attachment 73435

Thanks for your help on this and glad you say what you have as I see it that way as well. Two further questions if you can help:-
1) How do I best identify the Live and Live Switch wires out of the three on offer!

1)Is it critical which way the Live and Live Signal are wired in, if as you say it is just a switch device providing on/off facilities.

Thanks again, Frank

 

[RayPooley]

Thanks for your help on this and glad you say what you have as I see it that way as well. Two further questions if you can help:-
1) How do I best identify the Live and Live Switch wires out of the three on offer!

1)Is it critical which way the Live and Live Signal are wired in, if as you say it is just a switch device providing on/off facilities.

Thanks again, Frank

An ordinary light switch would do when you think about it from the boiler's perspective. It couldn't care less.

When you buy a thermostat or timer usually you will get a wiring diagram that shows the appropriate wiring
depending on what kind of CH installation you have. There are two types : Y-Plan and S-Plan. The main
difference is in the way that the hot water is controlled. Y-Plan you normally have a single diverter valve
that has 3 positions. HW only, CH only or HW and CH. S-Plan has two separate valves one that allows water from
the boiler to heat the hot water and one that does the same for the CH. So determine what type you have.
If you have a combi boiler it is probably a Y-Plan with a single diverter valve. Once you figure that out
just go with the appropriate wiring for the plan you have.

Assuming you have a Y-Plan this is a typical Y Plan wiring diagram. Your boiler terminals are the same as this schema but without the PL terminal for the pump. You should consult your boiler's manual for how the pump is connected. It might not even
need a separate line feed.

For the sake of neatness I would suggest you install a wiring center like the 8 term one in the schema so that you can
stick to the standard layout. If you don't already have a timer it would make it easier to install one later.

Anyway, compare the wiring diagram you get with your thermostat with this one.

I also include the wirin diagram for your boiler.

Room stat link is #2.

 

[Frank Noton]

An ordinary light switch would do when you think about it from the boiler's perspective. It couldn't care less.

When you buy a thermostat or timer usually you will get a wiring diagram that shows the appropriate wiring
depending on what kind of CH installation you have. There are two types : Y-Plan and S-Plan. The main
difference is in the way that the hot water is controlled. Y-Plan you normally have a single diverter valve
that has 3 positions. HW only, CH only or HW and CH. S-Plan has two separate valves one that allows water from
the boiler to heat the hot water and one that does the same for the CH. So determine what type you have.
If you have a combi boiler it is probably a Y-Plan with a single diverter valve. Once you figure that out
just go with the appropriate wiring for the plan you have.

Assuming you have a Y-Plan this is a typical Y Plan wiring diagram. Your boiler terminals are the same as this schema but without the PL terminal for the pump. You should consult your boiler's manual for how the pump is connected. It might not even
need a separate line feed.

For the sake of neatness I would suggest you install a wiring center like the 8 term one in the schema so that you can
stick to the standard layout. If you don't already have a timer it would make it easier to install one later.

Anyway, compare the wiring diagram you get with your thermostat with this one.

I also include the wirin diagram for your boiler.

Room stat link is #2.

View attachment 73477

Thanks Ray for that very helpful info, off to sort it all out.