Central heating timer replacement

[Ian Molyneux]

Hi all, hoping for some help with this one. I’m changing a danfoss 3060 timer controller to a Siemens RWB30E.

The wiring going to the existing timer are boiler, pump and room thermostat. The pictures and drawing show the existing timer wired up, drawing of existing wiring and new connections required.
Can anyone help with a diagram of the new wiring connections required.

 

[stem]

The Danfoss 3060 is a dual channel programmer. One channel provides control of the heating and the other channel is for the hot water.

The Siemens RWB30E is only a single channel timeswitch.....

.....so it's not compatible. The Siemens RWB2E is the dual channel version with separate heating and hot water control.

 

[Ian Molyneux]

Thanks for the reply stem
The boiler is only used for heating, not hot water, they use the immersion heater for hot water. So could this still be used. There’s no valve on the system, just wires to boiler, pump and thermostat

 

[stem]

This is all a bit odd

1) Looking at the existing wiring suggest that it is connected to a elderly gravity hot water / pumped central heating system. In which case, turning only the HW on starts the boiler running and heats the hot water via gravity circulation. When the CH is selected as well, then pump starts and circulates the hot water around the radiators. The drawback as you have pointed out is that it will not have motorised valves and so it will not be possible to have the heating on without the hot water being on at the same time.

The wiring does appear (as best I can see) to be connected to both channels of the existing programmer. In which case in the above scenario the boiler live will be connected to the hot water channel (Danfoss terminal 4) and the pump live to the central heating channel (Danfoss terminal 2).

It's easy to check. Turn on the HW only and just the boiler should operate. Then when the CH is selected as well, the pump will also start.

2) It would very bizarre for someone to install a system with a boiler and not connect it to the hot water cylinder. For one thing it would have been a simple and cheap thing to do when the system was being installed. Also, the cost to heat hot water with electricity is normally about 3 times that of using gas.

3) If the HW cylinder is connected to the boiler it will still be heating the hot water whenever the boiler is on anyway, regardless of the immersion heater.

If you can confirm that the boiler is in no way whatsoever connected to the hot water cylinder, and is solely piped to the radiators. I can provide you with the information you require.

 

[Ian Molyneux]

It is an old gravity hw and pumped ch system, oil fired boiler if that makes any difference.

I’m not at the house today but will check it as soon as I can. I believe the hot water is connected to the heating system but it wasn’t getting hot enough hence using the immersion.

 

[stem]

That would make sense, unfortunately gravity HW does rely on good pipework design.

Looking again at the wiring it does appear odd. From what I can see it appears that the room thermostat isn't connected in circuit. It appears to have a red wire connected to terminal 3 that has no function as far as the programmer is concerned, and if the two red wires in terminal 4 are to the boiler and pump they have been wired together so both channels aren't actually being used at the moment.

As it's wired I reckon that the room thermostat isn't working at all, and that the boiler and pump will both start up together when hot water is selected, and nothing will change when CH and HW are selected.

It's difficult to be certain as I don't know exactly what is on the other end of each of the cables. But your test will confirm.

 

[Ian Molyneux]

Wiring As it is, is as follows
Terminal 1 boiler and pump neutrals and blue from room stat
Terminal 2 yellow from room stat
Terminal 3 red from room stat
Terminal 4 boiler and pump lives
Terminal 5 empty
Terminals 6/7/8. Power from fused spur and earths
Now, following the room stat cables, they go into a connector block that has extended the cable. The cable from time switch to block are 3 core & earth, from lock to stat are 2 core & earth. The red, yellow and earth are connected and go to the stat, but the blue just terminates and ends there??
Am I right in thinking that red or yellow are live to the stat then comes back as switched live when the stat asks for heat?

 

[stem]

That's as I had figured it. So:

Terminal 1 boiler and pump neutrals and blue from room stat

All as it should be

Terminal 2 yellow from room stat

That's 'Heating on' and should be the live to the thermostat (usually the red wire is used for this, not yellow)

Terminal 3 red from room stat

This terminal has no function as far as the programmer is concerned so that is why I figured the room thermostat isn't working. However it can be used as a convenient junction box terminal and when it is, the pump live would also go here to connect it to the thermostat switched live (this would normally be the yellow wire, not red)

Terminal 4 boiler and pump lives

This is Hot water on. So the wires are wrong, the boiler only should go here and the pump live should be in 3 as per above, such that the room thermostat switches the pump on and off.

Terminal 5 empty

As per terminal 3 it has no function as far as the programmer is concerned.

Terminals 6/7/8. Power from fused spur and earths

That's OK. Terminal 6 is the Live supply and 7 is Neutral (Terminal 7 is for convenience and has an internal link to terminal 1]

Am I right in thinking that red or yellow are live to the stat then comes back as switched live when the stat asks for heat?

Yes. Normally, the wire from programmer 2 is the 'live supply' to the room thermostat (usually the red wire is used for this)
The 'switched live' wire back from the room thermostat (usually the yellow wire is used for this) goes to the pump live
The blue wire is used for the thermostat neutral supply (Not all room thermostats need a Neutral connection)

It appears to me that the red wire from the pump has been moved from terminal 3 to 4. This may be so that the heating and hot water always operate together regardless of the programmer selection, or the room thermostat wasn't working and the wiring mod was intended as temporary way of keeping the heating working until the thermostat was fixed.

 

[Ian Molyneux]

So is it possible to use the RWB30E on this system and what connections would need to be made?

 

[stem]

As it is just a single channel timeswitch it will of course switch on both central heating (pump) and hot water (boiler) together. However, the room thermostat would still control the heating by switching the pump off when the room temperature was reached.

So, if the instructions for the RWB30E that you have is as per this:

It would be connected as below. You may or may not need a room stat N connection.

 

[oilhead]

Given that you are not using the boiler for hot water only, then yes, it can be used. You need live and neutral to the L & N terminals, a link from l to terminal 2, then a feed from terminal 4 to both the boiler and pump via the room thermostat. The component neutrals need to be commonned with the live supply. You will find it easier to bring your wiring into a wiring centre or adaptable box with a connecting block as there is very little room on the RWB base plate.

 

[Ian Molyneux]

Just to confirm, only the pump via the room stat, not the boiler?

 

[stem]

That's normally how it's done. Most folks wouldn't want to have their radiators on in the summer in order to get some hot water.

Ideally the plumbing should be looked at so that the boiler heats the hot water properly (cheaper than elec ) so the immersion heater isn't used. Fudging / bodging the wiring to sort out a mechanical problem isn't good practice IMHO and I wouldn't do it.

Having said that, @oilhead appears ok with putting the boiler and pump though the room stat and I understand why you might want to control both via it if you don't want to use the boiler to heat hot water, because when the house was warm the room stat would turn off both the boiler and the pump.

 

[Ian Molyneux]

Sorry to keep going on about it.
I’ve got my head around the wiring now, thank you.
So if I put the pump via the room stat, and the boiler from the time switch term 4, then the time switch will control the hw and the room stat control the ch? And so could work independently, timer saying on so heats hw but stat saying temp reached so no ch?
Sorry if I’m complicating things

 

[Ian Molyneux]

There’s no thermostat on the hw cylinder by the way. I think there was one by the boiler but it’s been disconnected. Should there be one and could one be fitted somehow and where