Replace Heating Controller

Hi all,

My central heating runs off an old oil boiler which uses a Siemens RWB2E controller and doesn't have a room thermostat anywhere. By the looks of it the controller only controls the heating as the hot water is an immersion heater with a separate supply.

There are three cables to the controller. The first is the main supply which is fed directly from a junction box in the airing cupboard upstairs, the second goes to the CH pump and the third goes to the boiler itself.

I would like to replace the controller with a new smart one and was wondering whether there is a direct replacement available? I have seen the likes of Nest with a controller and a room thermostat but would a thermostat be of any use in my set up?

Is there any real benefit of adding the immersion heater to the controller? Looks like it might have been connected in the past but is now fed directly from the same JB as the controller (with no isolator etc).

What might be the best model for me to look at?

The kitchen is being rewired by the spark in a few weeks and I could get him to tidy this up too so it would be good to have it available for when he comes.

Regards

D

My oil fired central heating works on a C Plan. Basic idea is only boiler running and domestic hot water only, and pump and boiler central heating and domestic hot water (DHW) there is no way to have CH without DHW.

I used Nest Gen 3 but likely Hive is a better option. I found moving to Nest allowed ½ hour DHW every other day in summer, which was enough.

I had hoped for more info on your system.
I will agree that the wall thermostat is a minefield, there are so many options. My first and second house were so easy, I never thought about the problem until I returned to mothers old house. But that was so hard to control, I had a steep learning curve.

Many of the options don't apply to you with an oil boiler, so you want a simple on/off thermostat. But on moving here with oil, I found it was like turning the clock back 50 years, we still have steam trains here.

Latter oil boilers have a cool down sequence, but oil boilers tend to be twice the cost of gas, and also last twice as long, so many homes have 25 year old boilers. These often don't have a cool down sequence and rely on being able to get rid of the heat on switching off by heating the domestic hot water.

We talk about the Honeywell Plans, C, Y, W, S etc, and both the C and Y plans allow the boiler to transfer heat into DHW after being switched off, so widely used.

The C plan has three versions, the basic has no motorised valve, and no tank thermostat, some add a tank thermostat to assist summer use to heat the DHW and some even have a motorised valve so DHW can be turned off, but the latter stops the cooling by heating DHW so rare with oil.

So the idea today is to combine the thermostat and programmer, this allows you to turn down the heating at set times rather than turn it off, but it causes a problem with the C Plan. This is the old C Plan. once you combine the time controller and wall thermostat you have a problem. The time controller switched on the boiler and pump through the thermostat when central heating is required and only the boiler when DHW only is required. Hive wall thermostats can be also configured internally to do this, so one output to pump and one to boiler. But Nest has no internal setting however it does have volt free contacts so this allows it to still be done, but boiler fed from common not the normally open contact, which is reverse to normal, and often the common is internally linked to line, so many systems can't be wired to work with C Plan.

Many systems are designed for gas, and the gas boiler can reduce output (modulate) which is not really done with oil, so with the gas systems the TRV is king, but with oil although the TRV helps, as does setting the lock shield valve, it is not as important as with gas.

So I know Nest and Hive will work with oil, and likely Hive is best option, but it seems unlikely Wiser or Tado will work, I in error said Hive would not work in the past, I was unaware of the software option. So I am not sure others don't also have that software option.

The programmers (fancy word for timer) had typically 16 options (odd way of counting as they also counted off) but for C plan only 10 options, and there were often both mechanical and electrical switches which stop you selecting central heating only, as pipe work would not allow you to turn off DHW, but everything seems to be based on gas, so newer programmers let you select CH only but with C Plan still heated the DHW.

With old gas pre the modulating boiler often it was designed to just remove the chill, my parents system simply ran twice a day, warming up house in morning before the cooking stove was lit, and in evening before the solid fuel fire was lit. Likely my oil system was the same, with a 18 kW boiler for a three story house, but better insulation has resulted in it being ample as the only form of heating, I have an open fire, but never lit a fire in it.

What a comprehensive answer, thank you. I have added some photos of the controller and the wiring. There is no valve anywhere as far as I can tell. The boiler itself has a control to regulate the amount of heat output but there is no other thermostat. The hot water appears to only be an immersion heater but I can’t be sure. I think if there is no valve though this is most likely??

Regards

D

Looking at the Hive installation guide it shows a common live and then CH ON and CH OFF are at positions 2 and 4 (see image attached). In my set up it shows that there is a live at position 3 and position 4. One of these goes to the boiler (position 4 I think) and one goes to the pump. Would I therefore have to rewire this to take my position 3 to their position 2 and keep my position 4 going to their position 4?

Thanks

dunny said:

Is there any real benefit of adding the immersion heater to the controller? Looks like it might have been connected in the past but is now fed directly from the same JB as the controller (with no isolator etc).

Click to expand...

Hive is rated at 3A and an immersion heater at 13A so not a good idea to connect the immersion heater to the Hive unless you want to melt it. They are two totally separate and independent systems, Hive will control hot water heated by a boiler, and an immersion is an electric heating element that is inserted into the hot water cylinder as a back up for when the boiler isn't working. It's a similar situation to having a radiator connected to a boiler and an electric fire in the same room. Both will heat the same room, but they are in no way connected to each other. Likewise the boiler and the immersion heater will heat the same hot water tank but not connected to each other.

You should be able to isolate them both separately, so not both are 'on' at the same time. The heating system should also have a 3A fuse [occasionally 5A is specified for some boilers] protecting it somewhere.

If you don't have motorised valves it's likely that you have a system with pumped central heating and natural circulation of hot water around the cylinder like the diagram below. This type of system is much more common than a C Plan.




With this type of system, it's not possible to have the heating 'on' without the hot water being 'on' at the same time. A Hive Dual Channel version can be used, but it would need to be set to operate as below:



As you have wires in both terminals 3 & 4, it would appear that both heating and hot water are connected to your existing programmer. Terminal 3 is for the Hot Water and terminal 4 is for the Central Heating....



....which as you will notice, is exactly the same as the Hive Dual Channel diagram that you posted. Terminal 3 will control the boiler to heat the hot water by natural (gravity) circulation. terminal 4 will start the pump to heat the radiators.

Thanks

Pretty sure we get no hot water without the immersion heater though

I will follow the cables again and confirm.

D

One of the usual clues is the number of water pipes connected to the boiler, if yours is the type of system I described, then there will be 4 pipes. 2 for the central heating and 2 for the hot water, as per the diagram.

It would be odd for you to have a RWB2 programmer with Central Heating & Hot Water control....

.... and wires in the appropriate terminals and it not heat the hot water. Unless maybe it was originally like that and for some reason the Hot Water cylinder plumbing to the boiler is no longer working. If so, I would suggest getting it looked at so that the hot water is heated by the boiler, it costs twice as much than oil to heat the hot water with electricity.

dunny said:

I will follow the cables again and confirm.

Click to expand...

No follow the pipes, I have a C plan or thermo syphon or gravity which ever name you like, there is no electrical connection to the cistern other than the back up immersion heater.

If not sure turn off the immersion heater and see if you run out of hot water.

I am an electrician not a plumber, I employed a plumber to alter my system, he tells me if the pipe work is done correct, then the pump starves the thermo syphon and you only get central heating, but at least with mine this is not the case, in summer I have it set ½ hour every other day, four days a week, and in winter turn off the domestic hot water (DHW) and it heats it anyway.

I use Nest Gen 3, in hind sight likely Hive would have been better, main reason for Nest Gen 3 is I wanted it mains powered and only two wires between flat under house where boiler is, and main house where I want the thermostat, and Nest Gen 3 both charges the wall thermostat and takes info from it to heat link with just two wires, it was not fitted for Geofencing or any other internet connection, and nearly all the cleaver stuff has been disabled, in fact bit of a pain disabling it all. And the bit which would have been really handy, connection to TRV heads, does not work.

Yes now I have Nest Gen 3 I do use the geofencing, but it turns heating off around once a month so not worth having for that. Biggest improvement to my central heating was fitting electronic TRV heads, this means it only heats rooms being used, so heats the rooms faster.

However I will admit the electronic TRV head with oil fired boiler does not work as well as it did with gas fired boiler.

I have nine electronic TRV heads, 5 x eQ-3 and 4 x Energenie, the latter can also be set to geofence, however it uses IFTTT (if this then that) which was free, but now costs, so don't use it any more. Also the anti hysteresis software was OTT, and I have to cheat, set it to 22°C for one hour then 20°C, so the geofencing does not work too well anyway, so really the cheap eQ-3 work better than the Energenie.

My house is really too big for geofencing to work, it takes too long to heat and cool, simple time works better.