Usage of HT-Bus (D17) for Worcester Bosch

Cyrax

Hey,

I'm going to purchase heating & hot water control for my home which has a Worcester Greenstar 30CDi Classic Regular ErP boiler. I currently have a hot water tank and a honeywell programmer and wall thermostat.

I don't believe the boiler supports OpenTherm.

• So is the wireless starter kit suitable for controlling both heating (radiators) and hot water? Should I buy the OpenTherm version anyways?
• Is the wired thermostat not suitable for replacing my wired manual thermostat? Must I get a wireless version to control heating and hot water?
• Also, looking at install instructions, it says both relay on/off and HT-Bus (D17) is supported. What is the benefit of the latter?

Thanks

policywonk

In reverse order.
1. Your boiler has throttle controls (EMS-BUD) which enables it to be more efficient in its use of gas than when it is run in on/off switching mode. That can be enabled via the terminator on the pcb named ST19. This needs theTado interface running in HT-Bus mode which can handle the EMS-BUS.
2. Your boiler seems to be a system boiler with seperate outputs for CH and HW- is that correct? Or does it just produce heat, which is then routed by externally controlled valves to both?
3. Your existing wired thermostat for CH, is likely to be serving as a switch for the central heating motorised valve. If you wish to keep the valves you will need the replacement thermostat to also be wired. Otherwise it won't open when you need heat.
4. Do you have a HW tank? If so what specific thermostat model does it use? If it uses a simple switch thermostat, that probably drives another valve. So do you have an s plan in place?
5. If you want to save money on heating bills, the HT BUS/EMS BUS controls need to be activated.
* you still have to have a smart wired stat to drive the CH valve
* If you have a HW tank and a HW valve that needs thinking about.
* A wireless receiver may be needed to talk to the boiler in HT BUS mode, whilst the thermosats drive the valves.
Come back with the details of your existing setup, with models identified, including the exact boiler model

Cyrax

Thanks @policywonk. Very detailed.

Here are photos of the entire setup:

CH thermostat

boiler

HW cylinder

Programmer

Junction panel (hopefully can ignore!)

I don't know if I have s or y plan valve setup. I have a honeywell programmer (ST6400C) and CH is controlled by wired thermostat. Hot water goes to the hot water tank. I'm not sure if there is a tank for cold water in the loft. Need to check.

Certainly sounds like the EMS-BUS is the way to go so will look at that at install time.

My desire is to have a wired thermostat replace existing one that will call for CH. Hot water usage is controlled by the tado programmer on a schedule. The later I will add TRVs to control heating per room.

Also then, shall I not bother with the OpenTherm kit?

Cyrax

Something serious wrong with this forum. Replies and posts just randomly get deleted - https://community.tado.com/en-gb/discussion/26042/what-happened-to-my-post#latest

I'll try and post comment again.

Cyrax

Thanks @policywonk. Very detailed.

Here are photos of the entire setup:

1. CH thermostat
2. Boiler
3. HW cylinder
4. Programmer
5. Terminal panel (hopefully can ignore this fully!)

I don't know if I have s or y plan valve setup. I have a honeywell programmer (ST6400C) and CH is controlled by wired thermostat. Hot water goes to the hot water tank. I'm not sure if there is a tank for cold water in the loft. Need to check.

Certainly sounds like the EMS-BUS is the way to go so will look at that at install time.
My desire is to have a wired thermostat replace existing one that will call for CH. Hot water usage is controlled by the tado programmer on a schedule. The later I will add TRVs to control heating per room.

Also then, shall I not bother with the OpenTherm kit?

policywonk

The good news is that your boiler does not have a controller on its front panel. Makes the change easier. Could you check again at the tank? Was that two, two port motorised valves?

I think you'll need to check on the exact thermostat model used at the hot water tank.
1.Give the following exact model details to Tado sales:
* the boiler
* the hw tank stat
* the CH stat
* programmer
2. The fact that you have two two port valves, one for CH, one for HW, in what seems to be an S Plan.
3. Make sure they know you want to use HT-BUS for the EMS BUS link to the boiler and that you need detailed instruction links to complete the changes for each part.

They will probably recommend you adopt
1. An Internet bridge for Tado
2. A tado wired thermostat for the CH control.
3. No change to the HW side.
4. A Tado EU wireless receiver to replace the programmer.
5. Some rewiring of your wiring centre is undertaken by a professional because the wiring to the programmer will need to be altered.

Let us know if this needs explaining or their response is confusing

policywonk

Regarding point 5. Normally the wiring of an s plan two channel system is clearly set. Here is an example. http://wiki.diyfaq.org.uk/index.php/File:S-Plan-Wiring.gif
Now you'll see from that drawing that the programmer plays a significant role in arranging mains power through the wiring centre and then through the thermosats to open the zone valves. This must be changed when your programmer drives the boiler with HT-BUS. It won't be putting out mains voltage on the CH side. So the CH stat will operate the valves at mains voltage. So the wiring centre must not assume that the programmer is putting our mains voltage.
The thermostat for the hw may be one which is compatible with ems bus, but you have to check

Cyrax

Thanks @policywonk
. Very detailed.

Here are photos of the entire setup:

• 
  CH thermostat
• boiler
• HW cylinder
  
• Programmer
  
• Junction panel (hopefully can ignore!)

I don't know if I have s or y plan valve setup. I have a Honeywell programmer (ST6400C) and CH is controlled by wired thermostat. Hot water goes to the hot water tank. I'm not sure if there is a tank for cold water in the loft. Need to check.
Certainly sounds like the EMS-BUS is the way to go so will look at that at install time.

My desire is to have a wired thermostat replace existing one that will call for CH. Hot water usage is controlled by the tado programmer on a schedule. The later I will add TRVs to control heating per room.

Also then, shall I not bother with the OpenTherm kit?

policywonk

Got those photos. By the hot water tank there are two motorised valves.

• A two port motorised valve looks like this: Honeywell V4043H 1056 - 2 Port Motorised Zone Valve 22mm V4043H1056 - Specialists in plumbing & heating spares
• A three port motorise valve looks like this: Honeywell Home V4073A1039/U 3 Port Motorised Valve 22mm Compression - Screwfix

It is important to know whether you have one of each type, or whether both are two port valves.

Understood, that you want to replace your wired CH thermostat with a Tado wired thermostat. That wont be a problem.

There is a change which will probably need the attention of an electrician, ideally an electrician who works with a firm that installs Greenstar boilers and knows how to enable EMS-BUS controls. The alternative, if you are very comfortable handling wiring, is you do this yourself. The work will require no more than 1/2 day from a professional. You might need a whole day if you keep track of your process carefully.

Presently your central heating wiring centre is likely arranged to comply with the wiring given in this link Central Heating Controls and Zoning - DIYWiki (scroll down until you see what I believe you have - the S Plan - it gives both the plumbing layout and the wiring layout). The wiring must be altered so that:

a) The programmer point is altered in this way:

• It provides mains power to this control point (LIVE, NEUTRAL, EARTH)
• It takes back low voltage EMS-BUS instructions from that control point, to the boiler for central heating, and mains powered control for hot water.
• the wireless receiver now replaces the original honeywell.

b) The CH thermostat remains mains powered to it can drive the motorised valve when there is a call for heat. However it is told when to open and close, by receiving a wireless command from the wireless receiver.

c) The HW thermostat must remain mains powered so it can drive its motorised valve when there is a call for heat. It is also told when to open and close, by receiving a wireless command from the wireless receiver.

d) The wiring between the boiler and the wiring centre must be altered from running in mains-only, switching mode, to one where it only talks to the Tado wireless receiver via the EMS-BUS. It must replace

1. the old mains-voltage control wiring to the boiler which until now has used basic mains powered on-off power controls to control the boiler;
2. with one pair of wires running between the wireless receiver and the boiler's EMS-BUS point, controlling the boiler in digital (very low voltage) mode.
3. thus enabling you to replace the honeywell programmer with the Tado EU wireless receiver;
4. therefore this link Greenstar 30 CDi System Installation and Servicing Instructions | Worcester Bosch, pages 26-29 - need to be studied:

• they set out how this boiler can be wired for either (i) the original mains powred on-off control system for the thermostat and programmer, or (ii) the EMS-BUS layout, which is not mains powered;
• given that your boiler was first commissioned to be controlled in mains powered mode, one has to remove the mains-powered control wires for central heating and thermostat; then some wired bridges need to be reinserted at the connector points (shown in the installation drawings),
• then wire in the Tado wireless receiver control.

e) Ensure that the wiring centre is updated to comply with these changes. Double check.

@wateroakley do you agree with my reading of this? Need a second opinion because I dont think this boiler can run HW in on-off mode and CH in EMS-BUS mode - it has to have one EMS-BUS controller which then takes over both, hence my recommendation to add an EU Wireless Receiver and the need to update the wiring centre.

I also believe that Tado should set out the instructions clearly for the conversion. @Cyrax it is important to write to Tado sales, explaining what you have in place and asking for detailed written instructions to complete the migration, including the conversion to EMS-BUS/HT-BUS control mode. Tado will face a problem. Their X series doesnt interface with HT-BUS. The V3+ series does. They need to confirm that and explain that you dont need Opentherm.

wateroakley

@Cyrax @policywonk Some observations.

KISS. Keep It Simple ...

• At our rellies, we connected a Tado V3+ wireless starter kit (relay mode) to a similar vintage Worcester Greenstar combi boiler with an internal 24/7 mechanical timer.
• We disconnected the same wired thermostat and rewired the connections outside the boiler.
• The boiler power will modulate automatically from 30.9kW down to 8kW 😀. If it can already do this, why bother with the complications for EMS?

KISS. Suggestion …

• A Tado wireless starter kit (relay mode). The boiler will modulate automatically.
• The wireless receiver is an easy replacement for your Honeywell programmer and will default to S-Plan.
• Disconnect the old thermostat. Check for a wiring label on the back of the thermostat. Bridge two of the wires L and SL (?). Terminate the other wires safely. Either at the thermostat or at/near the Honeywell wiring centre.
• With TRVs, the boiler documentation says you may need an external bypass. Either an automatic bypass on the pipework, or keep one rad without a TRV.

Looking at the pictures …

• Two honeywell 2-port motorised valves are visible next to the Megaflo hot water tank.
• Unable to see an automatic bypass (nor a manual bypass) on the pipework?
• The Worcester boiler provides a sealed CH and open-vented HW.
• The Honeywell wiring boxes can be a tight fit for the wiring, and a pain when changing wiring. With the labels on the board, it should be easy to figure out what wire goes where for the thermostat and programmer. The choc-blocks and wires make it hard to see what's what.
• The Tado wireless receiver and wiring is an easy replacement for the Honeywell ST6400.
• The system appears to be wired as 'relay'. With two valves, that will be S-Plan wiring.
• Presumably, the grey box on the Megaflo is the HW thermostat connection? And power for the immersion heater?
• Recommend to add insulation on the pipework.

policywonk

Two very different views. Your call.